# Patent

## Title
Power amplifiers with push-pull transistors, capacitive coupling for harmonic cancellation, and inductive coupling to provide differential output signals

## Abstract

A differential power amplifier including a push-pull pair of transistors, a capacitance, a first inductance, and a second inductance. The push-pull pair of transistors includes first and second transistors. The first transistor includes control and output terminals. The second transistor includes input and control terminals. The control terminals of the first and second transistors collectively receive a differential input signal. The output and input terminals collectively provide a differential output signal. The capacitance is connected to the output and input terminals. The first capacitance cancels first harmonics at the output terminal of the first transistor with second harmonics at the input terminal of the second transistor. The first transistor and the first inductance are connected in series between a voltage source and a reference terminal. The second transistor and the second inductance are connected in series between the voltage source and the reference terminal.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
The present disclosure is a continuation of U.S. patent application Ser. No. 13/336,785 (now U.S. Pat. No. 8,680,924), filed Dec. 23, 2011. This application claims the benefit of U.S. Provisional Patent App. No. 61/426,991, filed Dec. 23, 2010. The entire disclosures of the applications referenced above are incorporated herein by reference.


BACKGROUND
Particular embodiments of the present invention generally relate to power amplifiers. More specifically, particular embodiments of the present invention relate to a power amplifier configured to cancel even harmonic signals.
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Power amplifiers are configured to amplify the power of a received AC signal, such as an RF signal. Traditional power amplifiers often include a single nMOS transistor or a cascode both with tank loading. FIG. 1 is a simplified schematic of a traditional power amplifier 100 that includes an nMOS transistor 105 coupled between an inductor 110 and ground. Inductor 110 may be coupled to a voltage source Vdd. Inductor 110 may be coupled to a capacitor 115 in a tank configuration for tuning the resonance of power amplifier 100. The nMOS transistor 105 may be in a common source configuration with the gate of the nMOS transistor 105 configured to operate as an input to receive an AC signal and the drain coupled to the output Vout of power amplifier 100. FIG. 2 is a simplified schematic of another traditional power amplifier 200 that may include first and second nMOS transistors 205 and 210 in series between an inductor 215 and ground. Inductor 215 may be coupled to a voltage source Vdd. Inductor 215 may also be coupled to a capacitor 220 in a tank configuration for tuning the resonance of power amplifier 200. The nMOS transistors 205 and 210 may be in a common source, common gate configuration with the gate of nMOS transistor 205 configured to operate as an input to receive an AC signal and a drain of transistor 210 coupled to the output of power amplifier 200.
In traditional power amplifiers, such as power amplifier 100 and 200 described above, there are typically a number of nonlinear components at the drain of the transistor adjacent to the inductor. For example, the even harmonics, and especially the 2nd harmonic, of a received AC signal tend to be fairly large at the drain of the transistor adjacent to the inductor. The averaging DC current from the even harmonics tends to be relatively large and flows in the inductor generating a relatively large amount of heat. As a result of the heat in the inductor, the inductance may change and in a worst case scenario the inductor may catastrophically fail.
Therefore, it would be desirable to provide new power amplifiers that are configured to reduce the nonlinear components and reduce the adverse effects of the nonlinear components in power amplifiers.
SUMMARY
Particular embodiments of the present invention generally relate to power amplifiers. More specifically, particular embodiments of the present invention relate to a power amplifier configured to cancel even harmonic signals.
According to one specific embodiment, a power amplifier includes a push-pull pair of transistors including a first transistor inductively coupled to a voltage source and coupled to a ground. A second transistor is inductively coupled to the ground and is coupled to the voltage source. Gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency. Drain regions of the first and the second transistors are, respectively, first and second output nodes. The power amplifier further includes a capacitor coupled between the first output node and the second output node where the capacitor is configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.
According to another specific embodiment, the power amplifier further includes a first inductor disposed between the first transistor and the voltage source. The first output node is between the first transistor and the first inductor. The power amplifier further includes a second inductordisposed between second transistor and the ground. The second output node is between the second transistor and the second inductor.
According to another specific embodiment, the first output node is between the drain of the first transistor and the first inductor, and the second output node is between the drain of the second transistor and the second inductor.
According to another specific embodiment, a source of the first transistor is coupled to the ground, and a source of the second transistor is coupled to the voltage source.
