In electronics, the Sziklai pair (also known as a "complementary feedback pair" (CFP) or "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, and so it is sometimes also called the "complementary Darlington". Current gain is similar to that of a Darlington pair, which is the product of the gains of the two transistors. The configuration is named for its early popularizer, George C. Sziklai.
One advantage over the Darlington pair is that the base turn-on voltage is only about 0.6V or half of the Darlington's 1.2V nominal turn-on voltage. Like the Darlington, it can saturate only to 0.6V, which is a drawback for high-power stages.
Sziklai-based output stages
Sziklai pairs are sometimes used in the push–pull output stage of power amplifiers (e.g., for audio) when the designer wants to use devices of the same type (e.g., NPN), instead of complementary types, which rarely match accurately. That is, rather than using a Darlington NPN push pair (i.e., two NPN transistors) and a Darlington PNP pull pair (i.e., two PNP transistors), the designer uses Sziklai pairs for both the upper push pair and the lower pull pair. By using two Darlington pairs, the designer is hoping that the combination of two NPN transistors will have similar characteristics of two PNP transistors. By using two Sziklai pairs which both have mixed NPN/PNP type, the intent is to improve push–pull matching.
Designers also sometimes use a "quasi-complementary" configuration, which uses a Darlington push pair (i.e., two NPN transistors) and a Sziklai pull pair (i.e., one PNP and one NPN transistor). This configuration, which uses three NPN transistors and one PNP transistor, is advantageous because:
- Silicon PNP transistors have historically been more expensive than their NPN counterparts (one factor is lower volume production and usage of PNP power transistors).
- The performance of the lower pull pair, which uses a single NPN transistor, more closely matches the performance of the upper push pair, which consists of two NPN transistors (PNP transistors have lower carrier mobility).
Recently, PNP and NPN transistors have become roughly equally available and have more closely matched performance characteristics, and so modern audio power amplifiers often use equivalent topologies for both pairs (e.g., both Sziklai pairs as described above). Some claim that using two Sziklai pairs gives a better sound than Darlington-based designs.
- 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
- ECE 327: Procedures for Output Filtering Lab — Section 4 ("Power Amplifier") discusses design of a BJT-Sziklai-pair-based class-AB current driver in detail.