2-23
with the class A amplifier. It operates for 360 degrees of the input signal and requires a relatively large
input from the power supply. Even with no input signal, the class A amplifier still uses power from the
power supply. Therefore, the output from the class A amplifier is relatively small compared to the total
input power. This results in low efficiency, which is acceptable in class A amplifiers because they are
used where efficiency is not as important as fidelity.
Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation of
the input, which results in less total input power than the class A amplifier. This leads to better efficiency.
Class B amplifiers are biased with little or no collector current at the dc operating point. With no
input signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higher still.
The efficiency of class C is the highest of the four classes of amplifier operations.
Q22. What amplifier class of operation allows collector current to flow during the complete cycle of the
input?
Q23. What is the name of the term used to describe the condition in a transistor when the emitter-base
junction has zero bias or is reverse biased and there is no collector current?
Q24. What two primary items determine the class of operation of an amplifier?
Q25. What amplifier class of operation is the most inefficient but has the least distortion?
TRANSISTOR CONFIGURATIONS
A transistor may be connected in any one of three basic configurations (fig. 2-16): common emitter
(CE), common base (CB), and common collector (CC). The term common is used to denote the element
that is common to both input and output circuits. Because the common element is often grounded, these
configurations are frequently referred to as grounded emitter, grounded base, and grounded collector.
