This article presents the basic analysis and design equations of a 2-transistor configuration using both polarities of transistors: the “complementary” two-BJT configuration. Transistor combinations can result in better performance than single transistors. Darlington and cascode 2-BJT or 2-FET configurations are commonly used because of their inherent advantages. The Darlington configuration has higher input resistance than a single common-collector stage. Cascode amplifiers have higher output resistance and a minimization of the Miller effect of base-collector capacitance.
The Darlington configuration can be viewed as a three-terminal device and compared to a single transistor. A commonly-used circuit in the output stages of power amplifiers is the “complementary” two-BJT configuration, shown below. It too is a three-terminal device and can be analyzed as though it were a single equivalent transistor.
NPN BJTs are preferred power devices just as n-channel MOSFETs are. The complementary PNP configuration is, like the Darlington, a two-transistor combination that has three terminals like a single BJT. Consequently, the complementary PNP behaves like a PNP for the input loop while using a NPN to implement the output loop.
The base of the equivalent PNP is the base of Q1, the Q1 emitter and Q2 collector node is the equivalent emitter, and the Q2 emitter is the equivalent collector. Apply KCL at the equivalent emitter to express the currents in IE2:
The first term,
This can be rearranged to become
where the second term,
An equivalent β, or β’ is then:
can also be solved for;
Thus, the formulas for β and α
are consistent with each other relative to a single BJT. We consequently have some assurance in regarding the complementary PNP as having at least quasistatic functional equivalency to a single PNP with the designated terminals corresponding to a single PNP.
Dynamically, the situation is somewhat more complicated in that the two BJTs form a tight feedback loop with a loop gain dependent upon external impedances. If the PNP is a smaller, faster transistor than the more powerful NPN, then the NPN places a dominant pole in the loop and this usually stabilizes the pair.