Audio Engineering

876 Chapter 30

use a high-speed analogue switch, operated by a control signal derived from a frequency
stable oscillator, to feed one or an other of a pair of preset voltages, alternately, to a
suitably fast output buffer stage.

An improved Wien bridge oscillator circuit layout of my own, shown in Figure 30.2
(Wireless World , May 1981, pp. 51–53) in which the gain blocks A 1 and A 2 are connected
as inverting amplifi ers, thereby avoiding “ common mode ” distortion, is capable of a THD
below 0.003% at 1 kHz with the thermistor-controlled amplitude stabilization layout
shown in Figure 30.2 , and about 0.001% when using the improved stabilization layout,
using an LED and a photo-conductive cell, described in the article.

As a general rule the time required (and, since this relates to a number of waveform
cycles, it will be frequency dependent) for an amplitude-stabilized oscillator of this kind
to “ settle ” to a constant output voltage, following some disturbance (such as switching
on, or alteration to its output frequency setting), will increase as the harmonic distortion
level of the circuit is reduced. This characteristic is a nuisance for general purpose use
where the THD level is relatively unimportant. In this case, an alternative output voltage









Op. amp. Op. amp.

U?

Output sinewave

0V

TH1 RA53

IC 1

R 3

R 1

R 2 C 2

C 1

1K0

Figure 30.2 : Improved Wien bridge oscillator.