TECHNICAL FIELD The present invention relates to a distributed amplifier. In particular, the invention is concerned with enhancing a component that terminates the output line of the distributed amplifier, which component is conventionally implemented by an output matching resistor. The distributed amplifier of the invention may, for example, be used for driving an electro- optical modulator (EAM) in an optical transmitter or may be applied in radar applications.

BACKGROUND

A conventional distributed amplifier is shown as a block-diagram in FIG. 8. It is composed of three main building blocks: an amplification part, which is typically based on transistors, an input matching resistor RG terminating an input line of the distributed amplifier, and an output matching resistor RD terminating the output line of the distributed amplifier. The output matching resistor RD is usually connected to a capacitor CD to avoid DC power consumption on the resistor RD. As a consequence, the DC power consumption of the distributed amplifier is determined by the current Idd used to bias the transistors composing the amplification part.

When implementing the conventional distributed amplifier, best trade-off between output voltage swing and output return loss requirements for a given DC power consumption needs to be found.

Most commonly a trade-off between the two cases described above is selected for implementing the conventional distributed amplifier, namely setting RD > RL. In this condition the output return loss is neither perfect nor absent, and the maximum output voltage that is possible to achieve is Vout_max = Idd · (RD//RL), i.e. it is greater than Idd (RL/2) but lower than Idd · RL. SUMMARY

In view of the above-mentioned shortcomings and trade-off, improving the conventional distributed amplifier is desirable. The objective is to provide a distributed amplifier with an increased maximum output voltage and, at the same time, an optimized or at least good output return loss. Further, the distributed amplifier should have reduced distortions, without affecting the DC power consumption.

In fact, the resistance value of an output matching resistor RD directly affects the maximum output voltage swing, which can be reached for a defined DC power consumption. For instance, in case of RD = RL, wherein RL is the resistance of an output load connected to the distributed amplifier, a perfect output return loss would be obtained. However, the maximum output voltage that is possible to achieve in this case is Vout_max = Idd · (RL/2). Conversely, in the case of RD = ¥ (i.e. open circuit), the output return loss is completely absent, but the maximum output voltage that is possible to achieve in this case is doubled, and thus is Vout max = Idd · RL.

The amplifier described in the following is particularly of interest for transmitters that require very wideband frequency responses, such as transmitters used in optical communications and radar applications. In most advanced solutions of transmitters for optical communications, it is required that the amplifier shows a matched output impedance. This is because the matched output impedance of the amplifier absorbs reflections coming from an interconnection between the amplifier itself and an EAM, wherein the interconnection may be implemented by bonding wires and bonding pads that reduce the quality of the transmitted signal. Similarly, most advanced radar technologies, which are based on multi-antennas beam forming, require amplifiers with a matched output impedance, in order to avoid loading effects between the antennas. Accordingly, the present disclosure also intends to provide a transmitter, particularly an optical transmitter or a transmitter for radar applications, in which the distributed amplifier of the invention is used for the signal amplification. Specifically, the distributed amplifier should provide a matched output impedance in the transmitter, in order to obtain higher performance transmitters both for optical communications and radar applications. The objective is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.

In particular a nonlinear component is used for terminating the output line, instead of the output matching resistor of the conventional distributed amplifier.

A first aspect provides a distributed amplifier, comprising an amplification part having an input line adapted to be connected to an AC voltage input and an output line adapted to be connected to a load, and a nonlinear component terminating the output line, said nonlinear component being equivalent to a resistance with increasing value according to increasing AC voltage across it.

A linear (resistive) component - like the linear output matching resistor of the conventional distributed amplifier - generally has a constant ratio between voltage and current independently by the amplitude of them. In contrast, a nonlinear (resistive) component - like the nonlinear component of the distributed amplifier of the first aspect - generally has a ratio between voltage and current, which depends on the amplitude of one of them. From a mathematical point of view, the equivalent resistance value of the nonlinear component is a monotonically increasing function of the AC voltage across the nonlinear component.

The nonlinear component leads to the distributed amplifier of the first aspect having a good or even optimized output return loss, without scarifying on the maximum output voltage (as it happens in the conventional distributed amplifier). Accordingly, the distributed amplifier of the first aspect is improved over the conventional distributed amplifier, and can e.g. be used for realizing higher performance transmitters.

Notably, the load connecting to the output line of the amplification part is not necessarily a part of the distributed amplifier. That is, the load may be connectable and disconnectable from the amplification part. In a transmitter, the load would e.g. be an EAM connected to the output line. In an implementation form of the first aspect, the nonlinear component has an equivalent resistance value that increases for an AC voltage across it that is above a first threshold voltage.

In an implementation, for instance the nonlinear component may have an equivalent resistance value higher than a characteristic impedance of the output line for an AC voltage across it that is above a first threshold voltage.

