The Push-Pull Output Stage and the Output Transformer

The Push-Pull Output Stage and the Output Transformer

We saw that the Class B stage introduced considerable distortion by half-wave rectifying the input signal. Clearly, this is a disadvantage for a Hi-Fi amplifier, since we require linearity.

Suppose, however, that we had two Class B valves, one fed directly with the input signal, and the other with an inverted signal. During time t ₁ the upper valve conducts, whilst the lower is cut off, and during t ₂, the situation is reversed. See Fig. 6.5.

Fig. 6.5: Summation of Class B signals in output transformer

So far all that we have achieved is to ensure that any one valve is switched on, no matter what the incoming phase. By inverting one output and summing it with the other in the output transformer, we can recreate the shape of the original input waveform. The inversion is performed by reversing the connection of one winding, and is marked on the diagram with + and ? symbols.

Whether achieved by a transformer or by a direct coupled series amplifier, such as a White cathode follower, this form of connection is known as push pull, and is the only way of approximating linearity in a Class B amplifier.

Unsurprisingly, this dissection of the signal and its subsequent restitching is rather less than perfect, and pure Class B is rarely used because of the distortion generated at the crossover region, where one valve takes over from the other.