Sine Wave Generator
Circuit : Andy Collinson
Email: anc@mitedu.freeserve.co.uk
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:
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 serail 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.
