# Patent

## Title
Class AB amplifiers

## Abstract

An amplifier including first, second, third, and fourth switches, each having first and second terminals. The first terminal of each switch communicates with a respective load. The second terminal of the first switch communicates with the second terminal of the second switch. The second terminal of the third switch communicates with the second terminal of the fourth switch. A first terminal of a first capacitance communicates with the second terminals of the first and second switches. A first terminal of a second capacitance communicates with the second terminals of the third and fourth switches. A first inductance communicates with second terminals of the first and second capacitances.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This is a continuation of U.S. patent application Ser. No. 13/769,700 (now U.S. Pat. No. 8,736,374), filed on Feb. 18, 2013 which is a continuation of U.S. patent application Ser. No. 13/044,183 (now U.S. Pat. No. 8,378,750), filed on Mar. 9, 2011, which claims the benefit of U.S. Provisional Application No. 61/312,167, filed on Mar. 9, 2010. The entire disclosures of the above applications are incorporated herein by reference.


FIELD
The present disclosure relates to amplifiers, and more particularly to class AB amplifiers.
BACKGROUND
The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
Class A amplifiers operate over an entire cycle of an input signal. An output signal is a scaled-up replica of the input signal. Class A amplifiers have a maximum efficiency of about 50% with inductive output coupling and about 25% with capacitive output coupling.
In Class A amplifiers, a transistor is biased such that the transistor is always conducting. The transistor is operated over a linear portion of the transistor's transfer characteristic. Because the transistor is always conducting, power is drawn from the power supply even when there is no input. If high output power is needed, power consumption (and the accompanying heat) may become significant.
Class B amplifiers amplify during half of an input cycle. As a result, Class B amplifiers tend to increase distortion but have higher efficiency than Class A amplifiers. Class B amplifiers have a maximum efficiency over 75%. Transistors are switched off half of the time and do not dissipate power at this time.
Class B amplifiers may use complementary transistor pairs (a “push-pull” transistor arrangement). Complementary devices amplify opposite halves of the input signal. Mismatch or crossover distortion may occur when re-joining the halves of the signal. One solution to the mismatch problem involves biasing the transistors to be just on, rather than completely off when not in use. This biasing approach is called Class AB operation. In other words, Class AB amplifying devices may include a class B output stage that is biased so that both transistors are conducting around a crossover point.
SUMMARY
A class AB amplifier includes a first inductor having a first terminal in communication with a voltage source terminal. A first transistor has a drain terminal in communication with a second terminal of the first inductor. A second transistor has a source terminal in communication with a source terminal of the first transistor. A second inductor has a first terminal in communication with a drain terminal of the second transistor and a second terminal in communication with a reference potential. The drain terminals of the first transistor and the second transistor are capacitively coupled together.
In other features, a first capacitance has a first terminal in communication with the source terminals of the first transistor and the second transistor. A second terminal of the first capacitance is in communication with a voltage input to the class AB amplifier.
In other features, a first capacitance has a first terminal in communication with gate terminals of the first transistor and the second transistor. A second terminal of the first capacitance is in communication with a voltage input to the class AB amplifier.
In other features, a first variable capacitance is connected in parallel with the first inductor. A second variable capacitance is connected in parallel with the second inductor. A first capacitance has a first terminal in communication with the drain terminal of the first transistor and a second terminal in communication with the drain terminal of the second transistor.
In other features, N capacitances have first terminals in communication with the drain terminal of the first transistor. N resistances have first terminals in communication with second terminals of respective ones of the N capacitances and have second terminals in communication with the drain terminal of the second transistor, wherein N is an integer greater than zero.
In other features, a third inductor has a first terminal in communication with the voltage source terminal. A third transistor has a drain terminal in communication with a second terminal of the third inductor. A fourth transistor has a source terminal in communication with a source terminal of the third transistor. A fourth inductor has a first terminal in communication with a drain terminal of the fourth transistor and a second terminal in communication with a reference potential. The drain terminals of the third transistor and the fourth transistor are capacitively coupled.
In other features, first and second capacitances are connected in series with each other and connected in parallel to the drain terminals of the first transistor and the second transistor, respectively. Third and fourth capacitances are connected in series with each other and connected in parallel to the drain terminals of the third transistor and the fourth transistor, respectively.
In other features, a fifth capacitance has a first terminal connected between the first and second capacitances and to the source terminals of the first transistor and the second transistor. The fifth capacitance has a second terminal connected between the third and fourth capacitances and to the source terminals of the third transistor and the fourth transistor. A sixth capacitance has one end that communicates with the first terminals of the first and second capacitances. A seventh capacitance has one end that communicates with the first terminals of the third and fourth capacitances.
