Audio Engineering

290 Chapter 9

The cascode layout is also used very widely as a voltage amplifi er stage, using a circuit
arrangement of the kind shown in Figure 9.7(a). As in the case of the valve amplifi er
stage, this circuit gives very good input/output isolation and an excellent HF performance
due to its freedom from capacitative feedback from output to input. It can also be
rearranged, as shown in Figure 9.7(b) , so that the input stage acts as an emitter–follower,
which gives a very high input impedance.

The long-tailed pair layout, shown in its simplest form in Figure 9.8(a) , gives a very
good input/output isolation; also, because it is of its nature a push–pull layout, it gives
a measure of reduction in even-order harmonic distortion. Its principal advantage, and
the reason why this layout is normally used, is that it allows, if the tail resistor ( R 1 ) is
returned to a –ve supply rail, both of the input signal ports to be referenced to the 0-V
line—a feature that is enormously valuable in DC amplifying systems. The designer
may sometimes seek to improve the performance of the circuit block by using a high
impedance (active) tail in place of a simple resistor, as shown in Figure 9.8(b). This will
lessen the likelihood of unwanted signal breakthrough from the –ve supply rail, as well
as ensuring a greater degree of dynamic balance, and signal transfer, between the two
halves.

Although like all solid-state amplifying systems it is free from the bugbears of hum and
noise intrusion from the heater supply of a valve amplifying stage—likely in any valve

R 3 R 5

Vcc

Output

Q 2

C 1

R 2

C 3

R 6

0V 0V

Input

Figure 9.6 : Grounded base stage.