FIELD OF THE INVENTION

The invention relates to a transimpedance amplifier, for instance a transimpedance amplifier of an active antenna comprising a loop aerial or a screened loop aerial.

The French patent applicationNo. FR2101610 of 18 February 2021, entitled “Amplificateur a transimpedance” is incorporated by reference.

PRIOR ART

In what follows, “coupled” always refers to an electrical coupling. When applied to two items such as terminals, conductors, nodes, etc, “coupled” may indicate that the items are directly coupled, that is to say connected (or, equivalently, in electrical contact) to one another, and/or that the items are indirectly coupled, in which case an electrical interaction different from direct coupling exists between the items, for instance through one or more components. When applied to two multi-terminal items, such as ports, connectors, etc, “coupled” may indicate that the items are directly coupled, in which case each terminal of one of the items is directly coupled to one and only one of the terminals of the other item, and/or that the items are indirectly coupled, in which case an electrical interaction different from direct coupling exists between the terminals of the items, for instance through one or more components. In what follows, in line with circuit theory, a port has exactly two terminals.

Loop aerials, more commonly referred to as “loop antennas”, and screened loop aerials, more commonly referred to as “shielded loop antennas”, are well known to specialists. They are used for radio reception applied to electromagnetic field measurements, to direction finding and to radio communications. Some characteristics and limitations of these antennas for these uses are explained in the article of F. Broyde and E. Clavelier entitled “Contribution to the Theory of Planar Wire Loop Antennas Used for Reception”, published in the journal IEEE Transactions on Antennas and Propagation, vol. 68, no. 3, in March 2020. In particular, this article explains to what extent it is possible to consider that these antennas measure a component of an incident magnetic field, that is to say a magnetic component of an incident electromagnetic field. As explained in this article and in paragraph 5-4 of chapter 5 of the book of R.C. Johnson entitled “Antenna Engineering Handbook, 3rd Edition”, published by McGraw-Hill in 1993, the screen (also referred to as “shield”) of a screened loop aerial typically operates as a loop aerial. Screened loop aerials used for radio reception provide better results than unscreened loop aerials, because they are not affected by a common-mode current flowing on the cable linking the antenna to a measuring instrument or a radio receiver, induced by an incident electromagnetic field received as a signal, or by electromagnetic disturbances. One uses “antenna factor” to designate an absolute value of a ratio of an intensity of an incident field (expressed in V/m for an electric field or in A/m for a magnetic field) to a voltage developed by an antenna across a specified impedance. F or electromagnetic field measurements, one often prefers, when it is possible, to use an antenna presenting an antenna factor which is substantially independent of the frequency, over a wide frequency band. The specialist knows that this result may be obtained, in a known frequency band, the known frequency band having a least upper bound, the least upper bound corresponding to a wavelength in vacuum, by utilizing, for radio reception, a prior art active antenna comprising: a passive antenna, the passive antenna being a single-turn loop aerial or a single-turn screened loop aerial, the passive antenna being small compared to said wavelength in vacuum, the passive antenna having a port, an impedance presented by this port being referred to as “impedance of the passive antenna”; an amplifier, the port of the passive antenna being coupled to an input of the amplifier, the input of the amplifier having an impedance referred to as “input impedance of the amplifier”, an absolute value of the input impedance of the amplifier being, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna, an output voltage of the amplifier being, at a given frequency in the known frequency band, equal to the product of an input current of the amplifier and a transimpedance, an absolute value of the transimpedance being substantially independent of the given frequency.

The specialist knows that a suitable amplifier could in principle be a transimpedance amplifier or a transresistance amplifier similar to the ones discussed in paragraph 8.11 of the book of P. Horowitz and W. Hill entitled “The Art of Electronics, Third Edition”, published by Cambridge University Press in 2015, or to the ones which are used in the international application number PCT/DE2006/000415 (WO 2006/097071) entitled “Active reception antenna system”.

Such a classical transimpedance amplifier, intended to provide a known transimpedance in a known frequency band, comprises: an input port having a first terminal and a second terminal, the second terminal of the input port being directly coupled to a reference conductor ; an operational amplifier, the first terminal of the input port being directly coupled to an inverting input terminal of the operational amplifier, a noninverting input terminal of the operational amplifier being coupled to the reference conductor; and one or more means for creating a feedback, the feedback determining the known transimpedance in the known frequency band, the feedback reducing an absolute value of an impedance presented by the input port in the known frequency band.

Typically, said one or more means for creating a feedback comprise a resistor having two terminals, a terminal of this resistor being directly coupled to an output terminal of the operational amplifier, the other terminal of this resistor being directly coupled to the inverting input terminal of the operational amplifier. Often, said one or more means for creating a feedback also comprise a small-value capacitor connected in parallel with the resistor.

