Bias in Push-Pull Stages

Author: N.H. Crowhurst

Bias in push-pull stages

Self bias using a resistor in the cathode circuit is found in push-pull stages as well as in "single-ended" stages. When the push-pull stage is operated class AB, the plate currents in the tubes change appreciably during different parts of the waveform. These currents are added in the common cathode resistor, not subtracted as in the output transformer primary. This results in a double-frequency current in the cathode resistor, because of the asymmetrical current waveforms in the tubes.

The usual operating point of the tubes is arranged so that when no signal is passing, the plate current is appreciably less than the maximum signal-current fluctuation in each direction. This current provides a voltage drop across the cathode resistor that establishes a bias that is quite close to cutoff. When signal current flows, this voltage drop increases, increasing the bias, and reducing the gain of the tube as in a single-ended stage. If a capacitor is connected across the cathode resistor, the fluctuation due to signal currents is smoothed out, resulting in an almost steady bias that is always higher than that present when no signal passes. This means that when the current waveform in the tubes falls toward zero (for which the bias should be at the no-signal level), the higher bias provided by the capacitor may cause premature cutoff and crossover distortion.

Bypass capacitor affects bias voltage

Push-pull stage using an unbypassed cathode resistor

Unfortunately, we cannot solve this problem by using a smaller cathode resistor because it would result in too high a current in the tubes when no signal was passing. The only solution is to omit the decoupling capacitor across the cathode resistor. If we wish to use, self bias at all, we must be willing to sacrifice some gain.

A separate bias supply is used to obtain an increased output from the push-pull stage.

For this reason, increased output can often be obtained from a push-pull stage by using a separate fixed-voltage bias supply. This supply is usually a simple rectifier that takes an alternating voltage from a suitable point (such as from a voltage divider connected across the high-voltage secondary of the power transformer) and rectifying it with a single diode. A simple resistor-capacitor combination will provide sufficient smoothing because there is no grid current requirement. This means the capacitors charge up to the peak alternating voltage, and the resistor merely provides additional filtering to prevent the leakage current pulses from being passed on to the grid circuits.