The Quasi-complementary output

The c200.1 is similar to the c200 except for the output stage. Notice that all the output transistors are NPN types. This output configuration is known as "Quasi-Complementary".

Why Quasi-complementary?

Quasi-complementary outputs came about in the early years of solid state designs where pnp transistors were few, costly and don't quite complement their npn counterparts. Nowadays, with modern complementary transistors sharing similar properties, this output topology is not often favored. One reason is probably due to the lack of performance of the early designs that left a bad impression. Nonetheless, quasi-complementary outputs do have their advantages, the most obvious being that all output transistors are identical, unlike npn and pnp, which in reality is never truly the same.

Implementing Quasi-complementary

The key is in Q8. Sometimes called a "complementary phase splitter", it is actually more of a level shifter. Together with the output transistors, they form a Sziklai pair.

Performance Factor

Is the Quasi-complementary inferior to the more common Emitter-follower configuration? I don't think so, at least not in THD terms. In amplifier stability, it is just as stable as any well designed amplifier with EF outputs, unlike the Compound Feedback Pairs, which can be tricky to make stable at times. Clip characteristics are similar to c200 (EF version), with no signs of distortion and instability when driving 8 and 4 ohms test loads up to rated power. Sonically, I am unable to tell the difference between the two. In short, it is just as good and reliable as an EF design. Actually, the thought of having all outputs identical is a very attractive proposition.

THD of c200.1 quasi-complementary output

Biasing of output transistors

All THD readings were done with outputs biased to 12mV across 0.22 ohms emitter resistor. This works out to approximately 55mA per output transistor in idling state.


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Sziklai Outputs

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