Power amplifiers are large-signal amplifiers. Compared to small-signal amplifiers, a much larger portion of the load line is used during signal operation. There are four classes of power amplifiers based on the percentage of the input cycle for which the amplifier operates in its linear region: class A, class B, class AB, and class C. Each class has unique circuit configurations and mode of operations. As the name suggests, the emphasis is on power amplification. Power amplifiers are normally used as the final stage of a communications receiver or transmitter to provide signal power to speakers or to a transmitting antenna. BJTs are used to illustrate power amplifier principles.
The Class A Power Amplifier
When an amplifier is biased such that it always operates in the linear region where the output signal is an amplified replica of the input signal, it is a class A amplifier; conduction over the full 360° of the cycle. The Class A amplifier uses a single switching transistor in the standard common emitter circuit configuration to produce an inverted output.
Class A amplifiers have excellent linearity, high gain and low signal distortion levels. For these advantages, they are used in high-fidelity audio amplifier designs. They are seldom used in high power amplifier applications due to thermal power supply considerations.
Since Class A amplifiers operate in the linear region, the transistors should be properly biased to ensure correct operation and low distortion. The output device is always “on” and is constantly carrying current, so there is a continuous loss of power in the amplifier. Due to this continuous loss of power the maximum efficiency of a capacitively coupled class A amplifier cannot be higher than 0.25, or 25%, and, in practice, is usually considerably less ( 10%).
In general, the low efficiency of class A amplifiers limits their usefulness to low power applications that usually require less than 1 W. Also, due to the high idling current of the amplifier, the power supply must be sized accordingly and well-filtered to avoid amplifier hum and noise. Due to the low efficiency and overheating problems of Class A amplifiers, more efficient amplifier classes have been developed.
The Class B and Class AB Push-Pull Amplifiers
When an amplifier is biased at cutoff so that it operates in the linear region for 180° of the input cycle and is in cutoff for 180°, it is a class B amplifier. Class AB amplifiers are biased to conduct for slightly more than 180°. In other words, a class B amplifier is biased to conduct over half the waveform. By using two transistors that conduct each half of a cycle, the complete signal is recovered. Compared to the class A amplifier, the theoretical efficiency of the class B amplifier is much higher at ~78%.
One main disadvantage of the class B amplifier is the cross-over distortion. This occurs due to nonlinearities close to switching, when one transistor turns on and the other turns off; and is more pronounced in low level signals below the 0.7V point (the turn ON VBE). Even if the transistors are specially matched, they do not exactly stop or start conducting at the cross-over point. There will be a time interval where both transistors are not conducting.
Class AB amplifiers seek to overcome cross-over distortion by slightly overlapping the conduction periods of the transistors so that there is a smoother transition at the cross-over point. This means that the two transistors conduct for slightly more than half of a cycle; one is already turned on while the other is just about to turn off.
This implementation is a little less efficient than the class B operation but yields better linearity and less distortion. Class AB amplifiers are a good compromise between efficiency (class B) and linearity (class A).
The Class C Amplifier
Class C amplifiers are biased so that conduction occurs for much less than 180° (around 90°). This results in very high levels of distortion, but also achieves very high efficiency levels. Class C amplifiers are more efficient than class A, class B or class AB, which means that more output power can be obtained from class C operation. The output amplitude is a nonlinear function of the input, so class C amplifiers are not used for linear amplification. They are generally used in radio frequency (RF) applications, such as oscillators that have a constant output amplitude, and modulators, where a high-frequency signal is controlled by a low-frequency signal.
The Class C amplifier circuit uses an LC resonant circuit at the output. The transistor produces current pulses according to the input signal and makes it flow through the resonant circuit. L and C values are selected so that the resonant circuit oscillates in the frequency of the input signal, and all other frequencies are attenuated. Harmonics or noise present in the output signal can be eliminated with additional filters. Theoretical efficiency can be as high as 80% but much lower values can be observed in practice.
The class A amplifier has the lowest distortion level but is the least efficient of these amplifier classes. The primary advantage of a class B or class AB amplifier over a class A amplifier is that either one is more efficient than a class A amplifier; you can get more output power for a given amount of input power. A disadvantage of class B/AB is that it is more difficult to implement the circuit in order to get a linear reproduction of the input waveform. The term push-pull refers to a common type of class B/AB amplifier circuit in which two transistors are used on alternating half-cycles to reproduce the input waveform at the output. The class AB amplifier offers less distortion than class B but is less efficient. The class C amplifier is the most efficient among these amplifier classes but it also introduces the highest distortion levels. While class C amplifiers are generally not used for linear applications, they can be used for frequency modulation with no issues.