Audio Engineering

Audio Amplifi ers 293

sit at a quiescent potential about 0.6 V more negative than that of the base, and this will
follow, quite accurately, any signal voltage excursions applied to the base. (There are
some caveats in respect of capacitative loads; these potential problems will be explored
under the heading of slew rate limiting.) The output impedance of this circuit is low
because this is approximately equal to 1/ gm , and the gm of a typical small-signal, silicon
BJT is of the order of 35 mA/V (35 mS) per mA of emitter current. So, if Q 1 is operated at
5 mA, the expected output impedance, at low frequencies, will be 1/(5.35) kilohms, or
5.7 ohms, a value that is suffi ciently smaller than any likely value for R 1 , that the presence
of this resistor will not greatly affect the output impedance of the circuit.

The output impedance of a simple emitter–follower can be reduced still further by the
circuit elaboration shown in Figure 9.10 , known as a compound emitter–follower. In
this, the output impedance is lowered in proportion to the effective current gain of Q 2 in
that, by analogy with the output impedance of an operational amplifi er with overall NFB,
any change in the potential of the Q 1 emitter, brought about by an externally impressed
voltage, will result in an opposing change in the collector current of Q 2. This layout
gives a comparable result to that of the Darlington pair, of two transistors, in cascade,
connected as emitter–followers, shown in Figure 9.11 , except that the arrangement of
Figure 9.10 will only have an input/output DC offset equivalent to a single emitter-base

0V 0V

Output

R 2

Q 1

R 1

Q 2

Input

Vcc

Figure 9.10 : Compound emitter–follower.