Quasi Complementary Output Stage:

The Quasi Complementary Output Stage was originally developed because complementary high-power transistors were not readily available. Despite the fact that such transistors are now available, the Quasi Complementary Output Stage circuit is still widely used.

Consider the arrangement in Fig. 18-26. Q₃ is a high-power npn transistor, and Q₅ is a low-power pnp device. When Q₅ base current (I_B5) flows, the collector current of Q₅ behaves (largely) as base current (I_B3) for transistor Q₃. This produces a Q₃ collector current flow (I_C3), which combines with I_E5 to constitute a current flow in the load. Because I_C3 ≫ I_E5, the output current can be taken to be approximately I_C3;

This is the same as the current gain with Darlington-connected transistors.

Now note that when a negative-going input voltage is applied to the base of Q₅ a negative-going output occurs, because the emitter of Q₅ (and the collector of Q₃) follow the input voltage. Thus, the combination of transistors Q₃ and Q₅ in Fig. 18-26 behave as a high-power pnp emitter follower, just like Darlington-connected transistors Q₃ and Q₅.

Because transistors Q₂ and Q₃ in Fig. 18-26 are both npn devices, they can be the same type of transistor. This eliminates any problem with finding suitable complementary high-power transistors.

Resistor R₉ In Fig. 18-26 ensures that Q₃ is biased off when Q₅ goes into cutoff.