The output stage in Figure 8.34 is a class-AB configuration composed of the complementary
transistor pair Q3 and Q4. Resistors R1 and R2 and the emitter-follower
transistors Q1 and Q2 establish the quiescent bias required in this configuration.
Resistors R3 and R4, used in conjunction with short-circuit protection devices not
shown in the figure, also provide thermal stability for the output transistors. The
input signal vI may be the output of a low-power amplifier. Also, since this is an
emitter follower, the output voltage is approximately equal to the input voltage.