Sziklai pair
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In electronics, the Sziklai pair is a configuration of two bipolar transistors, similar to a Darlington pair. In contrast to a Darlington pair however, the Sziklai pair are made up of transistors of two kinds, NPN and PNP, which is why this configuration is sometimes called the "complementary Darlington". It is also known as a "compound transistor". Current gains are similar to the Darlington arrangement, being the product of the gains of the individual devices. The configuration is named for engineer George C. Sziklai, who invented it.
One advantage of this arrangement compared to the normal Darlington pair is that the base voltage needs only to be ~0.6V to turn the device on, compared with ~1.2V. Like the Darlington, this configuration can only saturate to 0.6V when fully conducting, which is always a potential drawback, especially for high-power stages.
Sziklai pairs sometimes find an application in the output stages of power amplifiers (for audio, etc) where the designer wishes to use devices of the same type (NPN, say) for the final power transistors, instead of complementary types, which rarely match all that accurately. In such a design, one half of the output power pair would use a normal Darlington configuration, the other the Sziklai connection. This is sometimes called the "quasi-complementary" output configuration. This arrangement can simplify biasing and thermal stability issues. Sziklai pairs built with fully complementary devices are also very often found in audio power amplifier designs. Some claim [1] these give a better sound than normal Darlington designs.
As with a Darlington pair, a resistor between 100Ω and 1kΩ is usually connected between the emitter of Q2 and the collector of Q1 to improve the turn-off time of Q2 - this helps to improve the performance of the pair 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
