Push-Pull Amplifiers

Author: J.B. Hoag

A circuit diagram of a push-pull amplifier is shown in Fig. 13 0.

Fig. 13 O. A push-pull amplifier

This type of circuit is able to deliver larger outputs with less distortion than single-tube amplifiers. Assume that a sine wave signal is applied to the input terminals. During the first half-cycle, the top of the secondary of transformer T_t becomes positive and the bottom becomes negative. Then the grid of tube 1 becomes positive and the grid of tube 2 becomes negative. The plate current of tube 1 increases and that of tube 2 decreases. But the current increase in P₁ is up and that in P₂ is down. If an increase in plate current through P₁ makes the output positive at the top, then a decrease in current through P₂ will also make the output positive at the top. Thus, the outputs of tubes 1 and 2 add together.

It is easy to see that each tube need handle only one-half the secondary voltage of T₁. In other words, greater voltages may be applied to this circuit than to a single-tube circuit before serious overloading of the tubes and distortion begins.

There is some curvature, even in the " straight" part of a tube's curve, which causes distortion, i.e., harmonics. It can be shown that the push-pull circuit cancels all even-numbered distortion harmonics. Only the odd harmonics come out, i.e., the fundamental, the third, the fifth, etc.

If the upper and lower circuits and tubes of a push-pull amplifier are matched with each other, any change in the A-, B-, or C-battery voltages will cause equal but reversed changes in the plate currents in the upper and lower halves of the output transformer, and hence will not appear in the output. In other words, the circuit is notably stable against changing battery voltages.