Such a classical transimpedance amplifier unfortunately raises two problems when it is used in the active antenna considered above, for a known frequency band which is the band 9 kHz to 30 MHz: firstly, the input port of such a transimpedance amplifier cannot be directly coupled to the passive antenna, because the feedback becomes, at low frequencies and at the zero frequency, too weak to provide a suitable biasing of the operational amplifier (noting that an indirect coupling to the passive antenna, through a capacitor, is not a good option in this known frequency band); and, secondly, such a transimpedance amplifier is much more noisy than necessary, because it is not optimized for the impedance of the passive antenna.

Thus, for a passive antenna which is a single-tum loop aerial or a single-turn screened loop aerial, small compared to the wavelength in vacuum corresponding to the least upper bound of the known frequency band, the prior art does not disclose an amplifier such that: an absolute value of the input impedance of the amplifier is, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna; an absolute value of the transimpedance of the amplifier is substantially independent of the frequency, in the known frequency band; the input port of the amplifier can be directly coupled to the passive antenna; and the amplifier produces a very low noise in the known frequency band, when its input port is directly coupled to the passive antenna.

SUMMARY OF THE INVENTION

The purpose of the invention is a transimpedance amplifier, without the above-mentioned limitations of known techniques.

A base of a bipolar transistor is an electrode which may be used to control a collector current of the transistor and an emitter current of the transistor. Thus, in the patent of the United States of America No. 3851235 entitled “Bridge circuit for controlling a direct current motor”, a base of a bipolar transistor is referred to as “control electrode”. A gate of a field effect transistor is an electrode which may be used to control a drain current of the transistor and a source current of the transistor. Thus, in the patent of the United States of America No. 5426382 entitled “Complementary logic recovered energy circuit”, a gate of a field effect transistor is referred to as “control electrode”.

In what follows, “control electrode” means either a base of a bipolar transistor, or a gate of a field effect transistor. It is also the case in the patent of the United States of America No.4754233 entitled “Low noise ultra high frequency amplifier having automatic gain control”, and in the patent of the United States of America No. 5617056 entitled “Base current compensation circuit”.

In what follows, in line with the “IEC multilingual dictionary of electricity” edited by the Bureau Central de la Commission Electrotechnique Internationale in 1983, “closed-loop control” (which is also referred to as “feedback control”) means control in which the control action is made to depend on a measurement of the controlled variable.

An amplifier of the invention is an amplifier providing a transimpedance in a known frequency band, the amplifier comprising: an input port having a first terminal and a second terminal, the second terminal being directly coupled to a reference conductor; a first amplifying device comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a control electrode of the transistor, a voltage between the third terminal and the second terminal being positive, a current flowing out of the second terminal and a current flowing in the third terminal being positive and controlled by a voltage between the first terminal and the second terminal, the second terminal being coupled to the first terminal of the input port; a second amplifying device comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a control electrode of the transistor, a voltage between the third terminal and the second terminal being negative, a current flowing out of the second terminal and a current flowing in the third terminal being negative and controlled by a voltage between the first terminal and the second terminal, the second terminal being coupled to the first terminal of the input port; a third amplifying device comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a control electrode of the transistor, a voltage between the third terminal and the second terminal being negative, a current flowing out of the second terminal and a current flowing in the third terminal being negative and controlled by a voltage between the first terminal and the second terminal, the first terminal being coupled to the third terminal of the first amplifying device; a fourth amplifying device comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a control electrode of the transistor, a voltage between the third terminal and the second terminal being positive, a current flowing out of the second terminal and a current flowing in the third terminal being positive and controlled by a voltage between the first terminal and the second terminal, the first terminal being coupled to the third terminal of the second amplifying device, the third terminal being coupled to the third terminal of the third amplifying device; a first capacitor having a first terminal and a second terminal, the first terminal being coupled to the first terminal of the first amplifying device, the second terminal being coupled to the reference conductor; a second capacitor having a first terminal and a second terminal, the first terminal being coupled to the first terminal of the second amplifying device, the second terminal being coupled to the reference conductor; one or more means including a first differential amplifier comprising two or more transistors, for creating a first closed-loop control in which a controlled variable is a bias voltage between the third terminal of the third amplifying device and the reference conductor, an output of the first differential amplifier being in a first case coupled to the first terminal of the first amplifying device, or in a second case coupled to the first terminal of the second amplifying device; one or more means including a second differential amplifier comprising two or more transistors, for creating a second closed-loop control in which a controlled variable is in said first case a bias voltage between the third terminal of the second amplifying device and the reference conductor, or in said second case a bias voltage between the third terminal of the first amplifying device and the reference conductor, an output of the second differential amplifier being in said first case coupled to the first terminal of the second amplifying device, or in said second case coupled to the first terminal of the first amplifying device; and one or more means for creating a feedback, the feedback determining the transimpedance in the known frequency band, the feedback reducing an absolute value of an impedance presented by the input port in the known frequency band.

The specialist understands that, in the previous sentence: “voltage” means a potential difference; and “bias voltage”, which may also be referred to as “quiescent voltage”, means a dc voltage in the absence of a signal applied to the input port of the amplifier of the invention.

