Audio Engineering

288 Chapter 9

R 4 will then determine the emitter and collector currents, and the operating conditions
so provided will hold good for almost any broadly similar device used in this position.
Since the emitter resistor would cause a signifi cant reduction in stage gain, as seen in the
equivalent analysis of valve cathode bias systems, it is customary to bypass this resistor
with a capacitor, C 2 , which is chosen to have an impedance low in relation to R 4 and R 3.

9.3 Stage Gain .............................................................................................................

The stage gain of a BJT, used as a simple amplifi er, can be determined from the
relationship:

V

V

hR

Rr

out

in

fe L

Si



+

where Rs is the source resistance, RL is the collector load resistor, hfe is the small-signal
(AC) current gain, and ri is the internal emitter-base resistance of the transistor. An
alternative and somewhat simpler approach is similar to that used for a pentode valve gain
stage in which

VV gRout in m L

where the gm of a typical modern planar epitaxial silicon transistor will be in the range of
25–40 mS/mA of collector current. Because the gm of the junction transistor is so high, high
stage gains can be obtained with a relatively low value of load resistor. For example, a small-
signal transistor with a supply voltage of 15 V, a 4 k7 collector load resistor, and a collector
current of 2 mA will have a low frequency stage gain, for a relatively low source resistance,
of some 300 ̆. If some way can be found for increasing the load impedance, without also
increasing the voltage drop across the load, very high gains indeed can be achieved—up to
2500 with a junction FET acting as a high impedance constant current load.^1

A predictable, but interesting aspect of stage gain is that the higher the gain, which can be
obtained from a circuit module, the lower the distortion in this which will be due to the
input device. This is so because if increasingly small segments are taken from any curve,
they will progressively approach more closely to a straight line in their form. This allows
a very low THD fi gure, much less than 0.01% at 2 V rms output, over the frequency range
10 Hz–20 kHz, to be obtained from the simple NPN/PNP feedback pair shown in Figure 9.5 ,
which would have an open loop gain of several thousand. The distortion contributed by