Figure 1-9.Direct-coupled transistor amplifiers.
Notice that the output (collector) of Q1 is connected directly to the input (base) of Q2. The network
of R4, R5, and R6 is a voltage divider used to provide the bias and operating voltages for Q1 and Q2. The
entire circuit provides two stages of amplification.
Direct coupling provides a good frequency response since no frequency-sensitive components
(inductors and capacitors) are used. The frequency response of a circuit using direct coupling is affected
only by the amplifying device itself.
Direct coupling has several disadvantages, however. The major problem is the power supply
requirements for direct-coupled amplifiers. Each succeeding stage requires a higher voltage. The load and
voltage divider resistors use a large amount of power and the biasing can become very complicated. In
addition, it is 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 not very efficient and the losses increase as the number of stages
increase. Because of the disadvantages, direct coupling is not used very often.
The most commonly used coupling in amplifiers is RC coupling. An RC-coupling network is shown
in figure 1-10.