The specialist understands that said feedback is such that the amplifier of the invention is a transimpedance amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and characteristics will appear more clearly from the following description of particular embodiments of the invention, given by way of non-limiting examples, with reference to the accompanying drawings in which:

Figure 1 is a block diagram of a transimpedance amplifier of the invention (first embodiment);

Figure 2 is a block diagram of a trans impedance amplifier of the invention (second embodiment);

Figure 3 is a first part of a schematic diagram of a transimpedance amplifier of the invention (third embodiment); and

Figure 4 is a second part of the schematic diagram of a transimpedance amplifier of the invention (third embodiment). DETAILED DESCRIPTION OF SOME EMBODIMENTS

First embodiment.

As a first embodiment of a device of the invention, given by way of non-limiting example, we have represented in Figure 1 the block diagram of a transimpedance amplifier for a known frequency band, the transimpedance amplifier comprising: an input port ( 1 ) having a first terminal (11) and a second terminal (12), the second terminal of the input port being directly coupled to a reference conductor (ground); a first amplifying device (21) comprising a first terminal, a second terminal, a third terminal and at least one transistor, the first terminal of the first amplifying device being directly coupled to a control electrode of the at least one transistor of the first amplifying device, the first amplifying device being such that, if a voltage between the third terminal of the first amplifying device and the second terminal of the first amplifying device is greater than a given positive voltage, then a current flowing out of the second terminal of the first amplifying device and a current flowing in the third terminal of the first amplifying device are nonnegative and mainly depend on a voltage between the first terminal of the first amplifying device and the second terminal of the first amplifying device, the second terminal of the first amplifying device being coupled to the first terminal of the input port; a second amplifying device (22) comprising a first terminal, a second terminal, a third terminal and at least one transistor, the first terminal of the second amplifying device being directly coupled to a control electrode of the at least one transistor of the second amplifying device, the second amplifying device being such that, if a voltage between the third terminal of the second amplifying device and the second terminal of the second amplifying device is less than a given negative voltage, then a current flowing out of the second terminal of the second amplifying device and a current flowing in the third terminal of the second amplifying device are nonpositive and mainly depend on a voltage between the first terminal of the second amplifying device and the second terminal of the second amplifying device, the second terminal of the second amplifying device being coupled to the first terminal of the input port; a third amplifying device (23) comprising a first terminal, a second terminal, a third terminal and at least one transistor, the first terminal of the third amplifying device being directly coupled to a control electrode of the at least one transistor of the third amplifying device, the third amplifying device being such that, if a voltage between the third terminal of the third amplifying device and the second terminal of the third amplifying device is less than the given negative voltage, then a current flowing out of the second terminal of the third amplifying device and a current flowing in the third terminal of the third amplifying device are nonpositive and mainly depend on a voltage between the first terminal of the third amplifying device and the second terminal of the third amplifying device, the first terminal of the third amplifying device being coupled to the third terminal of the first amplifying device; a fourth amplifying device (24) comprising a first terminal, a second terminal, a third terminal and at least one transistor, the first terminal of the fourth amplifying device being directly coupled to a control electrode of the at least one transistor of the fourth amplifying device, the fourth amplifying device being such that, if a voltage between the third terminal of the fourth amplifying device and the second terminal of the fourth amplifying device is greater than the given positive voltage, then a current flowing out of the second terminal of the fourth amplifying device and a current flowing in the third terminal of the fourth amplifying device are nonnegative and mainly depend on a voltage between the first terminal of the fourth amplifying device and the second terminal of the fourth amplifying device, the first terminal of the fourth amplifying device being coupled to the third terminal of the second amplifying device, the third terminal of the fourth amplifying device being directly coupled to the third terminal of the third amplifying device; a first capacitor (31) having a first terminal and a second terminal, the first terminal of the first amplifying device being coupled to the first terminal of the first capacitor, the second terminal of the first capacitor being coupled to the reference conductor; a second capacitor (32) having a first terminal and a second terminal, the first terminal of the second amplifying device being coupled to the first terminal of the second capacitor, the second terminal of the second capacitor being coupled to the reference conductor; one or more means including a first differential amplifier (41) comprising two or more transistors, for creating a first closed-loop control in which a controlled variable is a bias voltage between the third terminal of the third amplifying device and the reference conductor, an output of the first differential amplifier being coupled to the first terminal of the first amplifying device; one or more means including a second differential amplifier (42) comprising two or more transistors, for creating a second closed-loop control in which a controlled variable is a bias voltage between the third terminal of the second amplifying device and the reference conductor, an output of the second differential amplifier being coupled to the first terminal of the second amplifying device; a power amplifier (6) having, in the known frequency band, a voltage gain greater than 8/10 and less than 1, and an input impedance which is much higher than its output impedance, an input of the power amplifier being coupled to the third terminal of the third amplifying device and to the third terminal of the fourth amplifying device; an output port (7) having a first terminal (71) and a second terminal (72), the first terminal of the output port being directly coupled to an output of the power amplifier, the second terminal of the output port being directly coupled to the reference conductor; a resistor (5) for creating a feedback, the feedback determining, in the known frequency band, a transimpedance of the transimpedance amplifier, the feedback reducing an absolute value of an impedance presented by the input port in the known frequency band; a positive power supply line (91), a voltage between the positive power supply line and the reference conductor being made positive and stable by a power supply which is not shown in Fig. 1; and a negative power supply line (92), a voltage between the negative power supply line and the reference conductor being made negative and stable by said power supply.

