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| Wien Bridge Oscillator Waveform Shape |
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| eev_carl:
Hi, I simulated a Wien Bridge Oscillator and implemented it on my breadboard. I'm getting oscillation, but it looks like there's clipping at the higher and lower ends that make this look more like a square wave. I'm using a 741. I tried changing the source voltage but it just produced a similar shape at a higher voltage. Is this expected for this old op amp or should the waveform be more rounded as in the sim? Thanks, Carl |
| Andy Watson:
To obtain a sinewave with the Wein Bridge circuit the op-amp needs to have a gain of exactly three. Any less and it does not oscillate. Any more and the oscillations will build up until the op-amp clips - as you've witnessed. The usual method of stablising the gain is to incorporate a voltage dependant element into the feedback of the op-amp - like a thermistor/small incandesent lamp. |
| Tim F:
The 741 isn't a problem. Instead, the sim doesn't match reality or you lucked out with the oscillation frequency and opamp bandwidth/slewrate specs of the simmed opamp. It would be bad practice to rely on the opamp being slow enough if that is indeed the case in the sim. As above, if the gain is too low you get no oscillation and if it's too high it keeps amplifying until it finds the rails or other limitations of the opamp. You need a variable gain element to stabilise it. I wrote an article about bulbs for wien bridges once, you can find it here: http://feleppa.com.au/wienosc.html It's also possible to add some diodes to 'soft clip' the output, similar to the waveform in your sim. Google image search 'wien bridge diodes' for ideas. |
| eev_carl:
--- Quote from: Andy Watson on October 25, 2018, 01:02:21 pm ---gain of exactly three --- End quote --- That's producing a better shape. I replaced R4 (the negative feedback amp part) with 2k which gives me A=1 + 2/1=3. Any insights as to the clipping at the bottom of the attached waveform? Also, When I used 2k for LT Spice, I didn't get any oscillation. However, when I used 2.1k, the sim did produce a sine wave. I went back and breadboarded 2.1k but that didn't oscillate. (These resistors are 5%.) |
| eev_carl:
--- Quote from: Tim F on October 25, 2018, 02:05:29 pm ---I wrote an article about bulbs for wien bridges once, you can find it here: http://feleppa.com.au/wienosc.html --- End quote --- Thanks will check out it out and post an update. |
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