In other features, an input driver includes a third transistor having a gate terminal in communication with an input signal, a tank circuit in communication with a terminal of the third transistor and a matching network in communication with the terminal of the third transistor and the source terminals of the first transistor and the second transistor.
In other features, a first capacitance has a first terminal in communication with the source terminals of the first transistor and the second transistor. A second capacitance has a first terminal in communication with the source terminals of the third transistor and the fourth transistor. A fifth inductor is in communication with second terminals of the first and second capacitances.
In other features, a fifth transistor has a gate terminal in communication with a first polarity of a differential input signal and a first terminal in communication with the second terminal of the first capacitance. A sixth transistor has a gate terminal in communication with a second polarity of the differential input signal and a first terminal in communication with the second terminal of the second capacitance.
In other features, a power combiner includes fifth, sixth, seventh and eighth inductors coupled to the first, second, third and fourth inductors, respectively. An antenna is connected to the power combiner. The first, second, third and fourth transistors and the first, second, third and fourth inductors are connected in a first loop. The fifth, sixth, seventh and eighth inductors are connected in a second loop that is arranged one of inside or outside of the first loop.
In other features, a first capacitance has a first terminal in communication with the source terminals of the first transistor and the second transistor. A second capacitance has a first terminal in communication with the source terminals of the third transistor and the fourth transistor. A fifth inductor is in communication with second terminals of the first and second capacitances. The first, second, third and fourth transistors and the first, second, third and fourth inductors are connected in a first loop. The fifth, sixth, seventh and eighth inductors are connected in a second loop that is arranged one of inside or outside of the first loop. The fifth inductor has a shape of number “8”. The fifth inductor is located inside of the first loop and the second loop.
In other features, the first and second capacitances and the fifth inductor have a first impedance at a center frequency of the class AB amplifier and have second and third impedances at second and third harmonic frequencies, respectively, of the class AB amplifier. The second and third impedances are greater than the first impedance. The first transistor is an NMOS transistor and the second transistor is a PMOS transistor.
A class AB amplifier includes a first inductor having a first terminal in communication with a voltage source terminal. A first transistor has a drain terminal in communication with a second terminal of the first inductor. A second transistor has a source terminal in communication with a source terminal of the first transistor. A second inductor has a first terminal in communication with a drain terminal of the second transistor and a second terminal in communication with a reference potential. A third inductor has a first terminal in communication with the voltage source terminal. A third transistor has a drain terminal in communication with a second terminal of the third inductor. A fourth transistor has a source terminal in communication with a source terminal of the third transistor. A fourth inductor has a first terminal in communication with a drain terminal of the fourth transistor and a second terminal in communication with a reference potential. The drain terminals of the first transistor and the third transistor are capacitively coupled. The drain terminals of the second transistor and the fourth transistor are capacitively coupled. A first polarity of a differential signal is input to gates of the first and third transistors and a second polarity of the differential signal is input to gates of the second and fourth transistors.
Further areas of applicability of the present disclosure will become apparent from the detailed description, the claims and the drawings. The detailed description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the disclosure.



BRIEF DESCRIPTION OF DRAWINGS
The present disclosure will become more fully understood from the detailed description and the accompanying drawings, wherein:
FIG. 1 is an electrical schematic of a first push-pull class AB amplifier according to the present disclosure;
FIG. 2 is an electrical schematic of second push-pull class AB amplifier according to the present disclosure;
FIG. 3 is an electrical schematic of third push-pull class AB amplifier according to the present disclosure;
FIG. 4 is an electrical schematic of fourth push-pull class AB amplifier according to the present disclosure;
FIG. 5 is an electrical schematic of fifth push-pull class AB amplifier according to the present disclosure;
FIG. 6 is an electrical schematic of sixth push-pull class AB amplifier according to the present disclosure;
FIG. 7 is an electrical schematic of seventh push-pull class AB amplifier according to the present disclosure;
FIG. 8 is an electrical schematic of eighth push-pull class AB amplifier according to the present disclosure;
FIG. 9 is an electrical schematic of ninth push-pull class AB amplifier according to the present disclosure;
FIG. 10 is an example partial layout of a push-pull class AB amplifier with a power combiner according to the present disclosure; and
FIG. 11 is an example partial layout of another push-pull class AB amplifier with a power combiner according to the present disclosure.