According to another specific embodiment, the first transistor and the second transistor are in a common source configuration.
According to another specific embodiment, the power amplifier further includes a first tank capacitor coupled in parallel with the first inductor and configured to tune the resonant frequency of a first inductor. The power amplifier further includes a second tank capacitor coupled in parallel with the second inductor and configured to tune the resonant frequency of a second inductor.
According to another specific embodiment, the capacitor is substantially not a pathway for cancellation of fundamental frequency of the AC signal.
According to another specific embodiment, the first output node and the second output node are the same output of the power amplifier and are configured to be combined by a combiner.
According to another embodiment, a power amplifier includes a first transistor, and a first inductor coupled in series with the first transistor between a voltage source and a ground. A first node between the first transistor and the first inductor is a first output. The power amplifier further includes a second transistor and a second inductor coupled in series with the second transistor between the voltage source and the ground. A second node between the second transistor and the second inductor is a second output. Gates of the first and the second transistors are AC inputs configured to receive an AC signal having a fundamental frequency. The power amplifier further includes a capacitor coupled between the first node and the second node and is configured as a pathway for cancellation of even harmonic signals of the fundamental frequency of the AC signal.
According to another embodiment, a fully-differential power amplifier includes first, second, third, and fourth transistor-inductor pairs each coupled in series between a voltage source and a ground. The fully-differential power amplifier further includes a first capacitor coupled between a first node, which is between the first transistor-inductor pair, and a second node, which is between the second transistor-inductor pair. The fully-differential power amplifier further includes a second capacitor coupled between a third node, which is between the third transistor-inductor pair, and a fourth node, which is between the fourth transistor-inductor pair. A first transistor of the first transistor-inductor pair is coupled to ground, and a first inductor of the first transistor-inductor pair is coupled to a voltage source. A second transistor of the second transistor-inductor pair is coupled to the voltage source, and a second inductor of the second transistor-inductor pair is coupled to the ground. A third transistor of the third transistor-inductor pair is coupled to ground, and a third inductor of the third transistor-inductor pair is coupled to a voltage source. A fourth transistor of the fourth transistor-inductor pair is coupled to the voltage source, and a fourth inductor of the fourth transistor-inductor pair is coupled to the ground. Gates of the first and the second transistors are plus input configured to receive an AC signal having a fundamental frequency, and gates of the third and the fourth transistors are minus input configured to receive the AC signal having the fundamental frequency. The first and second capacitors are configured to cancel even harmonics of the fundamental frequency of the AC signal.
According to one specific embodiment of the fully-differential power amplifier, the first and the third transistor are a first differential pair, and the second and fourth transistors are a second differential pair. Further, the first, second, third, and fourth transistors may be in a common source configuration.
According to another specific embodiment of the fully-differential power amplifier, the fully-differential power amplifier further includes a combiner having first, second, third, and fourth combiner inductors respectively in series. The first inductor and the first combiner inductor are inductively coupled. The third inductor and the second combiner inductor are inductively coupled. The fourth inductor and the third combiner inductor are inductively coupled. The second inductor and the fourth combiner inductor are inductively coupled.
According to another specific embodiment of the fully-differential power amplifier, the fully-differential power amplifier further includes a combiner having first and second combiner inductors in series. The first inductor, the second inductor, and the first combiner inductor are inductively coupled. The third inductor, the fourth inductor, and the second combiner inductor are inductively coupled.
The following detailed description and accompanying drawings provide a more detailed understanding of the nature and advantages of the present invention.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a simplified schematic of a traditional power amplifier that includes an nMOS transistor coupled in series with an inductor between a voltage source and ground;
FIG. 2 is a simplified schematic of another traditional power amplifier that may include first and second nMOS transistors coupled in series with an inductor between a voltage source and ground;
FIG. 3 is a simplified schematic of a power amplifier according to one embodiment of the present invention;
FIG. 4A is a simplified schematic of a fully-differential power amplifier according to one embodiment of the present invention;
FIG. 4B is an alternative simplified schematic of the fully-differential power amplifier shown in FIG. 4A; and
FIG. 5 is a simplified schematic of a fully-differential power amplifier according to an alternative embodiment.