The specialist understands that, in the previous sentence, “bias voltage”, which may also be referred to as “quiescent voltage”, means a dc voltage in the absence of a signal applied to the input port of the transimpedance amplifier.

The first closed-loop control and the second closed-loop control are configured to be such that: the voltage between the third terminal of the first amplifying device and the second terminal of the first amplifying device is greater than the given positive voltage; the current flowing out of the second terminal of the first amplifying device and the current flowing in the third terminal of the first amplifying device are positive and controlled by a voltage between the first terminal of the first amplifying device and the second terminal of the first amplifying device; the voltage between the third terminal of the second amplifying device and the second terminal of the second amplifying device is less than the given negative voltage; the current flowing out of the second terminal of the second amplifying device and the current flowing in the third terminal of the second amplifying device are negative and controlled by a voltage between the first terminal of the second amplifying device and the second terminal of the second amplifying device; the voltage between the third terminal of the third amplifying device and the second terminal of the third amplifying device is less than the given negative voltage; the current flowing out of the second terminal of the third amplifying device and the current flowing in the third terminal of the third amplifying device are negative and controlled by a voltage between the first terminal of the third amplifying device and the second terminal of the third amplifying device; the voltage between the third terminal of the fourth amplifying device and the second terminal of the fourth amplifying device is greater than the given positive voltage; and the current flowing out of the second terminal of the fourth amplifying device and the current flowing in the third terminal of the fourth amplifying device are positive and controlled by a voltage between the first terminal of the fourth amplifying device and the second terminal of the fourth amplifying device. For instance, it is possible that the first amplifying device is an NPN bipolar transistor having a base, an emitter and a collector, the first terminal of the first amplifying device being the base of the NPN bipolar transistor, the second terminal of the first amplifying device being the emitter of the NPN bipolar transistor, the third terminal of the first amplifying device being the collector of the NPN bipolar transistor; and that the second amplifying device is a PNP bipolar transistor having a base, an emitter and a collector, the first terminal of the second amplifying device being the base of the PNP bipolar transistor, the second terminal of the second amplifying device being the emitter of the PNP bipolar transistor, the third terminal of the second amplifying device being the collector of the PNP bipolar transistor. In this case, the specialist understands that the first amplifying device is an NPN transistor used in a common- base circuit, and that the second amplifying device is a PNP transistor used in a common-base circuit.

For instance, it is possible that the third amplifying device is a PNP bipolar transistor having a base, an emitter and a collector, the first terminal of the third amplifying device being the base of the PNP bipolar transistor, the second terminal of the third amplifying device being the emitter of the PNP bipolar transistor, the third terminal of the third amplifying device being the collector of the PNP bipolar transistor; and that the fourth amplifying device is an NPN bipolar transistor having a base, an emitter and a collector, the first terminal of the fourth amplifying device being the base of the NPN bipolar transistor, the second terminal of the fourth amplifying device being the emitter of the NPN bipolar transistor, the third terminal of the fourth amplifying device being the collector of the NPN bipolar transistor. In this case, the specialist understands that the third amplifying device is a PNP transistor used in a common-emitter circuit (with emitter degeneration), and that the fourth amplifying device is an NPN transistor used in a common-emitter circuit (with emitter degeneration).

For instance, if the first amplifying device and the fourth amplifying device are NPN transistors, the given positive voltage may be equal to one volt. For instance, if the second amplifying device and the third amplifying device are PNP transistors, the given negative voltage may be equal to minus one volt.

For instance, it is possible that at least one of said NPN bipolar transistors is replaced with a circuit having a similar behavior, for instance an NPN Darlington pair, or two NPN transistors used in a cascode circuit. For instance, it is possible that at least one of said PNP bipolar transistors is replaced with a circuit having a similar behavior, for instance a PNP Darlington pair, or two PNP transistors used in a cascode circuit.

The specialist understands that, if the input port of the transimpedance amplifier is directly coupled to a passive antenna which is a single -turn loop aerial or a single -turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the feedback created by the resistor (5) is negative (i.e., degenerative) in the known frequency band, and suitable for: determining, in the known frequency band, the trans impedance of the trans impedance amplifier, an absolute value of the transimpedance of the transimpedance amplifier being substantially independent of the frequency, in the known frequency band; and reducing, in the known frequency band, an absolute value of an input impedance of the transimpedance amplifier, the absolute value of the input impedance of the transimpedance amplifier being, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna. However, the feedback created by the resistor (5) is, at low frequencies and at the zero frequency, too weak to provide a suitable biasing of the third amplifying device and of the fourth amplifying device. The specialist understands that this is not a problem, because: the first closed-loop control is such that the bias voltage between the third terminal of the third amplifying device and the reference conductor is close to zero volt, for instance zero volt plus or minus ten millivolts; and the second closed- loop control is such that the bias voltage between the third terminal of the second amplifying device and the reference conductor is close to a wanted voltage applied to an inverting input terminal of the second differential amplifier. Thus, the first amplifying device, the second amplifying device, the third amplifying device and the fourth amplifying device are correctly biased.

