Sziklai Pairs

Quick Answer

A Sziklai pair is two complementary transistors — one NPN, one PNP — wired so the second device's collector current feeds back into the first's. Its composite gain is roughly the product of the two, it behaves like a single transistor of the input device's polarity, and it turns on with one base-emitter drop rather than a Darlington's two.

The arrangement is sometimes called a complementary feedback pair, which describes it better. The input device's collector current becomes the second device's base current, and the second device's much larger collector current is returned to the same output terminal — so a small base current at the input produces a large current at the output, with a composite gain near the product of the two individual gains, exactly as a Darlington achieves. What differs is the drive requirement. A Darlington stacks two base-emitter junctions in series and needs about 1.4 V to turn on; a Sziklai pair puts only the input device's junction in the drive path, so about 0.7 V is enough. The saturation voltage is lower too, because the output device's collector-emitter voltage is not propped up by a preceding emitter.

The place this matters most is a push-pull output stage. Building both halves from Darlingtons means the upper and lower composites behave differently — their turn-on voltages and thermal characteristics belong to devices of opposite polarity, and matching them is awkward. Building both halves from Sziklai pairs puts the same polarity of output device in both halves, with a small complementary driver setting the polarity of each composite, so the two halves match far better than they otherwise would. That symmetry is the usual reason to reach for it. Against that: the internal feedback loop makes the pair more inclined to oscillate than a Darlington, so a small base-emitter resistor across the output device and attention to layout are part of the standard recipe rather than optional extras.