Push-Pull Circuitry

Author: Leonard Krugman

Two Class B amplifiers connected as a push-pull stage, using two of the circuits illustrated in Fig. 5-12 (A), will not operate. One transistor will always be biased in the reverse direction by the input signal, thereby causing its input resistance to become very high. This condition can be eliminated by using a center-tapped input transformer and connecting the center tap to the common emitter electrodes. This circuit is characterized by a distorted output wave. The distortion is particularly evident when the signal generator resistance is low. However, the distortion can be reduced within limits by introducing base bias into the circuit.

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

Figure 5-12 (B) illustrates one possible form of this latter arrangement. The value of the base bias resistor R_F for minimum cross-over distortion can be determined by the conventional graphic methods of vacuum-tube Class B push-pull amplifiers when using the composite transistor characteristics. The proper bias setting may be determined experimentally by direct measurement with an oscilloscope or a distortion meter. If the experimental method is used, care must be taken to avoid setting the base bias too high. This would cause a relatively high quiescent d-c collector current to flow, and the circuit would perform in a manner similar to that of a Class AB amplifier in vacuum-tube circuits. Resistor R_c may be a thermistor or some other temperature sensitive device. R_c is usually required in stages, subject to large changes in temperature to prevent excessive variation in the collector d-c operating point.

Fig. 5-13. (A) Class & push-pull operation without input transformer. (b) Output waveforms.

Another arrangement for a transistor push-pull Class B stage is illustrated in Fig. 5-13(A). This circuit permits the elimination of the input transformers. The diodes D₁ and D₂ prevent each transistor from cutting off when it is biased in the negative (reverse bias) direction by the input signal, since the diodes effectively short out the signal-induced bias. The point at which this bypass action occurs is determined by the bias due to resistors R_F and R_c. These resistors also furnish base bias to the transistors to minimize cross-over distortion. Figure 5-13 (B) illustrates the effect of diodes and bias resistors on distortion of the output signal.

The detailed operating characteristics of a Class B transistor push-pull amplifier are determined by the same methods used in similar vacuum tube circuits. The approximate values of the major characteristics can be calculated as illustrated in the following example: Assume that the transistors to be used in the Class B push-pull circuit have a maximum collector dissipation rating of 100 milliwatts, and assume that a battery E_bb = 10 volts is specified. The collector dissipation P_c in each transistor is approximately where I_pc is the peak collector current.

Then

The required load for maximum power output is:

and the power output is approximately = 400 milliwatts, or four times the maximum collector dissipation of each transistor.