If the first amplifying device and the fourth amplifying device are each a well-chosen low- noise NPN transistor, and if the second amplifying device and the third amplifying device are each a well-chosen low-noise PNP transistor, then the transimpedance amplifier produces a very low noise in the known frequency band, when its input port is directly coupled to the passive antenna, because the two common-base circuits are, from the standpoint of the noise produced, optimized for the impedance of the passive antenna.

Thus, for a passive antenna which is a single-tum loop aerial or a single -turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the transimpedance amplifier is such that: an absolute value of the input impedance of the transimpedance amplifier is, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna; an absolute value of the transimpedance of the transimpedance amplifier is substantially independent of the frequency, in the known frequency band; the input port of the transimpedance amplifier can be directly coupled to the passive antenna; and the transimpedance amplifier produces a very low noise in the known frequency band, when its input port is directly coupled to the passive antenna. Consequently, the transimpedance amplifier is a solution to the two problems explained above in the section on prior art.

Second embodiment.

As a second embodiment of a device of the invention, given by way of non-limiting example, we have represented in Figure 2 the block diagram of a transimpedance amplifier for a known frequency band, the transimpedance amplifier comprising: an input port ( 1 ) having a first terminal (11) and a second terminal (12), the second terminal of the input port being directly coupled to a reference conductor; a first amplifying device (21) comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal of the first amplifying device being directly coupled to a control electrode of the transistor of the first amplifying device, a voltage between the third terminal of the first amplifying device and the second terminal of the first amplifying device being greater than a given positive voltage, a current flowing out of the second terminal of the first amplifying device and a current flowing in the third terminal of the first amplifying device being positive and controlled by a voltage between the first terminal of the first amplifying device and the second terminal of the first amplifying device, the second terminal of the first amplifying device being coupled to the first terminal of the input port; a second amplifying device (22) comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal of the second amplifying device being directly coupled to a control electrode of the transistor of the second amplifying device, a voltage between the third terminal of the second amplifying device and the second terminal of the second amplifying device being less than a given negative voltage, a current flowing out of the second terminal of the second amplifying device and a current flowing in the third terminal of the second amplifying device being negative and controlled by a voltage between the first terminal of the second amplifying device and the second terminal of the second amplifying device, the second terminal of the second amplifying device being coupled to the first terminal of the input port; a third amplifying device (23) comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal of the third amplifying device being directly coupled to a control electrode of the transistor of the third amplifying device, a voltage between the third terminal of the third amplifying device and the second terminal of the third amplifying device being less than the given negative voltage, a current flowing out of the second terminal of the third amplifying device and a current flowing in the third terminal of the third amplifying device being negative and controlled by a voltage between the first terminal of the third amplifying device and the second terminal of the third amplifying device, the first terminal of the third amplifying device being coupled to the third terminal of the first amplifying device; a fourth amplifying device (24) comprising a transistor, a first terminal, a second terminal and a third terminal, the first terminal of the fourth amplifying device being directly coupled to a control electrode of the transistor of the fourth amplifying device, a voltage between the third terminal of the fourth amplifying device and the second terminal of the fourth amplifying device being greater than the given positive voltage, a current flowing out of the second terminal of the fourth amplifying device and a current flowing in the third terminal of the fourth amplifying device being positive and controlled by a voltage between the first terminal of the fourth amplifying device and the second terminal of the fourth amplifying device, the first terminal of the fourth amplifying device being coupled to the third terminal of the second amplifying device, the third terminal of the fourth amplifying device being coupled to the third terminal of the third amplifying device; a first capacitor (31) having a first terminal and a second terminal, the first terminal of the first amplifying device being coupled to the first terminal of the first capacitor, the second terminal of the first capacitor being coupled to the reference conductor; a second capacitor (32) having a first terminal and a second terminal, the first terminal of the second amplifying device being coupled to the first terminal of the second capacitor, the second terminal of the second capacitor being coupled to the reference conductor; one or more means including a first differential amplifier (41), for creating a first closed- loop control in which a controlled variable is a bias voltage between the third terminal of the third amplifying device and the reference conductor, an output of the first differential amplifier being coupled to the first terminal of the second amplifying device, the first differential amplifier being a two-transistor differential amplifier; one or more means including a second differential amplifier (42), for creating a second closed-loop control in which a controlled variable is a bias voltage between the third terminal of the first amplifying device and the reference conductor, an output of the second differential amplifier being coupled to the first terminal of the first amplifying device, the second differential amplifier being a two-transistor differential amplifier; a power amplifier (6) having, in the known frequency band, a voltage gain greater than 9/10 and less than 11/10, and an input impedance which is much higher than its output impedance, an input of the power amplifier being coupled to the third terminal of the third amplifying device and to the third terminal of the fourth amplifying device; an output port (7) having a first terminal (71) and a second terminal (72), the first terminal of the output port being directly coupled to an output of the power amplifier, the second terminal of the output port being directly coupled to the reference conductor; one or more means for creating a feedback, said one or more means for creating a feedback including a resistor (5), the feedback determining, in the known frequency band, a transimpedance of the transimpedance amplifier, the feedback reducing an absolute value of an impedance presented by the input port in the known frequency band; a positive power supply line (91), a voltage between the positive power supply line and the reference conductor being made positive and stable by a first battery which is not shown in Fig. 2; and a negative power supply line (92), a voltage between the negative power supply line and the reference conductor being made negative and stable by a second battery which is not shown in Fig. 2.

