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.

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.

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