Audio Engineering

Audio Amplifi ers 291

amplifi er where there is a high impedance between cathode and ground—it is less good
from the point of view of thermal noise than a similar single stage amplifi er, partly
because there is an additional device in the signal line and partly because the gain of a
long-tailed pair layout will only be half that of a comparable single device gain stage.
This arises because if a voltage increment is applied to the base of Q 1 , then the Q 1 emitter
will only rise half of that amount due to the constraint fromQ 2 , which will also see, but in
opposite phase and halved in size, the same voltage increment. This allows, as in the case
of the valve phase splitter, a very close similarity, but in opposite phase, of the output
currents atQ 1 and Q 2 collectors.

9.5 Emitter–Follower Systems ....................................................................................

These are the solid-state equivalent of the valve cathode follower layout, although
offering superior performance and greater versatility. In the simple circuit shown in
Figure 9.9 (the case shown is for an NPN transistor, but a virtually identical circuit, but
with negative supply rails, could be made with a similar PNP transistor), the emitter will

Figure 9.7 : Cascode layouts.

C 1

Q 2

R 2

R 4

C 2

C 3

R 1

R 6

R 3 R 5

0V 0V

(a) Basic NPN/PNP cascode

Input

Output

Vcc

C 1

C 3

R 4

R 2

R 3

R 5

R 1

Q 1

Q 2

C 2

Output

0V 0V

(b) Complementary NPN/PNP cascode

Input

Vcc