For instance, it is possible that the first amplifying device is an N-channel field-effect transistor having a gate, a source and a drain, the first terminal of the first amplifying device being the gate of the N-channel field-effect transistor, the second terminal of the first amplifying device being the source of the N-channel field-effect transistor, the third terminal of the first amplifying device being the drain of the N-channel field-effect transistor; and that the second amplifying device is a P-channel field-effect transistor having a gate, a source and a drain, the first terminal of the second amplifying device being the gate of the P-channel field-effect transistor, the second terminal of the second amplifying device being the source of the P-channel field-effect transistor, the third terminal of the second amplifying device being the drain of the P-channel field-effect transistor. In this case, the specialist understands that the first amplifying device is an N-channel transistor used in a common-gate circuit, and that the second amplifying device is a P-channel transistor used in a common-gate circuit.

For instance, it is possible that the third amplifying device is a P-channel field-effect transistor having a gate, a source and a drain, the first terminal of the third amplifying device being the gate of the P-channel field-effect transistor, the second terminal of the third amplifying device being the source of the P-channel field-effect transistor, the third terminal of the third amplifying device being the drain of the P-channel field-effect transistor; and that the fourth amplifying device is an N-channel field-effect transistor having a gate, a source and a drain, the first terminal of the fourth amplifying device being the gate of the N-channel field-effect transistor, the second terminal of the fourth amplifying device being the source of the N-channel field-effect transistor, the third terminal of the fourth amplifying device being the drain of the N-channel field-effect transistor. In this case, the specialist understands that the third amplifying device is a P-channel transistor used in a common-source circuit (with source degeneration), and that the fourth amplifying device is an N-channel transistor used in a common-source circuit (with source degeneration).

For instance, if the first amplifying device and the fourth amplifying device are N-channel transistors, the given positive voltage may be equal to two volts. For instance, if the second amplifying device and the third amplifying device are P-channel transistors, the given negative voltage may be equal to minus two volts.

For instance, it is possible that at least one of said N-channel field-effect transistors is replaced with a circuit having a similar behavior, for instance two N-channel transistors used in a cascode circuit, or with a device having a similar behavior, for instance an N-channel dual gate MOSFET. For instance, it is possible that at least one of said P-channel field-effect transistors is replaced with a circuit having a similar behavior, for instance two P-channel transistors used in a cascode circuit, or with a device having a similar behavior, for instance a P-channel dual gate MOSFET. Said transimpedance of the transimpedance amplifier is a transimpedance between the input port and the output port. The specialist understands that, if the input port of the transimpedance amplifier is directly coupled to a passive antenna which is a single-tum loop aerial or a single- turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the feedback created by the resistor (5) is negative (i.e., degenerative) in the known frequency band, and suitable for: determining, in the known frequency band, the transimpedance of the transimpedance amplifier, an absolute value of the transimpedance of the transimpedance amplifier being substantially independent of the frequency, in the known frequency band; and reducing, in the known frequency band, an absolute value of an input impedance of the transimpedance amplifier, the absolute value of the input impedance of the transimpedance amplifier being, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna. However, the feedback created by the resistor (5) is, at low frequencies and at the zero frequency, too weak to provide a suitable biasing of the third amplifying device and of the fourth amplifying device. The specialist understands that this is not a problem, because: the first closed-loop control is such that the bias voltage between the third terminal of the third amplifying device and the reference conductor is close to zero volt; and the second closed-loop control is such that the bias voltage between the third terminal of the first amplifying device and the reference conductor is close to a wanted voltage applied to an inverting input terminal of the second differential amplifier. Thus, the first amplifying device, the second amplifying device, the third amplifying device and the fourth amplifying device are correctly biased.

Said one or more means for creating a feedback could also comprise, in addition to the resistor (5), other means, for instance a capacitor connected in parallel with the resistor (5). The specialist understands that this may sometimes be useful, for instance to improve a phase margin of this feedback, and/or to make the absolute value of the transimpedance of the trans impedance amplifier more independent of the frequency, in the known frequency band.

