The class of operation of an amplifier is defined by the percentage of the ac input cycle that produces an output current.
In a class A amplifier, collector current flows for the full 360° of the ac input cycle.
In a class B amplifier, collector current flows for only 180° of the ac input cycle.
In a class C amplifier, collector current flows for 120° or less of the ac input cycle.
A class A amplifier has very low distortion and very low power efficiency.
A class A amplifier sees two loads, an ac load and a dc load. Therefore, there are two load lines, a dc load line and an ac load line. The ac load line is always steeper than the dc load line.
In a class A amplifier, maximum transistor power dissipation occurs when there is no input signal applied to the amplifier.
A class B amplifier using a single transistor has very severe distortion and medium power efficiency.
A class B push-pull amplifier uses two transistors to get a linear reproduction of the input waveform being amplified. A class B push-pull amplifier has medium power efficiency.
A common way to bias a class B push-pull amplifier is with diode bias. With diode bias, the diode curves must match the emitter diode curves of the transistors to obtain a stable bias.
The transistors in a class B push-pull amplifier dissipate very little power when no ac signal is present at the input because the idling current in the transistors is quite low.
When large amounts of ac load power are required, a split power supply is often used with class B push-pull amplifiers.
Class C amplifiers cannot be used in audio circuitry because of their high distortion. However, class C amplifiers can be used as tuned rf amplifiers where the undesired harmonic frequencies can be filtered out.
A class C amplifier is more efficient than either a class A or class B amplifier; its power efficiency approaches 100%.
A tuned class C amplifier can be used as a frequency multiplier by tuning the LC tank circuit to a multiple of the input frequency.
