SZIKLAI PAIR? (2)

Enter the Sziklai Pair, sometimes called the complimentary pair for obvious reasons. This was invented by George Sziklai around the same time as the Darlington. It too behaves like a single transistor with a current gain equal to the product of the gains of the two transistors. Similarly, the Sziklai can’t saturate further than VBE for the same reasons as the Darlington. However, the Sziklai pair does not suffer as badly as the Darlington in terms of VBE. It is obvious that the overall VBE is equal just to Q1’s VBE. The Sziklai’s VBE is also more stable with load variations than that of the Darlington. This is because the overall VBE is not impacted by Q2’s collector current but defined only by Q1’s collector current, which does not vary nearly as much. Additionally, Q1 will typically operate over a lower temperature range since it does not see the same level of current as Q2, further improving VBE stability. The Sziklai pair therefore has a big advantage over the Darlington in terms of both the level and the stability of its VBE. This effect is often exploited in the push-pull output stages of Class AB audio amplifiers where the lower and more stable VBE makes biasing much easier.