09.RF power amplifiers

178 RF POWER AMPLIFIERS

is supplied through a resistor and max D 50% when the bias to the collector is supplied through an RF choke.

9.3THE CLASS C AMPLIFIER

The class C amplifier is useful for providing a high-power continuous wave (CW) or frequency modulation (FM) output. When it is used in amplitude modulation schemes, the output variation is done by varying the bias supply. There are several characteristics that distinguish the class C amplifier from the class A or B amplifiers. First of all it is biased so that the transistor conduction angle is <180°. Consequently the class C amplifier is clearly nonlinear in that it does not directly replicate the input signal like the class A and B amplifiers do (at least in principle). The class A amplifier requires one transistor, the class B amplifier requires two transistors, and the class C amplifier uses one transistor. Topologically it looks similar to the class A except for the dc bias levels. It was noted that in the class B amplifier, an output filter is used optionally to help clean up the output signal. In the class C amplifier, such a tuned output is necessary in order to recover the sine wave. Finally, class C operation is capable of higher efficiency than either of the previous two classes, so for the appropriate signal types they become very attractive as power amplifiers.

The class C amplifier shown in Fig. 9.9 gives the output circuit for a power bipolar transistor (BJT) with the required tuned circuit. An N-channel enhance- ment-mode metal oxide semiconductor field effect transistor (MOSFET) can be used in place of the BJT. The Q of the tuned circuit will determine the bandwidth of the amplifier. The large inductance RF coil in the collector voltage supply ensures that only dc current flows there. During that part of the input cycle when the transistor is on, the bias supply current flows through the transistor and the output voltage is approximately 90% of VCC. When the transistor is off, the supply current flows into the blocking capacitor. The current waveform at the

FIGURE 9.9 A simple class C amplifier where VBB determines the conduction angle.

180 RF POWER AMPLIFIERS

Evaluation of the integral makes use of the trigonometric identity, cos ˛ š ˇ D cos ˛ cos ˇ Ý sin ˛ sin ˇ. From Eq. (9.36) the dc current is

This gives the dc current from the power supply in terms of IOC and the conduction angle , so power supplied by the source is

The ac component of the current flows through the blocking capacitor and into the load. Harmonic current components are shorted to ground by the tuned circuit. The magnitude of the output voltage at the fundamental frequency is found using the Fourier method:

9.43 The quiescent current term, IC, is replaced by Eq. (9.37) again, and the trigonometric identity for sin ˛ cos ˇ is used:

The efficiency (neglecting the input power) is simply the ratio of the output ac power to the input dc power. The maximum output power occurs when VO O D

VCC. The maximum efficiency is then [2]

A plot of this expression (Fig. 9.11) clearly illustrates the efficiency in terms of the conduction angle for class A, B, and C amplifiers. The increased efficiency of the class C amplifier is a result of the collector current flowing for less than a half-cycle. When the collector current is maximum, the collector voltage is minimum, so the power dissipation is inherently lower than class B or class A operation.

Another important parameter for the power amplifier is the ratio of the maximum average output power where VO O D VCC, to the peak instantaneous output power:

FIGURE 9.11 Power efficiency for class A, B, and C amplifiers.

182 RF POWER AMPLIFIERS

The maximum average output power occurs when VO O D VCC and is given by

The maximum voltage at the collector is the output voltage ac voltage swing plus the bias voltage:

The ratio of the maximum average power to the peak power from (9.49) is [2]

A plot of this ratio as a function of conduction angle in Fig. 9.12 shows that maximum efficiency of the class C amplifier occurs when there is no output

FIGURE 9.12 Maximum output power to the peak power ratio.

184 RF POWER AMPLIFIERS

ψ

FIGURE 9.13 Conduction angle dependence on VBB.

is superimposed on the emitter bias voltage. The input voltage commences to rise above the turn on voltage of the transistor at

In this way the base bias voltage is determined.

9.5THE CLASS D POWER AMPLIFIER

Inspection of the efficiency and output power of a class C amplifier reveals that 100% efficiency only occurs when the output power is zero. A modification of class B operation shown in [3] indicates that judicious choice of bias voltages and circuit impedances provide a clipped voltage waveform at the collector of the BJT while, in the optimum case, retaining the half sine wave collector current. In the limit the clipped waveform becomes a square wave. This is no longer linear, and thus is distinguished from the class B amplifier.

The class D amplifier shown in Fig. 9.14 superficially looks like a class B amplifier except for the input side bias. In class D operation the transistors act as

1 : n Filter

M 2

FIGURE 9.14 Class D power amplifier.

near ideal switches that are on half of the time and off half of the time. The input ideally is excited by a square wave. If the transistor switching time is near zero, then the maximum drain current flows while the drain–source voltage, VDS D 0. As a result 100% efficiency is theoretically possible. In practice the switching speed of a bipolar transistor is not sufficiently fast for square waves above a few MHz, and the switching speed for field effect transistors is not adequate for frequencies above a few tens of MHz.

9.6THE CLASS F POWER AMPLIFIER

“A class F amplifier is characterized by a load network that has resonances at one or more harmonic frequencies as well as at the carrier frequency” [2, p. 454]. The class F amplifier was one of the early methods used to increase amplifier efficiency and has attracted some renewed interest recently. The circuit shown in Fig. 9.15 is a third harmonic peaking amplifier where the shunt resonator is resonant at the fundamental and the series resonator at the third harmonic. More details on this and higher-order resonator class F amplifiers are found in [4]. When the transistor is excited by a sinusoidal source, it is on for approximately half the time and off for half the time. The resulting output current waveform is converted back to a sine wave by the resonator, L1, C1. The L3, C3 resonator is not quite transparent to the fundamental frequency, but blocks the third harmonic energy from getting to the load. The drain or collector voltage, it would seem, will range from 0 to twice the power supply voltage with an average value of VCC. The third harmonic voltage on the drain or collector, if it has the appropriate amplitude and phase, will tend to make this device voltage more square in shape. This will make the transistor act more like a switch with the attendant high efficiency. It is found that maximum “squareness” is achieved if the third harmonic voltage is 1/9th of the fundamental voltage. This will give a maximum collector efficiency of 88.4% [2, pp. 454–456].

VCC

L 3

C B

FIGURE 9.15 Class F third harmonic peaking power amplifier.