DETAILED DESCRIPTION
Described herein are embodiments of a power amplifier and a method of operation for the power amplifier where the power amplifier is configured to cancel even harmonic signals present in the power amplifier.
In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. Particular embodiments as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.
Power amplifiers are configured to amplify the power of a received AC signal, such as an RF signal, for subsequent transmission of the amplified AC signal. Power amplifiers may be included in a variety of mobile devices, such as mobile telephones. A power-amplified AC signal may be directed through an antenna of a mobile device for transmission.
FIG. 3 is a simplified schematic of a power amplifier 300 according to one embodiment of the present invention. Power amplifier 300 includes a push-pull pair of transistors 305 and 310, (referred to as transistors 305 and 310). Power amplifier 300 further includes first and second inductors 315 and 320 and a capacitor 325. Power amplifier 300 may also include first and second tank capacitors 330 and 335. Transistors 305 and 310 may be metal-oxide-semiconductor field-effect transistors (MOSFETs), bipolar-junction transistors (BJTs), or other transistor types. For convenience, power amplifier embodiments are described herein as including MOSFETs. While power amplifier embodiments are described herein as including MOSFETs, it will be understood by those of skill in the art that BJTs or other types of transistors may be appropriately substituted for the MOSFETs, and these power amplifier embodiments that include BJTs or the like are considered to be within the scope and purview of the power amplifier embodiments of the present invention.
According to one embodiment, transistor 305 is an nMOS transistor with a drain 305a (sometimes referred to a drain region) coupled to a first end of inductor 315 where a second end of inductor 315 is coupled to a voltage source Vdd. A source 305b of transistor 305 may be coupled to ground. Specific configurations of the sources and the drains of transistors 305 and 310 are described herein for convenience of explanation of specific embodiments. Alternative embodiments of the power amplifiers may include alternative configurations of the sources and the drains of transistors 305 and 310 as will be understood by those of skill in the art. A gate 305c of transistor 305 may be a first AC input 340a configured to receive an AC signal.
Capacitor 330 is coupled to inductor 315 in parallel in a tank configuration. Capacitor 330 is configured to tune the resonant frequency of inductor 315.
According to one embodiment, transistor 310 is a pMOS transistor with a drain 310a coupled to a first end of inductor 320 where a second end of inductor 320 is coupled to ground. A source 310b of transistor 310 is coupled to the voltage source Vdd. A gate 310c of transistor 310 may be a second AC input 340b configured to receive the AC signal supplied to the first AC input 340a. Capacitor 335 may be coupled to inductor 320 in parallel in a tank configuration. Capacitor 335 is configured to tune the resonant frequency of inductor 320. Inductors 315 and 320 may be considered the respective loads of the push-pull pair of transistors 305 and 310.
Power amplifier 300 includes a first output node 345a disposed between inductor 315 and the drain 305 a of transistor 305. Power amplifier 300 further includes a second output node 345b disposed between inductor 320 and the drain 310a of transistor 310. According to one embodiment capacitor 325 is coupled between the first output node 345a and the second output node 345b. 
The even harmonics (e.g., 2nd harmonic, 4th harmonic, etc.) of the AC signal that are at the drain of transistor 305 are generally 180 degrees out of phase with the even harmonics of the AC signal that are at the drain of transistor 310. The capacitance of capacitor 325 is determined such that the circuit path between the two output nodes is substantially a short circuit for the even harmonics, but is not a short circuit for the fundamental frequency of the AC signal. According to one specific embodiment, the capacitance of capacitor 325 is approximately 20 picofarads for an approximately 2 gigahertz AC signal. As the even harmonics on either side of capacitor 325 are out of 180 degrees phase and as capacitor 325 is substantially a short for the even harmonics on either side of capacitor 325, the even harmonics on either side of capacitor 325 tend to cancel each other. Because the even harmonics on either side of capacitor 325 tend to cancel each other, capacitor 325 ensures that the even harmonics do not substantially pass into the inductors 315 and 320. As a result, unnecessary heating of the inductors 315 and 320 via the even harmonics is inhibited. Capacitor 325 may be replaced with alternative circuits that provide capacitance, such as a diode or the like according to one alternative embodiment.
The voltage at the first output node 345a (i.e., the voltage at the drain of transistor 305) may be expressed as: Vdn=aon+o.inv(/o)+a2nv(2/0)+ounv(3/o)+ . . . , and the voltage at the second output node 345b (i.e., the voltage at the drain of transistor 310) may be expressed as: Vdp=aop+aipv(/o)+a2pv(2/0)+ounv(3/o)+ . . . . The even coefficients of the voltage expressions for nMOS transistors and pMOS transistors have different polarities, providing for the substantial cancellation of the even harmonics across capacitor 325.
Power amplifier 300 is a single stage power amplifier that is not fully differential. Two power amplifiers 300 may be inductively coupled to form a fully-differential power amplifier according to one embodiment of the present invention.
FIG. 4A is a simplified schematic of a fully-differential power amplifier 400 according to one embodiment of the present invention. Fully-differential power amplifier 400 includes a first power amplifier 400a and a second power amplifier 400b. Power amplifiers 400a and 400b have substantially the same configuration as power amplifier 300 described above. Power amplifiers 400a and 400b are both configured to cancel the even harmonics of the fundamental frequency of a received AC signal. The numbering scheme of the power amplifiers used with respect to fully-differential power amplifier 400 is changed for convenience as fully-differential power 400 includes first and second stages where each stage includes a power amplifier 300.
