Figure 5-8. Direct-coupled Transistor Amplifier
5-40. Direct coupling has several disadvantages. The major problem is the power supply
requirements for direct-coupling amplifiers. Each succeeding stage requires higher voltage.
The load and voltage divider resistors use a large amount of power and the biasing can
become very complicated. It is also difficult to match the impedance from stage to stage
with direct coupling. Impedance matching is covered a little later in this chapter. The
direct-coupled amplifier is also not very efficient and the losses increase as the number of
stages increase. Because of the disadvantages, direct coupling is not used very often.
5-41. The most commonly used coupling in amplifiers is RC coupling. Figure 5-9 shows
an RC-coupling network. The network of R1, R2, and C1 enclosed in the dashed lines of
the figure is the coupling network. You may notice that the circuitry for Q1 and Q2 is
incomplete. That is intentional so that you can concentrate on the coupling network.
Figure 5-9. RC-coupled Transistor Amplifier
23 June 2005