Quiescent Point

Author: Leonard Krugman

Fig. 5-12. (A) Class B circuit (constant voltage). (B) Class B push-pull operation.

While the efficiency of a Class A amplifier is good under operating conditions, the collector dissipation is approximately the same whether or not a signal is applied. Its efficiency for intermittent or standby operation is poor.

For standby operation, as in the case of the vacuum tube, Class B operation is preferred, and the operating point of a Class B transistor amplifier should be on the E_c = 0 line. This bias condition, however, would require an extremely high resistance in series with the battery. Thus most of the available supply power would be lost in the series resistor, the only function of which was to convert the voltage source into a current source. As an alternate method, a constant voltage battery is used. This sets the d-c operating point at the collector voltage E_c = E_bb on the I_c = 0 axis. Figure 5-12 (A) shows a typical Class B transistor amplifier with a constant voltage source, using the same transistor as in previous calculations.