AM Pocket Radio -- Audio Output Stage

The audio output stage of an early pocket radio usually consisted of a pair of audio transformers, which you might remember seeing in the photographs in the previous pages of this multi-page article. Here is a circuit diagram extract of a typical class A push-pull design found in vintage radios. Back then, only PNP transistors were readily available and therefore all the transistors were operated with common emitters. In this type of configuration, we get greater audio output compared to single-ended designs with one transistor. However a disadvantage of this design is that the average collector current flow is steady whether or not an input signal is present.

Class B Push-Pull

As you can imagine, power consumption is an important factor for battery operated portable devices, and therefore designers swiftly moved on to the class B push-pull output stage. If you remember your analogue electronics class, then you might remember that in a class B push-pull stage, the efficiency is usually closer to 75 %. This is because the stage is biased near cut-off, and the collector current is extremely low when there is no input signal.

The audio driver stage has one transistor energising the primary of the first transformer, which provides coupling and impedance matching to the final class B audio output stage. This transformer also does phase splitting; hence each end of the coil, on the secondary side, produces signals 180-degrees out of phase with each other. These signals then drive the final audio output transistors. Due to its greater efficiency, this type of class-b push-pull stage became a very typical feature in pocket radios.

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