Sine Wave Oscillator Circuit

To maintain oscillation the op. amp. must have a gain equal or exceeding
this attentuation which is in fact x3. The desired gain is obtained by
selecting the ratio of feedback resistance to input resistance of the
inverting input (RV2 + R3)/R2.
lf the overall gain, including feedback, exceeds
unity the circuit will produce sine wave oscillation at a frequency set
by the Wien network. Stabilisation of the gain is brought about by the
action of diodes D1 and Dl% then  the instantaneous output voltage is
close to zero, neither diode conducts, since even a germanium diode
requires 0.4 volts or so forward voltage to bias it on. Consequently,
the negative feedback loop is open (giving maximum gain) and, under the
action of the positive feedback via the Wien network, oscillations build
up rapidly. As soon as their amplitude is  sufficient to bias on either
Dl or D2 (depending on the polarity of the output voltage swing), then
R2, R3 and RV2 provide negative feedback, so limiting oscillations to a
convenient level. Reinforcement of such oscillations takes place close
to each zero crossing when D1 and D2 are open ire. non·conducting; the
setting of RV2 determines the final amplitude. This method of
stabilisation does give rise to a very small amount of crossover
distortion, but the effect of this can be minimised by setting VR2 for
the largest possible sine wave without clipping. ln any event, some
distortion is a small price to pay for such a simple,
easy-to~get—working sine wave oscillator and, further, it is a low level
of distortion — some class B audio amplifiers are worse! Range
switching is confined to a choice of two ranges, in the interest of
simplicity and cheapness, but more ranges could easily be provided if
the  constructor is so inclined. A simple emittentouower output stage
completes the unit, with a logarithmic potentiometer as a level control,
enabling the output to be set from 1 V rms down to 10 mV rms or so.
With wiring up completed and thoroughly checked, switch on and, if
possible, monitor the output on an oscilloscope. No ’scope? Then a pair
of headphones, of reasonably high impedance, can be used instead. Set
RV4 about half way, S1 to “low” and RVI about half way. lf there is no
output, adjust RV2; clockwise rotation should give increased output.
With an ac meter, measure the signal level at the junction of D1, D2 and
RV2. Adjust RV2 for 3 volts rms. This will ensure the highest output
level (thus reducing the effect of crossover distortion) consistent with
sine wave operation (no`clipping). This should provide about one volt
rms at the output.