Sziklai pair

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In electronics, the Sziklai pair (also known as a "compound transistor") is a configuration of two bipolar transistors, similar to a Darlington pair. In contrast to the Darlington arrangement, the Sziklai pair has one NPN and one PNP transistor. Hence, it is sometimes called the "complementary Darlington". Current gain is similar to that of a Darlington pair, being the product of the gains of the two transistors. The configuration is named for its inventor George C. Sziklai.

One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6V (compared to about 1.2V). Like the Darlington, it can saturate only to 0.6V, which is a drawback for high-power stages.

Sziklai pairs are sometimes used in the output stage of power amplifiers (for audio etc) where the designer wants to use devices of the same type (e.g., NPN), instead of complementary types (which rarely match accurately). In such designs, half the output pair is a Darlington configuration, the other the Sziklai connection. This is sometimes called the "quasi-complementary" configuration. It can simplify biasing and thermal stability issues. Audio power amplifiers also often have Sziklai pairs built with fully complementary devices. Some claim [1] that these give a better sound than Darlington designs.

As with a Darlington pair, a resistor (100Ω-1kΩ) is usually connected between Q2 emitter and base, to improve its turn-off time and hence performance at high frequencies.

[edit] References

• U.S. Patent 2,762,870  Push-pull complementary type transistor amplifier. G.C.Sziklai, September 11, 1956
• U.S. Patent 2,791,644  Push-pull amplifier with complementary type transistors. G.C.Sziklai, May 7, 1957

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