Sziklai Pair - Sziklai-based Output 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.