Here and in the following the characteristic impedance of the output line may be the impedance of the components constituting the output side of the amplification part of the distributed amplifier. In an implementation, the characteristic impedance of the output line may be the impedance of the output inductors and output parasitic capacitors of the amplification part of the distributed amplifier.

In a further implementation form of the first aspect the nonlinear component has an equivalent resistance value that is constant for an AC voltage across it that is below a second threshold voltage. In an implementation, the constant value of the resistance may match the characteristic impedance of the output line.

In a further implementation, the nonlinear component may have an equivalent resistance value that matches the characteristic impedance of the output line for an AC voltage across it that is below a second threshold voltage.

That is, the nonlinear component may match the resistance of the output load at small voltages and assumes resistance values larger than the resistance of the output load for higher voltages. For instance the equivalent resistance of the nonlinear component may have an exponential of power behavior for larger voltages.

In a further implementation form of the first aspect, the nonlinear component is a nonlinear resistor.

This is the most simple implementation form of a nonlinear component. In this case the equivalent resistance of the nonlinear component is the resistance value at a given voltage across it. In a further implementation form of the first aspect, the nonlinear component has a continuous dependency to the AC voltage across it.

This allows the simplest realization of the nonlinear component. The dependency to the AC voltage can also be discreet, but the nonlinear component is in this case somewhat more complex to be realized.

In a further implementation form of the first aspect, the nonlinear component comprises a transistor, particularly a Field Effect Transistor (FET).

The transistor can, however, also be based on a bipolar junction transistor (BJT) for obtaining similar results.

In a further implementation form of the first aspect, the nonlinear component further comprises a first resistor and a second resistor.

These resistors serve to adjust the equivalent resistance of the nonlinear component, and may avoid loading effects.

In a further implementation form of the first aspect, an input terminal of the transistor is connected via a capacitor to ground and an output terminal of the transistor is connected to the amplification part, and a control terminal of the transistor is connected to the input terminal via the first resistor and to the output terminal via the second resistor.

This provides an implementation of the distributed amplifier that can easily be used with Monolithic Microwave Integrated Circuit (MMIC) technologies.

In a further implementation form of the first aspect, an input terminal of the transistor is connected to a DC voltage supply and an output terminal of the transistor is connected to the amplification part, and a control terminal of the transistor is connected to the input terminal via the first resistor and to the output terminal via the second resistor.

This implementation of the distributed amplifier allows avoiding the use of a capacitor connected between the nonlinear component and ground. In a further implementation form of the first aspect, a resistance of the first resistor and the second resistor, respectively, is higher than the equivalent resistance of the nonlinear component. .

In this way, loading effects are avoided.

In a further implementation form of the first aspect, the distributed amplifier is implemented in a MMIC.

A second aspect provides a transmitting device, particularly optical transmitter, comprising a distributed amplifier according to the first aspect or any of its implementation forms.

The distributed amplifier based on the first aspect can also be used in a transmitters oriented to other applications (not only, but for example also radar transmitters). With the distributed amplifier of the first aspect, a better output impedance matching can be achieved, thus increasing the performance of the transmitter.

In a further implementation form of the second aspect, the transmitting device further comprises an EAM as the load connected to the AC voltage output of the distributed amplifier.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be performed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof. BRIEF DESCRIPTION OF THE DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which:

FIG. 1 shows a distributed amplifier according to an embodiment of the present invention. FIG. 2 shows a distributed amplifier according to an embodiment of the present invention.

FIG. 3 shows a distributed amplifier according to an embodiment of the present invention.

FIG. 4 shows an example behavior of a nonlinear component of a distributed amplifier according to an embodiment of the present invention.

FIG. 5 shows a distributed amplifier according to an embodiment of the present invention.

FIG. 6 shows a distributed amplifier according to an embodiment of the present invention. FIG. 7 shows a transmitter including a distributed amplifier according to an embodiment of the present invention.

FIG. 8 shows a conventional distributed amplifier. DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 shows a distributed amplifier 100 according to an embodiment. The distributed amplifier 100 may, for instance, be used in a transmitter, particularly in an optical transmitter for driving an EAM. The distributed amplifier 100 comprises an amplification part 101 having an input line 102 for being connected to an AC voltage input, and an output line 103 for connecting to a load 104. The load 104 may, for example, be the EAM in an optical transmitter. Thus, the load

104 is usually not a part of the distributed amplifier 100. The amplification part 101 may particularly be based on a plurality of transistors, which may be implemented as in the conventional distributed amplifier.

