Audio Engineering

Fundamentals and Instruments 881

These diffi culties are lessened if the oscillator is only required to operate at a range of
fi xed “ spot ” frequencies, and a further circuit of my own of this kind ( Wireless World ,
July 1979, pp. 64–66) is shown in simplifi ed form in Figure 30.7. The output voltage
stabilization used in this circuit is based on a thermistor/resistor bridge connected across a
transistor, Q 1. The phase of the feedback signal derived from this, and fed to A 1 , changes





R 10

C 5

100 μF

C 2

(Cz)

R 6

(Rz)

R 2

A 470R

1

R 1

RV1

2K2

(Rx)

R 6

C 4

R 7

470R

TH 1

100 μF

C 3

R 3

R 11

R 6

1K

C 1

R 4 (Cx) (Cy)

(Ry)

Q 1

Vbias NPN

0V

VCC

Sinewave output

Figure 30.7 : Level stabilizer circuit.