DESCRIPTION
The following description is merely illustrative in nature and is in no way intended to limit the disclosure, its application, or uses. As used herein, the phrase at least one of A, B, and C should be construed to mean a logical (A or B or C), using a non-exclusive logical OR. It should be understood that steps within a method may be executed in different order without altering the principles of the present disclosure.
Referring now to FIGS. 1 and 2, single-ended arrangements of push-pull class AB amplifiers 50 and 100 are shown. In FIG. 1, the amplifier 50 is arranged in a common gate configuration. The amplifier 50 includes a first inductor L1, a first transistor T1, a second transistor T2 and a second inductor L2, which are connected in series. The first transistor T1 may be an NMOS transistor and the second transistor T2 may be a PMOS transistor, although other types of transistors can be used. Inputs of the transistors T1 and T2 may be connected to AC ground or another bias or reference signal.
Variable capacitances C1 and C2 may be connected in parallel with the inductors L1 and L2, respectively. The inductor L1 may be connected to a reference potential Vdd. The inductor L2 may be connected to a ground potential Vss.
A capacitance Ccm may be connected to drain terminals of the first and second transistors T1 and T2. An input signal may be applied via an input capacitance Cin to source terminals of the first and second transistors T1 and T2. Output signals Vo1 and Vo2 may be taken across terminals of the capacitance Ccm.
In FIG. 2, the push-pull class AB amplifier 100 is shown. The amplifier 100 is arranged in a common source configuration. The source terminals of the transistors T1 and T2 may be connected to AC ground or another bias or reference signal. The amplifier 100 is similar to the amplifier 50 except that the input signal Vin is applied to gates of the first and second transistors T1 and T2.
In both FIGS. 1 and 2, the capacitance Ccm removes even harmonics from the output signals Vo1 and Vo2 due to cancelling of the opposite phase of the even harmonics. The capacitance Ccm tends to help transistor mismatch and to reduce distortion. Unlike conventional power amplifiers, the amplifiers 50 and 100 can have a voltage swing that is greater than 2Vdd. The source node between the transistors T1 and T2 is floating and a low side can go below ground. The value of the capacitance Ccm may be selected to be larger than the capacitances C1 and C2. The output signals Vo1 and Vo2 may be recombined in any suitable manner. In one example, inductive coupling via transformers may be used to recombine the output signals Vo1 and Vo2, as will be described below.
Referring now to FIG. 3, another push-pull class AB amplifier 150 is shown. The capacitance Ccm is replaced by one or more series-connected capacitance and resistance pairs, which are connected in parallel across the first and second transistors T1 and T2. In particular, capacitances C31, C32, . . . , and C3N are connected in series with resistances R11, R12, . . . , and Rin, respectively, where N is an integer greater than zero. In some implementations, the capacitances C31, C32, . . . , and C3N are selected to have the same or different values. In some implementations, the resistances R11, R12, . . . , and Rin are selected to have the same or different values. Using the capacitances and resistances may tend to reduce oscillation.
Referring now to FIG. 4, another push-pull class AB amplifier 200 is shown. The amplifier 200 is a differential implementation of the amplifier 150 of FIG. 3. The amplifier 200 further includes a third inductor L3, a third transistor T3, a fourth transistor T4 and a fourth inductor L4, which are connected in series. The fourth transistor T4 may be an NMOS transistor and the third transistor T3 may be a PMOS transistor, although other types of transistors can be used. Inputs of the transistors T1 and T2 and T3 and T4 may be connected to AC ground or another bias or reference signal.
Variable capacitances C3 and C4 may be connected in parallel with the inductors L3 and L4, respectively. The inductor L4 may be connected to the reference potential Vdd. The inductor L3 may be connected to the ground potential Vss. A differential input signal may be applied via an input capacitance Cin to source terminals of the third and fourth transistors T3 and T4. Output signals Vo3 and Vo4 may be taken across terminals of the third and fourth transistors T3 and T4.
Capacitances C51, C52, . . . , and C5N are connected in series with resistances R11, R12, . . . , and Rin, respectively, where N is an integer greater than zero. One or more pairs of the capacitances C51, C52, . . . and C5N and the resistances R11, R12, . . . and Rin are connected in parallel across the third and fourth transistors T1 and T2. Capacitances C61, C62, . . . , and C6N are connected in series with resistances R21, R22, . . . , and R2N, respectively, where N is an integer greater than zero. One or more pairs of the capacitances C61, C62, . . . and C6N and the resistances R21, R22, . . . and R2N are connected in parallel across the third and fourth transistors T3 and T4.
Referring now to FIG. 5, another push-pull amplifier 250 is shown. Additional capacitances C5 to C11 may be provided. The capacitances C5 and C6 replace one of the common mode capacitances Ccm and are connected in series with each other and in parallel across the transistors T1 and T2. The capacitances C7 and C8 replace the other common mode capacitance Ccm and are connected in series with each other and in parallel across the transistors T3 and T4. One end of a capacitance C10 is connected to first terminals of the capacitances C5 and C6 and to the source terminals of the transistors T1 and T2. Another end of the capacitance C10 is connected to first terminals of the capacitances C7 and C8 and to the source terminals of the transistors T3 and T4.
One end of a variable capacitance C11 is connected to a second terminal of the capacitance C6 and to the drain terminal of the transistors T2. Another end of the variable capacitance C11 is connected to a second terminal of the capacitance C8 and to the drain terminal of the transistor T3. One end of a variable capacitance C9 is connected to a second terminal of the capacitance C5 and to the drain terminal of the transistors T1. Another end of the variable capacitance C9 is connected to a second terminal of the capacitance C7 and to the drain terminal of the transistor T4. For example, an inductive coupling loop may couple with the inductors L1, L2, L3 and L4 to drive an output such as an antenna.
