Push-Pull Operation

Author: Leonard Krugman

Whenever possible, transistor power amplifiers should be operated as push-pull stages. Push-pull operation has several desirable features, including the elimination of the even-order harmonics and the d-c component in the load. The first factor is particularly fortunate, insofar as transistor applications are concerned. It was noted previously that operation at high values of collector current introduces a distortion due to crowding of the collector current-voltage lines. Thus, for a given value of allowable distortion, push-pull operation will allow the transistors to be driven into the higher I_c regions. In turn, each transistor delivers more power to the load than when it is connected for single-ended operation.

The operating point, load, and biasing resistors for the Class A push-pull stage are determined for each transistor exactly as if it were a single-ended type. A typical push-pull transistor amplifier is illustrated in Fig. 5-11, based on the same transistor characteristics used previously. The separate biasing arrangement indicated in this illustration permits a more exact match of the transistor characteristics. Notice that the load is twice the value computed for the single-ended stage.

Fig. 5-11. Class A push-pull amplifier.