Description:
A classic Wien Bridge oscillator using an Op-Amp covering a
frequency range of 15 to 150kHz in four switched steps.
Notes:
Two conditions exist for a sinusoidal oscillator.
Regenerative or positive feedback, and a closed loop gain of
unity. The losses in the wien feedback circuit, are such
that the open loop gain of the amplifier must also exceed
29. The following link also provides the wien oscillator
theory:
Wien Oscillator Theory
In this circuit the gain is provided by a FET type op-amp. I
have used an LF351, which may be hard to obtain, but the
TL071CN or TL081CN may be used and have a faster slewing
rate than the LF351. The
Maplin
order codes are RA67X and RA70M respectively. The wien
network is a parallel combination of resistor and capacitor,
in series with a serial R-C network. Regenerative feedback
is applied from the op-amp output, to the serial R-C input
and continues. Stabilization is required to prevent the
otherwise uncontrolled oscillation from building up and
becoming unstable.
Stability and Distortion
There are two common methods of stabilizing a wien type
oscillator. A thermistor with a NTC in the series leg of the
feedback loop or an incandescent lamp (with a positive
temperature coefficient) in the shunt leg of the feedback
loop. The bulb used here is a 6V 60mA type Maplin code
BT99H. A 12 Volt bulb rated 60mA or 40mA will also work. The
feedback arrangement works as follows. As a bulb heats up
its filament resistance increases. This will decrease the
overall gain of the amplifier, as the output signal is fed
back to the input. Similarly, if the output amplitude
decreased the signal appearing at the bulb would be less,
its filament resistance would drop and gain would be
increased. Therefore a stable output amplitude is produced.
The 1k preset is adjusted for minimum distortion. Note that
split supplies are used and a ganged 10k potentiometer
controls frequency with a 10:1 range.
