Audio Engineering

Fundamentals and Instruments 877

stabilizing circuit, such as the simple back-to-back connected silicon diode peak limiter
circuit shown in Figure 30.3 , would be preferable, despite its relatively modest (0.5% at
1 kHz) performance in respect of waveform distortion.

Rather greater control of the output signal amplitude can be obtained by more elaborate
systems, such as the circuit shown in Figure 30.4. In this circuit, the output sinewave is
fed to a high-input impedance rectifi er system ( A 2 / D 1 / D 2 ), and the DC voltage generated
by this is applied to the gate of an FET used as a voltage-controlled resistor. The values
chosen forR 6 / R 7 and C 3 / C 4 / C 5 determine the stabilization time constant and the output
signal amplitude is controlled by the ratio ofR 8 : R 9. In operation, the values of R 4 and R 3
are chosen so that the circuit will oscillate continuously with the FET ( Q 1 ) in zero-bias
conducting mode. Then, as the –ve bias on the Q 1 gate increases as a result of the rectifi er
action ofQ 1 / Q 2 , the amplitude of oscillation will decrease until an equilibrium output
voltage level is reached.

In commercial instruments, a high-quality small-power amplifi er would normally be
interposed between the output of the oscillator circuit and the output take-off point to





C 2

C 1 R 1

A 1

Op. amp.

R 2

R 4

R 3

1K0

R 5

D 1

D 2

X

Y

0V

Output sinewave

Figure 30.3 : Diode-stabilized oscillator.