Referring now to FIG. 6, another push-pull class AB amplifier 270 is shown and includes a power combiner 280. The amplifier 270 includes a first inductor L1, a first transistor T1, a second transistor T2 and a second inductor L2, which are connected in series.
The amplifier 270 further includes a third inductor L3, a third transistor T3, a fourth transistor T4 and a fourth inductor L4, which are connected in series. Capacitances C1, C2, C3 and C4 may be variable capacitances that are arranged in parallel with the inductors L1, L2, L3 and L4. Common mode capacitances C5 and C6 are arranged in parallel with transistors T1 and T2 and T3 and T4, respectively.
The power combiner 280 includes first, second, third and fourth inductors S1, S2, S3 and S4, respectively, which are coupled to the first, second, third and fourth inductors L1, L2, L3 and L4, respectively, to create first, second, third and fourth transformers. In some examples, the output may be coupled to an antenna (not shown) or another load.
Referring now to FIG. 7, an example of an input driver 300 for the single ended amplifier 50 is shown. A matching network 304 includes the capacitance Cin and an inductor L3. A tank circuit 306 includes an inductor L4 and a capacitance C3. An input signal Vin is input to a gate of a transistor T3. The tank circuit 306 and the matching network 304 couple the input signal to the sources of the transistors T1 and T2.
Referring now to FIG. 8, an example of an input driver 340 for a differential amplifier 350 is shown. The amplifier 350 includes the components of the amplifier 50. The amplifier 350 further includes a third inductor L3, a third transistor T3, a fourth transistor T4 and a fourth inductor L4, which are connected in series. Inputs of the transistors T1, T2, T3, and T4 may be connected to AC ground or another bias or reference signal. Variable capacitances C3 and C4 may be connected in parallel with the inductors L3 and L4, respectively. The inductor L4 may be connected to the reference potential Vdd. The inductor L3 may be connected to the ground potential Vss.
Capacitance C5, inductor Lin and capacitance C6 are connected in series between the sources of the transistors T1 and T2 and the sources of transistors T3 and T4. Drains (or sources) of the transistors T5 and T6 are connected between the inductor Lin and the capacitances C5 and C6, respectively. Sources (or drains) of the transistors T5 and T6 are connected to Vss. One polarity of the differential input signal Vin+ is coupled to a gate of the transistor T5 and the other polarity of the differential input signal Vin− is coupled to a gate of the transistor T6. The inductor Lin may have a center tap that may be connected to a bias signal, a reference potential or a ground potential.
The connection provided by the capacitances C5 and C6 and the inductor Lin provides source degeneration. The connection provides a low impedance connection such as a short circuit at a center frequency and a higher impedance connection at other frequencies. For example, the connection has high impedance at second and third harmonic frequencies.
Referring now to FIG. 9, another push-pull class AB amplifier 600 is shown and includes a power combiner 630. The amplifier 600 includes a first inductor L1, a first transistor T1, a second transistor T2 and a second inductor L2, which are connected in series. The amplifier 600 further includes a third inductor L3, a third transistor T3, a fourth transistor T4 and a fourth inductor L4, which are connected in series. A first capacitance C1, an inductor Lin and a second capacitance C2 are connected in series. The first capacitance C1 is also connected to source terminals of the first and second transistors T1 and T2. The second capacitance C2 is also connected to source terminals of the third and fourth transistor T3 and T4. Capacitance C3 is connected to drains of the transistors T1 and T3. Capacitance C4 is connected to drains of the transistors T2 and T4. A first polarity of the differential signal Vin+ is input to gates of transistors T1 and T3. A second polarity of the differential signal Vin− is input to gates of transistors T2 and T4.
The power combiner 630 includes first, second, third and fourth inductors S1, S2, S3 and S4, respectively, which are coupled to the first, second, third and fourth inductors L1, L2, L3 and L4, respectively, to create first, second, third and fourth transformers. In some examples, the output may be coupled to an antenna (not shown) or another load.
Referring now to FIG. 10, an example layout of the amplifier 600 and the power combiner 630 is shown. A first loop 704 includes the first, second, third and fourth inductors S1, S2, S3 and S4, respectively. A second loop 708 provides connections to the transistor pairs, Vdd and Vss. The first and second loops 704 and 708 may have a circular, elliptical, rectangular, square or other generally closed shape. The inductor Lin may be arranged inside or outside of the first and second loops 704 and 708. The inductor Lin may have a shape of number “8”. The first loop 704 may be arranged inside or outside of the second loop 708 in a plan view. The current in the first loop 704 may flow through the inductors S1, S2, S3 and S4 in the same direction.
While two legs are shown for example in FIGS. 5 and 6, additional legs with additional transistor pairs can be used. Referring now to FIG. 11, an example layout of an amplifier 730 and a power combiner 740 for four pairs of transistors is shown. A first loop 744 includes inductors that couple with inductors in a second loop 748. For example only, the first and second loops 744 and 748 may have a circular, elliptical, rectangular, square or other generally-closed shape. The inductor Lin may be arranged inside or outside of the first and second loops 744 and 748. The first loop 744 may be arranged inside or outside of the second loop 748 in a plan view.
The broad teachings of the disclosure can be implemented in a variety of forms. Therefore, while this disclosure includes particular examples, the true scope of the disclosure should not be so limited since other modifications will become apparent upon a study of the drawings, the specification, and the following claims.