The distributed amplifier 100 further comprises a nonlinear component 105 terminating the output line 103. The nonlinear component 105 replaces the output matching resistor that is in the conventional distributed amplifier (as shown in FIG. 8). The nonlinear component

105 is equivalent to a resistance with increasing value for an increasing AC voltage across it. For instance, the nonlinear component 105 may be a nonlinear resistor, whose resistance value changes with applied AC voltage, or may be a component including one or more transistors and/or resistors, in order to have an equivalent resistance with nonlinear behavior to the applied AC voltage.

The distributed amplifier 100 of FIG. 1 is designed according to a new scheme with the nonlinear component 105, in order to have greater output voltage swing than the conventional distributed amplifier, without compromising on output return loss.

FIG. 2 shows a distributed amplifier 100 according to an embodiment, which builds on the distributed amplifier 100 shown in FIG. 1. Same elements are labelled with the same reference signs and function likewise. In particular, also the distributed amplifier 100 shown in FIG. 2 comprises the amplification part 101, the input line 102, the output line 103 connecting to a load 104, and the nonlinear component 105 instead of the conventional output matching resistor.

In FIG. 2, the nonlinear component 105 is specifically connected to ground via a capacitor 201 (capacitor CD). That means, the capacitor CD is connected on the terminating end of the output line 103 between the nonlinear component 105 of the distributed amplifier 100 and ground. The capacitor CD may be a part of the distributed amplifier 100. Further, the distributed amplifier 100 shown in FIG. 2 has an input matching resistor RG 202 terminating the input line 102. The block-diagram of the distributed amplifier 100 shown in FIG. 2 includes the nonlinear component 105 as replacement for a conventionally used output matching resistor. The voltage vl is the AC voltage across the nonlinear component 105, and the current il is the AC current flowing in the nonlinear component 105. The DC current flowing in the nonlinear component 105 is zero, due to the capacitor CD.

The nonlinear component 105 may particularly have an equivalent resistance value that increases for an AC voltage vl across the nonlinear component 105 that is above a first threshold voltage. For instance the equivalent resistance value may be higher than a characteristic impedance of the output line. For instance, the equivalent resistance value may be higher than the resistance RL of the load 104. Further, it may have an equivalent resistance value that is constant for an AC voltage across the nonlinear component 105 that is below a second threshold voltage. For instance the equivalent resistance value may match characteristic impedance of the output line. In an example, the equivalent resistance value may match the resistance RL of the load 104.

In particular, the constitutive equation of the nonlinear component 105 may thus be NLRD = vl / il = RL for a small amplitude of vl. In this case, also NLRD = RD (as it would typically be selected for the conventional distributed amplifier of FIG. 8). Further, the constitutive equation of the nonlinear component 105 becomes NLRD = vl / il > RL for increasing amplitude of vl. In this case, also NLRD > RD (as it would typically be selected for the conventional distributed amplifier of FIG. 8).

The nonlinear behavior of the nonlinear component 105 allows to simultaneously obtain an output return loss that is the same as in the conventional distributed amplifier shown in FIG. 8 at small signal level, and a maximum output voltage greater than the one of the conventional distributed amplifier with the same DC power consumption. The output return loss at small signal level is usually enough, because the reflected signal from the load resistance RL is usually a small portion of the transmitted signal. Moreover, especially in optical transmitters, also the output load (such as Electro-absorption Modulator Laser (EML)) has a nonlinear behavior that prevents a good output return loss at higher output AC voltage levels also in case of the conventional distributed amplifier. The most convenient scheme to implement the distributed amplifier 100 proposed by the invention is shown in FIG. 3. In particular, FIG. 3 shows a distributed amplifier 100 according to an embodiment of the present invention, which builds on the distributed amplifier 100 shown in FIG. 1 and FIG. 2. Same elements in FIG. 3 and in FIG. 1 and 2, respectively, are labelled with the same reference signs and have likewise functions.

The nonlinear component 105 in the distributed amplifier 100 shown in FIG. 3 is realized by means of a transistor 300 (transistor Ql), here exemplary assumed to be a FET, and two resistors 301 and 301 (resistors RT and R2). An output terminal (here exemplarily a Drain (D) terminal) and an input terminal (here exemplarily a source (S) terminal) of the transistor Q 1 are completely interchangeable, and one of the terminals is connected to the capacitor CD, while the other terminal is connected to the amplification part 101 of the distributed amplifier 100. The control terminal (here exemplarily a gate (G) terminal) is connected to the resistors RT and R2, whose values may be very high compared to the value of RL. No DC current flows in Ql, R1 and R2.

