Quasi Complementary Symmetry-Transistor Output

- circuit and design details for the quasi or pseudo complementary output stage used in many audio amplifiers.

It is possible to use the Compound pair along with a Darlington to provide a high gain output stage.

This form of output configuration is known as a quasi-complementary symmetry output or a pseudo complementary output.

The reason for this is that for a fully complementary output the transistor configurations on each side of the output would need to be the exact mirror of each other. In other words one side would need to be PNP and the other NPN.

In the past, NPN power transistors offered a much better alternative to PNP power transistors and as a result the quasi complementary output was widely used. Both output transistors were the same and often transistors such as the 2N3055 could be used.

Circuit diagram of a quasi complementary output circuit using both Darlington and Sziklai pairs
Quasi Complementary Output

In this quasi complementary symmetry amplifier, the transistor pair formed from Q2 and Q3 is a Darlington, and the transistor pair formed from Q$ and Q5 is a Sziklai or complementary pair.

The diodes provide the correct bias between the two pairs of transistors, accommodating the required number of base emitter junctions. The variable resistor, VR1 enables the correct bias current to be set through the diodes to ensure that crossover distortion is minimised.

Although the circuit has been widely used in the past, quasi complementary symmetry amplifiers are seldom used these days because excellent NPN and PNP pairs are widely available.

By Ian Poole


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