Audio Engineering

Audio Amplifi ers 287

Figure 9.4 : Biasing circuits.

R 2

R 1

Vb

Vcc

C 1

Q 1

0V

Output

Input

(a)

R 2

R 1

Vcc

C 1

Q 1

0V

Output

Input

(b)

(c)

C 1

Input Q 1

R (^2) R
4
R 3
R 1
C 2
Vcc
0V 0V
Output
The method almost invariably used in competently designed circuitry is that shown in
Figure 9.4(c) , or some equivalent layout. In this, a potential divider ( R 1 , R 2 ) having an
output impedance low in relation to the base impedance of Q 1 is used to provide a fi xed
DC base potential. Since the emitter will, by emitter–follower action, sit at a potential,
depending on emitter current, which is about 0.6 V below that of the base, the value of