**Darlington Connected Output Transistors:**

High-power transistors usually have low current gains, so relatively large base currents must flow into Q_{2} and Q_{3} to supply a high load current. This means that the quiescent current through Q_{1} 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 Q_{4} and Q_{5} in Fig. 18-24(a) are low-power devices that supply base current to output transistors Q_{2} and Q_{3}, 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 Q_{4} and Q_{5} 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 R_{8} and R_{9} in Fig. 18-24(a) are included to bias Q_{2} and Q_{3} off when Q_{4} and Q_{5} are in cutoff. The largest possible resistance values should normally be selected for R_{8} and R_{9}. When Q_{2} and Q_{3} are off, the collector-base leakage current I_{CBO} flows in R_{8} and R_{9}, [see Fig. 18-24(b)],

The voltage drop across the resistors (I_{CBO} R_{8}) should be much smaller than the normal transistor base-emitter voltage. Selecting I_{CBO} R_{8} equal to 0.01 V normally gives satisfactory resistor values. R_{8} and R_{9} are not required when power Darlington Connected Output Transistors are used.