The invention relates to a class A amplifier comprising at least a first and a second transistor.

The invention also relates to an audio system which comprises such a class A amplifier.

A prior-art class A amplifier is known from US 5,343, 166, which discloses an efficient high-fidelity audio amplifier.

The known class A amplifier is disadvantageous, inter alia, because it is complex and costly.

It is, inter alia, an object of the invention to provide a simple efficient class A amplifier. The invention is defined by the independent claims. The dependent claims define advantageous embodiments.

The class A amplifier according to the invention comprises at least a first transistor and a second transistor, each transistor having a first main electrode, a second main electrode and a control terminal; a first reference terminal and a second reference terminal; a first coil coupled between the first main electrode of the first transistor and the first reference terminal; a second coil coupled between the first main electrode of the second transistor and the first reference terminal ; a current source coupled between the second main electrode of the first transistor and the second reference terminal, a second main electrode of the second transistor being coupled to the second main electrode of the first transistor, at least one of the first main electrodes being coupled to an output of the class A amplifier, at least one of the control electrodes being coupled to an input of the class A amplifier, and the first and the second coil being inductively coupled to each other. The first reference terminal may be coupled to a first power supply for supplying a first reference voltage like, for example, a positive voltage or a ground voltage. The second reference terminal may be coupled to a second power supply for supplying a second reference voltage like, for example, a ground

voltage or a negative voltage. Such a class A amplifier has a high overall performance and comprises fewer components, so that it is simpler than the prior-art class A amplifier.

Moreover, the class A amplifier is more efficient than the prior-art class A amplifier.

The inductive coupling of the first and the second coil is preferably maximized. As a result, the class A amplifier according to the invention may have an efficiency of, for example, about 50% at maximum undistorted load, which is a great improvement compared to a standard class A amplifier comprising two transistors and having a maximum efficiency of about 19%.

A first embodiment of the class A amplifier according to the invention is defined by claim 2. If field effect transistors (FETs) are used, the first main electrodes will be drains and the second main electrodes will be sources, and if bipolar transistors are used, the first main electrodes will be collectors and the second main electrodes will be emitters.

Independently of the kind of transistors being used, both will be either n-channel (FETs) or npn (bipolar) or p-channel (FETs) or pnp (bipolar), which allows the advantageous selection of transistors having substantially equal characteristics.

A second embodiment of the class A amplifier according to the invention is defined by claim 3. By coupling the current source to the second main terminal of the first transistor via a first resistor and to the second main terminal of the second transistor via a second resistor, the class A amplifier according to the invention will have less non-linear distortion.

A third embodiment of the class A amplifier according to the invention is defined by claim 4. By creating the current source through using a third coil, the class A amplifier according to the invention has a simple construction.

A fourth embodiment of the class A amplifier according to the invention is defined by claim 5. By creating a class A amplifier without feedback, slew rate distortion is kept to a minimum due to the fact that delay via a feedback path is no longer present. In fact, in the class A amplifier according to the invention, any slew rate distortion will result solely from parasitic capacitances.

A fifth embodiment of the class A amplifier according to the invention is defined by claim 6. By using the first main electrodes as outputs of the class A amplifier according to the invention and using the control electrodes as inputs of the class A amplifier according to the invention, a balanced class A amplifier has been created. This balanced class A amplifier according to the invention does not require a stable (balanced or single-ended) power supply and is not sensitive to ripples in reference voltages or supply voltages.

A sixth embodiment of the class A amplifier according to the invention is defined by claim 7. The adjustable resistor provides a simple DC setting of the DC current through the transistors.

A seventh embodiment of the class A amplifier according to the invention is defined by claim 8. By coupling the other terminals of the adjustable resistor to the first and the second reference terminal, the amplifier is less sensitive to fluctuations of the supply voltages.

The invention provides a simple and efficient class A amplifier, and is advantageous, inter alia, in that the class A amplifier according to the invention has a high overall performance. Furthermore, non-linear distortion is minimized easily and slew rate distortion is minimized automatically, while the construction and the dc setting can be kept simple. The balanced class A amplifier according to the invention does not require a stable balanced or single-ended power supply and is not sensitive to ripples of the power supply.

These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments (s) described hereinafter.

In the drawings: Fig. 1 is a block diagram of a class A amplifier according to the invention; Figs. 2A to 2H show four voltage graphs and four current graphs corresponding to the amplifier of Fig. 1 ; Fig. 3 is a block diagram of an audio system according to the invention; and Fig. 4 shows inductively coupled first and second coils in Fig. 4a and a third coil in Fig. 4b.

