AMPLIFIER CLASSES OF OPERATION
5-22. The class of operation of an amplifier is determined by the amount of time (in
relation to the input signal) that current flows in the output circuit. This is a function of the
operating point of the amplifying device. The operating point of the amplifying device is
determined by the bias applied to the device. There are four classes of operation for an
amplifier. These four classes are A, B, AB, and C. Each class of operation has certain uses
and characteristics. No one class of operation is "better" than any other class. The selection
of the "best" class of operation is determined by the use of the amplifying circuit.
Class A Operation
5-23. Figure 5-3 shows the class A operation of a simple transistor amplifier. Since the
output signal is a 100 percent (or 360) copy of the input signal, current in the output
circuit must flow for 100 percent of the input signal time. This is the definition of a class A
amplifier. Amplifier current flows for 100 percent of the input signal.
Figure 5-3. Simple Class A Transistor Amplifier
5-24. The class A amplifier has the characteristics of good fidelity and low efficiency.
Fidelity means that the output signal is just like the input signal in all respects except
amplitude. It has the same shape and frequency. In some cases, there may be a phase
difference between the input and output signal (usually 180), but the signals are still
considered to be "good copies". The signal is said to be distorted if the output signal is not
like the input signal in shape or frequency. Distortion is any undesired change in a signal
from input to output.
23 June 2005