Darlington Connected Output Transistors:

High-power transistors usually have low current gains, so relatively large base currents must flow into Q₂ and Q₃ to supply a high load current. This means that the quiescent current through Q₁ must be large, and consequently resistor R_C is small. The small value of R_C keeps the amplifier voltage gain low. To improve on this situation, Darlington Connected Output Transistors may be used, as illustrated in Fig. 18-24(a).

Transistors Q₄ and Q₅ in Fig. 18-24(a) are low-power devices that supply base current to output transistors Q₂ and Q₃, respectively. Note the four biasing diodes in Fig. 18-24(a) to bias the four transistor base-emitter junctions.

When peak load current (I_p) flows, the peak base current to Q₄ and Q₅ is,

This reduced base current allows I_RC(min) to be smaller, giving a larger resistance for R_C, and resulting in a larger voltage gain.

Resistors R₈ and R₉ in Fig. 18-24(a) are included to bias Q₂ and Q₃ off when Q₄ and Q₅ are in cutoff. The largest possible resistance values should normally be selected for R₈ and R₉. When Q₂ and Q₃ are off,  the collector-base leakage current I_CBO flows in R₈ and R₉, [see Fig. 18-24(b)],

The voltage drop across the resistors (I_CBO R₈) should be much smaller than the normal transistor base-emitter voltage. Selecting I_CBO R₈ equal to 0.01 V normally gives satisfactory resistor values. R₈ and R₉ are not required when power Darlington Connected Output Transistors are used.