The class A amplifier 1 according to the invention shown in Fig. 1 comprises a first transistor 11 (in this case an n-channel MOSFET, but p-channel FETs and bipolar transistors can be used alternatively), and comprises a second transistor 12 (in this case an n- channel MOSFET, but p-channel FETs and bipolar transistors can be used alternatively). A first main electrode (the drain, alternatively the collector) of first transistor 11 is coupled via a first coil 13 to a first reference terminal 17 (for supplying, for example, a positive voltage or a ground voltage), and a first main electrode (the drain, alternatively the collector) of second transistor 12 is coupled via a second coil 14 to the first reference terminal 17. Both

coils 13 and 14 are so-called coupled coils (like, for example, a transformer) coupled to each other via an inductive coupling 15 and are shown in Fig. 4a in more detail and have a value of, for example, 64 mH each.

A second main electrode (the source, alternatively the emitter) of first transistor 11 is coupled via a first resistor 19 and via a current source 16 to a second reference terminal 18 (for supplying, for example, a ground voltage or a negative voltage), and a second main electrode (the source, alternatively the emitter) of second transistor 12 is coupled via a second resistor 20 and via the current source 16 to the second reference terminal 18. Current source 16, for example, comprises a third coil and is shown in Fig. 4b in more detail and has a value of, for example, 100mH. Such a value provides sufficient filtering of even the lowest frequency of an audio signal, so as to keep the current through the coil sufficiently constant.

A control electrode (the gate, alternatively the base) of first transistor 11 is coupled to first input 21, and a control electrode (the gate, alternatively the base) of second transistor 12 is coupled to second input 22. The first main electrode of first transistor 11 is coupled to a first output 23, and the first main electrode of second transistor 12 is coupled to a second output 24, with a load 25 being coupled between outputs 23 and 24.

The control electrode of first transistor 11 is coupled via a third resistor 33 to an adjustable resistor 35 coupled between the first reference terminal 17 and the second reference terminal 18. The control electrode of second transistor 12 is coupled to the adjustable resistor 35 via a fourth resistor 34. The control electrode of first transistor 11 is coupled to the second reference terminal 18 via a fifth resistor 31 and the control electrode of second transistor 12 is coupled to the second reference terminal 18 via a sixth resistor 32.

The class A amplifier 1 according to the invention shown in Fig. 2 corresponds to the one shown in Fig. 1. Additionally, four voltage graphs and four current graphs are disclosed in Fig. 2. These graphs illustrate waveforms in the situation where the amplifier delivers the largest possible undistorted sinewave output.

The graph of Fig. 2A shows the current IL1 a flowing through first coil 13.

This current IL 1 a has a fixed value which is equal to half the value of the current ISS of the current source 16.

Fig. 2B shows the voltage Vdl at the drain of first transistor 11. This voltage Vdl starts at a value which is equal to the voltage Vcc at the first reference terminal 17, and is pulled to ground voltage (the voltage at the second reference terminal 18) in the first half cycle of the first input signal Vinl shown in Fig. 2D, and increases to twice the value of the

voltage Vcc at the first reference terminal 17 in the second half cycle of the first input signal Vinl.

Fig. 2C shows the current Idl flowing through the main electrodes of first transistor 11. This current starts with an average value which is equal to half the value of the current ISS current source 16, and increases to the value of the current ISS of the current source 16 in the first half cycle of the first input signal Vinl, and decreases to zero in the second half cycle of the first input signal Vinl.

The graph in Fig. 2E shows the current ILlb flowing through second coil 14.

This current IL 1 b has a fixed value which is equal to half the value of the current referenced by current source 16.

Fig. 2F shows the voltage Vd2 at the drain of second transistor 12. This voltage Vd2 starts at a value which is equal to the voltage Vcc at the first reference terminal 17, and increases to twice the value of the voltage Vcc at the first reference terminal 17 in the first half cycle of the second input signal Vin2 shown in Fig. 2H, and is pulled to ground voltage (the voltage at the second reference terminal 18) in the second half cycle of the second input signal Vin2.

Fig. 2G shows the current Id2 flowing through the main electrodes of second transistor 12. This current Id2 starts with a fixed value which is equal to half the value of the current Iss of the current source 16, and decreases to zero in the first half cycle of the second input signal Vin2, and increases to the value of the current Iss of the current source 16 in the second half cycle of the second input signal Vin2.

As a result, the drive current flowing through load 25 (from second output 24 to first output 23) will start at zero, and will increase to half the value of the current ISS of current source 16 in the first half cycle, and will decrease to minus half the value of the current ISS of current source 16 in the second half cycle. The drive voltage between outputs 24 and 23 will start at a value of zero volts, and will increase to twice the value of the voltage Vcc at the first reference terminal 17 in the first half cycle, and will decrease to minus twice the value of the voltage at Vcc at the first reference terminal 17 in the second half cycle.

