need some explanation

Thread Starter


Joined Jul 14, 2004
Hi ,

do you know something about Oscilator?Now i learn it as part of OP-AMP.
If you know how oscilator work i will be really happy.
I am really confused in Wien Bridge Oscilator.

Thanks for your help


Joined Apr 20, 2004

Op amps can make the basis for a good Wien Bridge oscillator. I have a schematic of one I'm using below.

Circuit operation is fairly simple. When cold, the filament of the 7387 lamp will be a low resistance. That means that the gain of the op amp is high - possibly araound 20 (I never checked the cold resistance of the filament). Any disturbance of the output tends to be magnified by the voltage gain, and result in a significant voltage excursion.

However, current passing through R1 and then the 7387 filament will cause it to heat up, and become more resistive. This lowers the gain significantly, and will cause the output to fall back towards zero. The C1/R2 series tank circuit and the C2/R3 parallel tank store enough energy that the output will continue past zero and head for the other supply rail. The bulb filament will cool and raise the gain as the output approaches zero, aiding the swing.

Once again, though, the filament will heat and decrease the gain enough to stop the voltage swing. That is the defininition of an oscillator - a device that constantly varies between two extremes, but never reaches either.

The tank circuits are the means of setting the oscillator frequency. 16.0K resistors run a bit fast - the oscillator runs above 1000 Hz. 16.2K gets very close with 10% capacitors.

The bulb is a bi-pin type, so it can solder into a pc board easily. The filament is rated 40 mills at 24 volts, so any similar bulb will work. A 784 bulb will work when R1 is changed to 200 ohms. It's rated at 80 mills and 24 volts. It's the non-linear resistance of the lamp filament that make the Wien Bridge oscillator work.