If the first amplifying device and the fourth amplifying device are each a well-chosen low- noise N-channel transistor, and if the second amplifying device and the third amplifying device are each a well-chosen low-noise P-channel transistor, then the transimpedance amplifier produces a very low noise in the known frequency band, when its input port is directly coupled to the passive antenna, because the two common-gate circuits are, from the standpoint of the noise produced, optimized for the impedance of the passive antenna.

Thus, for a passive antenna which is a single-tum loop aerial or a single -turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the transimpedance amplifier is such that: an absolute value of the input impedance of the transimpedance amplifier is, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna; an absolute value of the transimpedance of the transimpedance amplifier is substantially independent of the frequency, in the known frequency band; the input port of the transimpedance amplifier can be directly coupled to the passive antenna; and the transimpedance amplifier produces a very low noise in the known frequency band, when its input port is directly coupled to the passive antenna. Consequently, the transimpedance amplifier is a solution to the two problems explained above in the section on prior art.

Third embodiment (best mode).

As a third embodiment of a device of the invention, given by way of non-limiting example and best mode of carrying out the invention, we have represented in Figure 3 and Figure 4 a simplified schematic diagram of an amplifier of the invention providing a transimpedance in a known frequency band, the known frequency band being the band 9 kHz to 30 MHz, the amplifier comprising: an input port ( 1 ) having a first terminal (11) and a second terminal (12), the second terminal being directly coupled to a reference conductor; a first amplifying device (21) comprising an NPN transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a base of the NPN transistor, the second terminal being directly coupled to an emitter of the NPN transistor, the third terminal being directly coupled to a collector of the NPN transistor, the second terminal being coupled to the first terminal of the input port; a second amplifying device (22) comprising a PNP transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a base of the PNP transistor, the second terminal being directly coupled to an emitter of the PNP transistor, the third terminal being directly coupled to a collector of the PNP transistor, the second terminal being coupled to the first terminal of the input port; a third amplifying device (23) comprising a PNP transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a base of the PNP transistor, the second terminal being directly coupled to an emitter of the PNP transistor, the third terminal being directly coupled to a collector of the PNP transistor, the first terminal being coupled to the third terminal of the first amplifying device; a fourth amplifying device (24) comprising an NPN transistor, a first terminal, a second terminal and a third terminal, the first terminal being directly coupled to a base of the NPN transistor, the second terminal being directly coupled to an emitter of the NPN transistor, the third terminal being directly coupled to a collector of the NPN transistor, the first terminal being coupled to the third terminal of the second amplifying device, the third terminal being coupled to the third terminal of the third amplifying device; a first capacitor (31) having a first terminal and a second terminal, the first terminal being coupled to the first terminal of the first amplifying device, the second terminal being coupled to the reference conductor; a second capacitor (32) having a first terminal and a second terminal, the first terminal being coupled to the first terminal of the second amplifying device, the second terminal being coupled to the reference conductor; one or more means including a first differential amplifier comprising two NPN transistors (4101) (4102) and a resistor (4103), for creating a first closed-loop control in which a controlled variable is a bias voltage between the third terminal of the third amplifying device and the reference conductor, an output of the first differential amplifier being coupled to the first terminal of the first amplifying device; one or more means including a second differential amplifier comprising two PNP transistors (4201) (4202) and a resistor (4203), for creating a second closed-loop control in which a controlled variable is a bias voltage between the third terminal of the second amplifying device and the reference conductor, an output of the second differential amplifier being coupled to the first terminal of the second amplifying device; one or more means for creating a feedback, the one or more means for creating a feedback comprising three resistors (51) (52) (53), the feedback determining the transimpedance in the known frequency band; an output port (7) having a first terminal (71) and a second terminal (72), the second terminal of the output port being directly coupled to the reference conductor; a positive power supply line (91), a voltage between the positive power supply line and the reference conductor being positive and stable; and a negative power supply line (92), a voltage between the negative power supply line and the reference conductor being negative and stable.

Figure 3 and Figure 4 each comprise three continuation symbols, a direct coupling existing between continuation symbols containing the same letter. Thus, the two continuation symbols containing the letter A (93) are connected to one another, the two continuation symbols containing the letter B (94) are connected to one another, and the two continuation symbols containing the letter C (95) are connected to one another.

The specialist sees that the circuit shown in Figure 4 is a power amplifier having, in the known frequency band, a voltage gain less than or equal to 1 , and an input impedance which is much higher than its output impedance, the first terminal of the output port being directly coupled to an output terminal of the power amplifier, an input terminal of the power amplifier being coupled to the third terminal of the third amplifying device and to the third terminal of the fourth amplifying device.