This corresponds to a perfect class A amplification. Due to the fact that the voltages at the first main electrodes of transistors 11 and 12 reach twice the level of the voltage at the reference terminal 17 being the supply voltage, the class A amplifier 1 can also be used advantageously in car radios supplied by car batteries operating at limited voltage levels. The efficiency of this class A amplifier 1 is equal to P2/P1 * 100%, with P2 being equal to Vcc * Iss/2 and with PI being equal to Vcc * Iss. As a result, the efficiency is equal

to 50%, which is a great improvement compared to the efficiency of about 19% of standard class A amplifiers which also comprise two transistors.

MOSFETs or more generally MOS transistors are favourable, due to bipolar transistors transferring emitter-base voltages into collector-emitter currents logarithmically, and due to bipolar transistors introducing odd harmonics. Both transistors should preferably be of the same channel type (thereby guaranteeing most optimal matching), with n-channel transistors being superior to p-channel transistors.

The balanced class A amplifier 1 shown in Fig. 1 does not require a stable (balanced or single-ended) supply voltage and is not sensitive to ripples in reference voltages.

Non-linearities are suppressed by first and second resistors 19 and 20. Non-linearities in the BH-graph of core material of transformer 13,14 (coupled coils 13 and 14) do not lead to non- linearities in the output signal due to the transformer 13,14 being driven with a constant current. As a consequence of being without feedback, slew rate distortions are kept to a minimum (any delay via a feedback signal will no longer be present) and will result solely from parasitic capacitances.

First resistor 19 and second resistor 20 have a value of, for example, 0.56 Ohm. Third resistor 33 and fourth resistor 34 have a value of, for example, 10 kOhm, and fifth resistor 31 and sixth resistor 32 have a value of, for example, 4.7 kOhm. Adjustable resistor 35 has a value of, for example, 10 kOhm. The voltage at first reference terminal 17 has a value of, for example, 15 volt, and the voltage at second terminal 18 corresponds to, for example, ground. Via current source 16, the balanced class A amplifier 1 is biased with, for example, 5 ampere.

The audio system 2 shown in Fig. 3, such as, for example, an audio amplifier/receiver or a car radio or a television receiver or a mobile terminal or a computer, etc. comprises a class A amplifier 1, an input of which is coupled to an output of a unit 4 comprising, for example, a pre-amplifier and/or an optical-to-electrical converter and/or one or more filters, etc. An output of class A amplifier 1 is coupled to an input of a unit 5 comprising, for example, one or more loudspeakers or other reproducing or non-reproducing circuits to be driven by the class A amplifier. Alternatively, unit 5 may comprise output terminals for connecting loudspeakers to the amplifier 1. Audio system 2 further comprises a power supply 3 for supplying unit 4 and class A amplifier 1. Unit 5 may or may not form part of audio system 2, and further units may be present, such as, for example, tuners, processors, displays, separators for separating audio from video, etc. Consequently, audio system 2 may correspond to an audio/video system.

The expression"for"in"for A"and"for B"does not exclude that other functions"for C"are performed as well, simultaneously or not. The expressions"X coupled to Y"and"a coupling between X and Y"and"coupling/couples X and Y", etc. do not exclude that an element Z is in between X and Y (wherein a coupling may comprise a wired coupling and a non-wired coupling like an inductive or a capacitive coupling, etc. and wherein two or more elements may be coupled in a wired manner and non-wired manner like <BR> <BR> inductively or capacitively, etc. ). The expressions"P comprises Q"and"P comprising Q", etc. do not exclude that an element R is comprised/included as well.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb"comprise"and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article"a"or"an"preceding an element does not exclude the presence of a plurality of such elements or steps. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

The invention is based upon an insight, inter alia, that prior-art class A amplifiers are too complex due to the fact that they comprise too many components, and is based upon a fundamental idea, inter alia, that two branches should be sufficient for creating a simple class A amplifier: a first branch comprising a first transistor with a first main electrode being coupled to a first coupled coil, a second branch comprising a second transistor with a first main electrode being coupled to a second coupled coil, with both branches comprising (coming together in or having an overlap through) a current source coupled to both second main electrodes of both transistors.

The invention solves the problem, inter alia, of providing a simple efficient class A amplifier, and is advantageous, inter alia, in that the class A amplifier according to the invention has a high overall performance. Furthermore, non-linear distortion is minimized easily and slew rate distortion is minimized automatically, and the construction and the dc setting can be kept simple. The balanced class A amplifier according to the invention does not require a stable balanced or single-ended power supply and is not sensitive to ripples.