## Claims

1. An amplifier, comprising:
a first switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal communicates with a first tank circuit;
a second switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the second switch communicates with a second tank circuit, and wherein the second terminal of the second switch communicates with the second terminal of the first switch;
a third switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the third switch communicates with a third tank circuit;
a fourth switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the fourth switch communicates with a fourth tank circuit, and wherein the second terminal of the fourth switch communicates with the second terminal of the third switch;
a first capacitance connected in series to a second capacitance, wherein the first capacitance is connected across the first and second terminals of the first switch, and wherein the second capacitance is connected across the first and second terminals of the second switch;
a third capacitance connected in series with a fourth capacitance, wherein the third capacitance is connected across the first and second terminals of the third switch, and wherein the fourth capacitance is connected across the first and second terminals of the fourth switch;
a fifth capacitance having a first terminal and a second terminal, wherein the first terminal of the fifth capacitance communicates with the first terminal of the first switch, and wherein the second terminal of the fifth capacitance communicates with the first terminal of the fourth switch; and
a sixth capacitance having a first terminal and a second terminal, wherein the first terminal of the sixth capacitance communicates with the first terminal of the second switch, and wherein the second terminal of the sixth capacitance communicates with the first terminal of the third switch.

2. The amplifier of claim 1, further comprising:
the first, second, third, and fourth tank circuits,
wherein each of the first, second, third, and fourth tank circuits comprises (i) an inductance and (ii) a variable capacitance.