Power amplifier 400a includes a push-pull pair of transistors 405a and 410a, which are referred to herein as transistors 405a and 410a. Power amplifier 400a further includes first and second inductors 415a and 420a and a capacitor 425a. Power amplifier 400 may also include first and second tank capacitors 430a and 435a. Transistors 405a and 410a are MOSFETs, BJTs, or other transistors types. According to one embodiment, transistor 405a is an nMOS transistor, and transistor 410a is a pMOS transistor. The electronic components of power amplifier 400a are configured substantially the same as the corresponding electronic components of power amplifier 300. For example, transistor 405a may be in a common source configuration and coupled in series with inductor 415a. The series pair of transistor 405a and inductor 415a are coupled between the voltage source Vdd and the ground where transistor 405a is coupled to the ground and inductor 415a is coupled to the voltage source Vdd. Transistor 410a is similarly in a common source configuration and is coupled in series with inductor 420a. The series pair of transistor 410a and inductor 420a are coupled between the voltage source Vdd and the ground with transistor 410a coupled to the voltage source Vdd and inductor 420a coupled to the ground.
Power amplifier 400b includes a push-pull pair of transistors 405b and 410b, which are referred to herein as transistors 405b and 410b. Power amplifier 400b further includes first and second inductors 415b and 420b and a capacitor 425b. Power amplifier 400 may also include first and second tank capacitors 430b and 435b. Transistors 405b and 410b may be MOSFETs, BJTs, or other transistor types. According to one embodiment, transistor 405b is an nMOS transistor, and transistor 410b is a pMOS transistor. The electronic components of power amplifier 400a are configured substantially the same as the corresponding electronic components of power amplifier 300. For example, transistor 405b may be in a common source configuration and coupled in series with inductor 415b. The series pair of transistor 405b and inductor 415b are coupled between the voltage source Vdd and the ground where transistor 405b is coupled to the ground and inductor 415b is coupled to the voltage source Vdd. Transistor 410b is similarly in a common source configuration and is coupled in series with inductor 420b. The series pair of transistor 410b and inductor 420b are between the voltage source Vdd and the ground with transistor 410b coupled to the voltage source Vdd and inductor 420b coupled to the ground.
The first and second output nodes of power amplifier 400a are coupled by capacitor 425a. The first and second output nodes of power amplifier 400b are coupled by capacitor 425b. Capacitor 425a is configured to provide a circuit path for the cancellation of even harmonics on opposite sides of capacitor 425a. Similarly, capacitor 425b is configured to provide a circuit path for the cancellation of even harmonics on opposite sides of capacitor 425b. 
The gates of transistors 405a and 410a are the AC inputs for power amplifier 400a and are the “plus” inputs of fully-differential power amplifier 400. The gates of transistors 405b and 410b are the AC inputs for power amplifier 400b and are the “minus” inputs of fully-differential power amplifier 400. The plus inputs of the fully-differential power amplifier are designated with “+” symbols in FIG. 4A, and the minus inputs are designated with symbols in FIG. 4A. The nMOS transistors 405a and 405b are a fully differential n-pair and the pMOS transistors 410a and 410b are a fully differential p-pair.
According to one embodiment of the present invention, a combiner 460 is configured to combine the outputs of fully-differential power amplifier 400 to deliver an amplified AC signal to an antenna 465 or the like. Combiner 460 includes a first, second, third, and fourth inductors 470a, 470b, 470c, and 470d coupled together in series and coupled to antenna 465. Inductors 470a, 470b, 470c, and 470d are inductively coupled, respectively, to inductors 415a, 420a, 415b, and 420b where each pair of inductively coupled inductors is a transformer.
FIG. 4B is an alternative simplified schematic of the fully-differential power amplifier 400 shown in FIG. 4A. Transistors 405a and 405b are shown as a driver 405 with positive and negative differential outputs, and transistors 410a and 410b are shown as a driver 410 also with positive and negative differential outputs. The fully-differential power amplifier 400 as shown in FIG. 4B clearly shows the serial nature of the first, the second, the third, and the fourth inductors 470a, 470b, 470c, and 470d. As shown in FIG. 4B, the laterally adjacent inductors 415a and 470a are inductively coupled, the laterally adjacent inductors 415b and 470b are inductively coupled, the laterally adjacent inductors 420a and 470d are inductively coupled, and the laterally adjacent inductors 420b and 470c are inductively coupled. The node between inductors 415a and 415b may be coupled to a reference voltage, such as Vdd. The node between inductors 420a and 420b may be coupled to ground.
FIG. 5 is a simplified schematic of a fully-differential power amplifier 500 according to an alternative embodiment. The same reference numeral schema used for the foregoing described figures is used in FIG. 5 to identify the same elements or substantially similar elements. Fully-differential power amplifier 500 is substantially similar to fully-differential power amplifier 400 but differs in that a combiner 480 of fully-differential power amplifier 500 differs from combiner 460 of fully-differential power amplifier 400. Combiner 480 includes the first inductor 470a inductively coupled to both inductors 415a and 420a, and the second inductor 470b inductively coupled to both inductors 415b and 420b. Inductors 470a and 470b are disposed in series. Inductors 470a, 415a, and 420a are substantially parallel, and inductors 470b, 415b, and 420b are also substantially parallel. Combiner 480 then applies the outputs onto antenna 465.
The above description illustrates various embodiments of the present invention along with examples of how aspects of the present invention may be implemented. The above examples and embodiments should not be deemed to be the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the above disclosure and the following claims, other arrangements, embodiments, implementations, and equivalents may be employed without departing from the scope of the invention as defined by the claims.

