# Problems Building Variable Frequency Wien-Bridge Oscillator

I have been doing some experiments with Wien-Bridge Sine Wave Oscillators, specifically trying to make one where frequency can be controlled via a single knob, using a dual ganged potentiometer. I am basing my design on the following circuit from page 29 of Analog Devices App Note 43:

I am replacing the two 1.6k resistors with each of the gangs of a 100k dual potentiometer.

I have been noticing some strange behavior. Rather than creating a variable frequency oscillator the circuit only oscillates when the potentiometer is set to a very specific range of values (~1k ohms). As I turn the knob away from this range the oscillation first becomes unstable (very jittery in oscilloscope) then disappears. Does anyone know the reason for that this is happening?

• You should replace the 1.6K resistors with a 2.5K or 5K dual pot instead of 100K, to give you a more reasonable control range. Jun 14, 2017 at 6:27
• I'm trying to build an oscillator that can go to just below audio range. Based on the formula f = 1/(2 π RC) 100k resistance will create a ~16Hz frequency, which is what I'm looking for in terms of range. Jun 14, 2017 at 14:22
• Increase the capacitor value rather than increase the resistor value. Peter Bennet is giving you good advice. Also note that you may want to ensure that the capacitor values match to within 5% or better. Feb 10, 2018 at 4:23

Are you sure you have the two gangs of the pot wired the same way (i.e. so they both increase together)?

If the two resistors are not matched well the gain required to oscillate may increase beyond the range of control of the 430 ohm resistor and #327 lamp.

The classic Wien bridge needs a gain of 3.

• How can I calculate gain in this circuit? I know the lamp works a kind of variable resistor, which makes the side with the 430 ohm resistor and the #327 a lot like a standard non-inverting amplifier topology. What kind of resistance value would I use for the lamp to calculate gain, and how does this relate to the part of the circuit attached to the inverting input? Jun 14, 2017 at 14:28

You're not doing anything wrong per se. Just don't use a Wien bridge.

In many respects a Wein bridge is a theoretical construct. Yes you can just about get it to oscillate but it's very twitchy. No one really uses them seriously, and certainly not as variable frequency oscillators. And you've picked the simplest quintessential example using a bulb component that's very loosely defined. I think that's meant to be a 28V bulb, but what's the circuit's voltage?

Someone mentioned the success of Bill Hewlett's original Wien generator. This is it, the model 200A:-

It could vary it's output by three decades, but it's not simple like you're attempting to replicate. The frequency was set with a double range switch (S1) and a quad tuning capacitor (C2). But also notice C1, C6, C7, and R26, all hand adjusted at the factory. Also notice pot R14 which sets the output level which can compensate for loss of signal amplitude. And for interest, this bulb was a 3W Mazda.

Also check out Figure 40 your datasheet. That's meant for multiple frequencies. You'll see that it also has more adjustment points than just a pot.

Have a poke around YouTube. People there have built them with alternate arrangements and you'll see that they're still difficult to use. I'm not sure if those designs are much of an improvement...

• I completely disagree with your answer. You may have heard of a company called "Hewlett and Packard". The start of their company was a sine-wave oscillator built using a Wein Bridge. And - although they were possibly the first commercial venture to build Wein-Bridge oscillators, many other companies also began manufacturing this type of oscillator. Feb 10, 2018 at 4:17
• Regarding the light bulb: note that it is NOT supposed to illuminate brightly. The circuit is designed such that the filament temperature is barely lit, if at all. That's because that is where the non-linear slope is the greatest (small change in current results in large change in resistance). The 327 and 387 lamps are readily available because they are used in industrial indicators all over the entire globe. Feb 10, 2018 at 4:20
• At equilibrium, the resistance of the "lamp" must be equal to 430/2 Ohm (condition of oscillation of the assembly) ... Where it is placed, if the amplitude of the oscillation is low ... its resistance is "weak" (cold filament) ... As soon as the amplitude of the oscillation increases ... it becomes "larger" (hotter filament)! Beware of the "time constant" of the temperature of the filament ... which means that it does not work very well for frequencies that are too "low" (no sufficient inertia) ... Jul 10, 2021 at 15:41