Most of us have seen the classic Darlington Pair in which two transistors of the same type are connected as shown in Figure 1. It was invented by Sydney Darlington and patented by Bell Labs in 1953. This arrangement behaves like a single transistor with a current gain given by the product of the gains of the two transistors. Gains of several thousand are easily achieved—really useful when driving high-powered loads such as in audio amplifiers and the like.
As usual with electronics, there is a price to pay for this extra gain. Firstly, the Darlington transistor cannot fully saturate further that one VBE drop (about 0.6V) no matter how hard we drive it. This is because Q1’s emitter must always be one VBE above Q2’s emitter (or below in the case of the PNP arrangement), and even if Q1 could saturate to zero volts, the Q2’s collector can never fall below this.
Secondly, the base-emitter voltage of the Darlington will be the sum of Q1 and Q2’s VBE or around 1.2V. Further this VBE will change significantly as the collector currents of the two transistors changes. The Ebers-Moll model tells us that VBE changes by about 60mV per decade change in IC. The Ebers-Moll impact will be most significant in Q2’s VBE as it varies from virtually zero to the full load current.
Enter the Sziklai Pair as shown in Figure 2, 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.
Let’s see how this works in practice. Figure 3 is a hypothetical example. Most of the analysis is identical for the Darlington & Sziklai cases. We want an output current that varies over the range of 10mA to 10A and have transistors with current gains of 100 and base resistors of 50W. The 1000:1 current range in Q2 gives an approximate change in its VBE of 180mV at room temperature (it would be way worse if we took temperature into account). In each case we will see approximately 0.6V across the base resistor, which means about 12mA will flow through it. Q2’s base current will vary over the range 10mA to 10mA, so Q1’s collector current will be this plus the current through the base resistor, or 12-22mA which is a 1.8:1 range. Q1’s VBE will therefore only vary by about 15mV over the full current range.
In the case of the Darlington, both Q1 and Q2’s VBE are in play so the overall VBE will change by 195mV over the 10mA to 10A range. In the case of the Sziklai, only Q1’s VBE matters, with a variation of only 15mV over the same load range—more than an order of magnitude better than the Darlington.
“George Clifford Sziklai.” In Wikipedia, June 19, 2019. https://en.wikipedia.org/w/index.php?title=George_Clifford_Sziklai&oldid=902521130
Horowitz, Paul. The Art of Electronics. Third edition. New York, NY: Cambridge University Press, 2015.
Horowitz, Paul, Winfield Hill, and Paul Horowitz. The Art of Electronics: The x-Chapters. Cambridge ; New York, NY: Cambridge University Press, 2020.
“Sidney Darlington.” In Wikipedia, October 2, 2020. https://en.wikipedia.org/w/index.php?title=Sidney_Darlington&oldid=981403895
“Complementary Feedback Pair.” In Wikipedia, August 30, 2020. https://en.wikipedia.org/w/index.php?title=Complementary_feedback_pair&oldid=975801447