## Claims

1. A differential power amplifier comprising:
a first push-pull pair of transistors including
a first transistor comprising a control terminal and an output terminal, and
a second transistor comprising an input terminal and a control terminal, wherein the control terminal of the first transistor and the control terminal of the second transistor are configured to collectively receive a differential input signal, wherein the output terminal of the first transistor and the input terminal of the second transistor are configured to collectively provide a differential output signal;

a first capacitance connected to (i) the output terminal of the first transistor, and (ii) the input terminal of the second transistor, wherein harmonics at the output terminal of the first transistor combine with harmonics at the input terminal of the second transistor via the first capacitance such that the harmonics at the output terminal of the first transistor and the harmonics at the input terminal of the second transistor cancel;
a first inductance, wherein the first transistor and the first inductance are connected in series between a voltage source and a reference terminal;
a second inductance, wherein the second transistor and the second inductance are connected in series between the voltage source and the reference terminal; and
a second capacitance connected in parallel with the first inductance or the second inductance.

2. The differential power amplifier of claim 1, further comprising a third capacitance, wherein:
the second capacitance is connected in parallel with the first inductance; and
the third capacitance is connected in parallel with the second inductance.

3. The differential power amplifier of claim 1, wherein
the first transistor includes an input terminal;
the input terminal of the first transistor and the second inductance are connected to the voltage source;
the second transistor includes an output terminal; and
the output terminal of the second transistor and the first inductance are connected to the reference terminal.

4. A circuit comprising:
a differential power amplifier comprising
a first push-pull pair of transistors including
a first transistor comprising a control terminal and an output terminal, and
a second transistor comprising an input terminal and a control terminal, wherein the control terminal of the first transistor and the control terminal of the second transistor are configured to collectively receive a differential input signal, wherein the output terminal of the first transistor and the input terminal of the second transistor are configured to collectively provide a differential output signal,

a first capacitance connected to (i) the output terminal of the first transistor, and (ii) the input terminal of the second transistor, wherein harmonics at the output terminal of the first transistor combine with harmonics at the input terminal of the second transistor via the first capacitance such that the harmonics at the output terminal of the first transistor and the harmonics at the input terminal of the second transistor cancel,
a first inductance, wherein the first transistor and the first inductance are connected in series between a voltage source and a reference terminal, and
a second inductance, wherein the second transistor and the second inductance are connected in series between the voltage source and the reference terminal, and

a second power amplifier comprising
a differential input configured to receive the differential output signal, and
a differential output configured to output a second differential output signal.