An example of a nonlinear il-vl curve of a nonlinear component 105 that may be used in the distributed amplifier 100 shown in FIG. 3 is shown in FIG. 4. It may have the form of a hyperbolic tangent behavior and/or may show asymptotic behavior to certain constant i- v curves. In particular, FIG. 4 shows that the equivalent resistance of the nonlinear component 105 is equivalent to a 50 Ohm resistor for a smaller amplitude level of vl. Increasing the vl amplitude level, the il-vl curve of the nonlinear component 105 changes its slope, at some point becoming equivalent to a 140 Ohm resistor. As a consequence, the distributed amplifier 100 with such a nonlinear component 105 allows obtaining a perfect output return loss, assuming e.g. RL = 50 Ohm, and at the same time a maximum output voltage Vout max = Idd · (50 Ohm // 140 Ohm) = Idd · 37 Ohm. In contrast, in the case of a conventional distributed amplifier with a perfect output return loss, the maximum output voltage would be only Vout_max = Idd · (50 Ohm // 50 Ohm) = Idd · 25 Ohm. This means that the distributed amplifier 100 according to the present invention allows increasing the maximum output voltage by over 50% with respect to the conventional distributed amplifier, without degrading the output return loss and with the same DC power consumption. Generally speaking, compared to the conventional distributed amplifier as e.g. shown in FIG. 8, the distributed amplifier 100 of the present invention allows a higher maximum output voltage without degrading other key performance parameters such as: DC power consumption, output return loss, gain, bandwidth, input return loss, circuit complexity, integrability, and required bias voltages.

FIG. 5 shows a distributed amplifier 100 according to an embodiment of the present invention, which builds on the distributed amplifier 100 shown in the FIGs. 1, 2 and 3. Same components are labelled with the same reference signs and function likewise. The distributed amplifier 100 of FIG. 5 comprises again the amplification part 101, and an input matching resistor RG 202, which may be implemented as in the conventional distributed amplifier, and the nonlinear (output matching) component 105, which is exemplarily composed again of a transistor 300 (transistor QN+l) and two resistors 301 and 302 (resistors Rl and R2).

The size of the transistor QN+l composing the nonlinear component 105 may be selected to obtain a desired output return loss. The value of the resistors Rl and R2 may be selected to be much higher than the output load resistance RL, in order to avoid loading effects. Rl and R2 can be of the same value or may be of different values. FIG. 5 is particularly an easy way to build the distributed amplifier 100 such that it can be fully and simply integrated in MMIC technologies.

FIG. 6 shows a distributed amplifier 100 according to an embodiment of the present invention, which builds on the distributed amplifier 100 shown in the FIGs. 1, 2 and 3, and is an alternative to the distributed amplifier 100 shown in FIG. 5. In this implementation of the distributed amplifier 100, both terminals of the nonlinear component 105 are connected to a VDD voltage supply from a DC point of view. This connection allows to avoid the use of the capacitor CD (as in FIG. 5).

In both FIG. 5 and FIG. 6 the distributed amplifier 100 includes a plurality of inductors labelled LG1, LG2 ... LGN+l and respectively LD1, LD2 ... LDN+l. The inductors LD1, LD2 ... LDN+l are particularly connected in series on the output line 103 between the load 104 and the nonlinear component 500. The inductors LG1, LG2 ... LGN+l are particularly connected in series on the input line 102 between the input matching resistor 202 and a voltage input (Vin). Further, the distributed amplifier 100 includes a plurality of transistors 302 connected in parallel. The transistors are labelled Q 1, Q2 ... QN, and are here indicated as being FETs. However, the may also be BJTs. Notably, N is a natural number. The output terminals D of the transistors (‘drain’ for FETs and‘collector’ for BJTs) are connected to the output line 103. In particular, the output terminal D of each transistors is connected to the output line 102 in-between two of the inductors LD1, LD2 ... LDN+l. The control terminals G of the transistors (‘gate’ for FETs and‘base’ for BJTs) are connected to the input line 102. In particular, the control terminal G of each transistor is connected to the input line 103 in-between two of the inductors LG1, LG2 ... LGN+l. The input terminals S of the transistors (‘source’ for FETs and‘emitter’ for BJTs) are all grounded.

FIG. 7 shows a transmitting device 700 according to an embodiment of the present invention. The transmitting device 700 may particularly be an optical transmitter for optical communications or a transmitter for radar applications. In any case, the transmitting device 700 comprises a distributed amplifier 100 according to an embodiment of the present invention. The distributed amplifier 100 may particularly be as shown in FIG. 1, 2, 3, 5 or 6.

The transmitting device 700, if it is an optical transmitter, may further include an electro- EAM 701 as the load 104, i.e. the EAM 701 is connected to the AC voltage output line 103 of the distributed amplifier 100. In FIG. 7 the EAM 701 is shown in dashed lines, since it is not essential to the transmitting device 700, and could be a different kind of load 104.

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word“comprising” does not exclude other elements or steps and the indefinite article“a” or“an” does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.