The operating point also determines the operating class of an amplifier.
In a Class A amplifier, the grid bias is set so that the vacuum tube conducts and amplifies the entire input signal. For instance, if the operating point is set in correspondence of the red spot in Figure 8, the obtained amplifier operates in class A, since it can amplify the entire input signal. The plot at the top of Figure 10 shows a signal amplified by a Class A amplifier. In a Class A amplifier, significant current traverses the vacuum tube also in the quiescent state, that is when no input signal is provided to the grid.
A Class B amplifier amplifies just half input signal. Suppose the bias is set so that it is very close to the intersection of the green loadline and the horizontal axis in Figure 8. In this case, just the upper part of the signal is amplified, and the vacuum tube quits conducting in correspondence of the remaining part of the signal. The plot in the middle of Figure 10, shows a signal amplified by a Class B amplifier. Class B amplifiers are more efficient, since there is no current traversing the vacuum tube when there is no input signal.
Class AB amplifiers are a compromise between class A and Class B. The bias is set so that most of, but not all, the signal is amplified. Suppose the operating point is set in correspondence of the violet spot in Figure 8. In this case, the upper part of the signal is entirely amplified. The lower part of the signal is amplified until the intersection of the green loadline and the horizontal axis is reached. At this point the vacuum tube quits conducting, so part of the output signal is missing. Lower plot in Figure 10, depicts a signal amplified by a Class AB amplifier. Class AB amplifiers are more efficient than Class A amplifiers and less than Class B amplifiers, since just a small current traverses the tube when no signal is amplified.