3. The amplifier of claim 1, further comprising:
the first, second, third, and fourth tank circuits,
wherein the first and third tank circuits are connected to a first potential, and
wherein the second and fourth tank circuits are connected to a second potential.

4. The amplifier of claim 1, further comprising:
a seventh capacitance having a first terminal and a second terminal,
wherein the first terminal of the seventh capacitance is connected between the first and second capacitances and to the second terminals of the first and second switches, and
wherein the second terminal of the seventh capacitance is connected between the third and fourth capacitances and to the second terminals of the third and fourth switches.

5. An amplifier, comprising:
a first switch having a first terminal and a second terminal, wherein the first terminal communicates with a first load;
a second switch having a first terminal and a second terminal, wherein the first terminal of the second switch communicates with a second load, and wherein the second terminal of the second switch communicates with the second terminal of the first switch;
a third switch having a first terminal and a second terminal, wherein the first terminal of the third switch communicates with a third load;
a fourth switch having a first terminal and a second terminal, wherein the first terminal of the fourth switch communicates with a fourth load, wherein the second terminal of the fourth switch communicates with the second terminal of the third switch;
a first capacitance having a first terminal and a second terminal, wherein the first terminal of the first capacitance is in communication with the second terminals of the first and second switches;
a second capacitance having a first terminal and a second terminal, wherein the first terminal of the second capacitance is in communication with the second terminals of the third and fourth switches; and
a first inductance in communication with second terminals of the first and second capacitances.

6. The amplifier of claim 5, wherein:
each of the first, second, third, and fourth loads comprises a tank circuit;
wherein the tank circuit includes (i) a second inductance and (ii) a variable capacitance;
wherein the first and fourth tank circuits are connected to a supply voltage; and
wherein the second and third tank circuits are connected to a reference voltage.

7. The amplifier for claim 5, further comprising:
a fifth switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the fifth switch is in communication with the second terminal of the first capacitance, wherein the second terminal of the fifth switch is in communication with a reference voltage, and wherein the control terminal of the fifth switch is in communication with a first input of the amplifier; and
a sixth switch having a first terminal, a second terminal, and a control terminal, wherein the first terminal of the sixth switch is in communication with the second terminal of the second capacitance, wherein the second terminal of the sixth switch is in communication with the reference voltage, wherein the control terminal of the sixth switch is in communication with a second input of the amplifier.

8. The amplifier of claim 5, wherein:
each of the first, second, third, and fourth loads comprises a second inductance;
wherein the first and fourth loads are connected to a supply voltage; and
wherein the second and third loads are connected to a reference voltage.

9. The amplifier of claim 5, wherein:
each of the first, second, third, and fourth switches includes a control terminal,
the control terminals of the first and third switches receive a first input of the amplifier, and
the control terminals of the second and the fourth switches receive a second input of the amplifier.

10. The amplifier of claim 5, further comprising:
a third capacitance having a first terminal and a second terminal, wherein the first terminal of the third capacitance is in communication with the first terminal of the first switch, and wherein the second terminal of the third capacitance is in communication with the first terminal of the third switch; and
a fourth capacitance having a first terminal and a second terminal, wherein the first terminal of the fourth capacitance is in communication with the first terminal of the fourth switch, and wherein the second terminal of the fourth capacitance is in communication with the first terminal of the second switch.

11. The amplifier of claim 5, wherein:
the first and second capacitances and the first inductance have a first impedance at a center frequency of the amplifier;
the first and second capacitances and the first inductance have second and third impedances respectively at second and third harmonic frequencies of the amplifier; and
the second and third impedances are greater than the first impedance.

12. The amplifier of claim 5, wherein the first inductance has a shape of number “8”.

13. The amplifier of claim 5, further comprising:
four series-connected inductances;
wherein the four series-connected inductances are coupled respectively to the first, second, third, and fourth loads; and
wherein the four series-connected inductances couple an output of the amplifier to an antenna.

14. The amplifier of claim 13, wherein:
the first, second, third, and fourth loads are connected in a first loop; and
the four series-connected inductances are connected in a second loop.

15. The amplifier of claim 14, wherein the first loop and the second loop are concentric.

16. The amplifier of claim 14, wherein the first inductance is arranged inside the first loop and the second loop.