The specialist sees that said one or more means including a first differential amplifier comprising two NPN transistors (4101) (4102) and a resistor (4103), for creating a first closed- loop control, also include two resistors (8101) (8102), a capacitor (8103), and said power amplifier. In said power amplifier, two resistors (8104) (8105) are used to sense, at the node corresponding to the two continuation symbols containing the letter C (95), a potential very close to a potential of the third terminal of the third amplifying device. The specialist therefore understands that a bias voltage between the third terminal of the third amplifying device and the reference conductor is indeed a variable controlled by the first closed-loop control.

The specialist sees that said one or more means including a second differential amplifier comprising two PNP transistors (4201) (4202) and a resistor (4203), for creating a second closed-loop control, also include two resistors (8201) (8202), and a capacitor (8203). The resistor (8201) is used to sense, at the second terminal of the fourth amplifying device, a potential presenting an almost constant difference with a potential at the third terminal of the second amplifying device. The specialist therefore understands that a bias voltage between the third terminal of the second amplifying device and the reference conductor is indeed a variable controlled by the second closed-loop control.

The specialist understands that said transimpedance is measured between the input port and the output port. The specialist understands how said feedback determines the transimpedance in the known frequency band. At any frequency in the known frequency band, the feedback is such that an absolute value of the transimpedance is close to 257 ohms.

At 1 MHz, an absolute value of an impedance presented by the input port is about 78 milliohms, but if said feedback is removed by removing the resistor (51), the absolute value of the impedance presented by the input port is about 2 ohms. Consequently, we can say that the feedback reduces an absolute value of an impedance presented by the input port in the known frequency band.

The specialist understands that, if the input port of the amplifier of the invention is directly coupled to a port of a passive antenna which is a single -turn loop aerial or a single-turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the feedback created by the three resistors (51), (52) and (53) is negative (i.e., degenerative) in the known frequency band, and suitable for: determining, at any frequency in the known frequency band, the transimpedance of the amplifier of the invention, an absolute value of the transimpedance of the amplifier of the invention being substantially independent of the frequency, in the known frequency band; and reducing, at some frequencies in the known frequency band, an absolute value of an input impedance of the amplifier of the invention, the absolute value of the input impedance of the amplifier of the invention being, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna. However, the feedback created by the three resistors (51), (52) and (53) is, at low frequencies and at the zero frequency, too weak to provide a suitable biasing of the third amplifying device and of the fourth amplifying device. The specialist understands that this is not a problem, because: the first closed-loop control is such that the bias voltage between the third terminal of the third amplifying device and the reference conductor is close to zero volt, for instance zero volt plus or minus ten millivolts; and the second closed-loop control is such that the bias voltage between the third terminal of the second amplifying device and the reference conductor is close to a wanted voltage determined by the resistors (8201) and (8202), and by the voltage between the positive power supply line and the reference conductor. Thus, the first amplifying device, the second amplifying device, the third amplifying device and the fourth amplifying device are correctly biased.

Said one or more means for creating a feedback could also comprise, in addition to the three resistors (51), (52) and (53), other means, for instance a capacitor connected in parallel with the resistor (51). The specialist understands that this may sometimes be useful, for instance to improve a phase margin of this feedback, and/or to make the absolute value of the transimpedance of the amplifier of the invention more independent of the frequency, in the known frequency band.

If the first amplifying device and the fourth amplifying device are each a well-chosen low- noise NPN transistor, and if the second amplifying device and the third amplifying device are each a well-chosen low-noise PNP transistor, then the amplifier of the invention produces a very low noise in the known frequency band, when its input port is directly coupled to the port of the passive antenna, because the common-gate circuits comprising the first amplifying device and the second amplifying device are, from the standpoint of the noise produced, optimized for the impedance of the passive antenna.

Thus, for a passive antenna which is a single-tum loop aerial or a single-turn screened loop aerial, small compared to a wavelength in vacuum corresponding to a least upper bound of the known frequency band, the amplifier of the invention is such that: an absolute value of the input impedance of the amplifier of the invention is, at any frequency in the known frequency band, much less than an absolute value of the impedance of the passive antenna; an absolute value of the transimpedance of the amplifier of the invention is substantially independent of the frequency, in the known frequency band; the input port of the amplifier of the invention can be directly coupled to the port of the passive antenna; and the amplifier of the invention produces a very low noise in the known frequency band, when its input port is directly coupled to the port of the passive antenna. Consequently, the amplifier of the invention is a solution to the two problems explained above in the section on prior art.

INDICATIONS ON INDUSTRIAL APPLICATIONS

The amplifier of the invention is suitable to be used in an active antenna comprising a single-turn loop aerial or a single-tum screened loop aerial. The amplifier of the invention is also suitable to be used in the active antenna disclosed in the French patent application No. FR2002047 of 28 February 2020, entitled “Antenne active comportant un cadre blinde”, and to be used in the active antenna disclosed in the French patent application No. FR2004969 of 18 May 2020, entitled “Antenne active incluant un cadre blinde”. Such active antennas comprising the amplifier of the invention are particularly suitable for electromagnetic field measurements, and for direction finding.