5. The circuit of claim 4, wherein the second power amplifier comprises a second push-pull pair of transistors.

6. The circuit of claim 5, further comprising a third inductance and a fourth inductance, wherein:
the second push-pull pair of transistors comprises a third transistor and a fourth transistor;
the second power amplifier comprises a fifth inductance and a sixth inductance, wherein the fifth inductance is connected in series with the fourth transistor, and wherein the sixth inductance is connected in series with the third transistor;
the third inductance is inductively coupled to the first inductance or the fifth inductance; and
the fourth inductance is inductively coupled to the second inductance or the sixth inductance.

7. The circuit of claim 6, wherein the fifth inductance and the sixth inductance collectively provide the second differential output signal.

8. The circuit of claim 6, wherein the third inductance and the fourth inductance collectively provide the second differential output signal.

9. The circuit of claim 6, wherein the third inductance and the fourth inductance are connected in series.

10. The circuit of claim 6, further comprising a seventh inductance and an eighth inductance, wherein:
the third inductance is inductively coupled to the first inductance;
the fourth inductance is inductively coupled to the second inductance;
the seventh inductance is inductively coupled to the fifth inductance; and
the eighth inductance is inductively coupled to the sixth inductance.

11. The circuit of claim 10, wherein the third, fourth, seventh and eighth inductances are connected in series.

12. The circuit of claim 6, wherein:
the third inductance is inductively coupled to the first inductance and the fifth inductance; and
the fourth inductance is inductively coupled to the second inductance and the sixth inductance.

13. A power amplifier comprising:
a first differential amplifier comprising a differential input, a first output terminal and a second output terminal, wherein the differential input is configured to receive a differential input signal, and wherein the first output terminal and the second output terminal are configured to collectively output a first differential output;
a first inductance;
a second inductance, wherein the first inductance and the second inductance are connected in series between the first output terminal and the second output terminal of the first differential amplifier, and wherein the first inductance and the second inductance are connected to each other at a voltage source terminal or a ground reference terminal;
a third inductance inductively coupled to the first inductance;
a fourth inductance inductively coupled to the second inductance, wherein the third inductance and the fourth inductance provide a second differential output;
a second differential amplifier comprising a third output terminal and a fourth output terminal, wherein the third output terminal and the fourth output terminal are not connected to the first output terminal and the second output terminal;
a first capacitance connected between the first output terminal and the second third output terminal, wherein harmonics at the first output terminal combine with harmonics at the third output terminal such that the harmonics at the first output terminal and the harmonics at the third output terminal cancel out each other; and
a second capacitance connected between the second output terminal and the fourth output terminal, wherein harmonics at the second output terminal combine with harmonics at the fourth output terminal such that the harmonics at the second output terminal and the harmonics at the fourth output terminal cancel.

14. The power amplifier of claim 13, wherein the third inductance and the fourth inductance are connected in series.

15. The power amplifier of claim 13, further comprising:
a fifth inductance; and
a sixth inductance, wherein the fifth inductance and the sixth inductance are connected in series between terminals of the differential output of the second differential amplifier.

16. The power amplifier of claim 15, wherein:
the third inductance is inductively coupled to the fifth inductance; and
the fourth inductance is inductively coupled to the sixth inductance.

17. The power amplifier of claim 15, further comprising:
a seventh inductance inductively coupled to the fifth inductance; and
an eighth inductance inductively coupled to the sixth inductance.

18. The power amplifier of claim 17, wherein the third inductance, the fourth inductance, the seventh inductance and the eighth inductance are connected in series.

19. The power amplifier of claim 15, wherein:
the first differential amplifier comprises a first transistor and a second transistor;
the second differential amplifier comprises a third transistor and a fourth transistor;
the first transistor and the third transistor are a first push-pull pair of transistors; and
the second transistor and the fourth transistor are a second push-pull pair of transistors.

20. The power amplifier of claim 19, wherein:
the first transistor and the first inductance are connected in series between the voltage source terminal and the ground reference terminal;
the second transistor and the second inductance are connected in series between the voltage source terminal and the ground reference terminal;
the third transistor is connected in series with the fifth inductance; and
the fourth transistor is connected in series with the sixth inductance.

21. The differential power amplifier of claim 1, wherein:
the first inductance is connected to the output terminal of the first transistor and to a reference terminal; and
the second inductance is connected to a voltage source and to the input terminal of the second transistor.

22. The differential power amplifier of claim 1, wherein the differential output signal is provided at terminals of the first capacitance.

23. The differential power amplifier of claim 1, wherein the first capacitance is directly connected to (i) the output terminal of the first transistor, and (ii) the input terminal of the second transistor.