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12
A
B
c
-tO
SK3
BUl
S2
Fig.
2. Block diagram
C. THE AMPLIFlER
The Wien bridge is coupled to an amplitude-stabilised amplifier.
In the block diagram (Fig. 2) this amplifier is indicated by the
letter B, and consisrs of a differential amplifier which is coupled
to
a common emitter circuit. This circuit in turn is coupled to a
White emitter follower.
The differential amplifier TSl has a high effective input impedance
and its
shunting
dfect upon the bridge is very
small.
Feedback is used in the common emitter circuit TS2, to reduce the
Miller effect
at
high frequencies
.
The White emitter folIower has
a low output impedance, which is necessary for
supplying
the Wien
bridge and the output potentiometer.
The collector of TS 1 is connected to a resistor (R 4),
selected
on
test by which the d.c. voltage conditions can be slightly varied, in
order to compensate for transistor
spreads,
if these cause
excessive
distortion.
In the
amplifier,
both positive feedback
and
negative feedback
are
applied.
The negative feedback prevails at all frequencies, except at the oscil
lation frequency f
o •
At the oscillation frequency, the positive feed
back is maximum and the phase-shift angle equals zero. The output
voltage is kept constant with the aid of thermistor R16, which is
connected in one of the bridge arms. This thermistor regulates
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