After yesterday’s failed attempt to generate a clean since wave, I started experimenting a bit further. How could the Op-amp book be so wrong about the quadrature oscillator circuit?
The nice thing about op-amps in DIP-8 packages, is that most of them use the same pinout, so it’s very easy to swap them out and test different brands and types. The TLV2472 only supports up to 6V as power supply, most of the other ones go much higher – usually above 30V, i.e. ±15V.
Here’s the list of op-amp chips I tried (yeah, got quite a bunch of them in my lab, for various reasons):
- TLV2474
- LM358N
- LM833N
- NE5532ANG
- OP2340
- NJM14558D
- MCP6023
- LT1413
All of them had similar behavior, i.e. clipping at both limits of the voltage range, except for the LT1413:
Still nowhere near a sine wave, BTW. But what’s more interesting, is the the voltage swing of this signal was just 4.5 Vpp, while the op-amp was being driven from a ±15V power supply in this particular case. So for some reason, it was “oscillating” at 1.25 KHz (about 8x higher than the other mode).
I have no idea what was going on. When trying to reproduce this a second time, I couldn’t get this behavior back. I suspect a loose connection, or perhaps some odd interaction due to the breadboard.
I’m not really interested in tracking down this issue, since it looks like this quadrature oscillator circuit is not suitable for a Component Tester – not without some sort of amplitude regulation anyway.
So there you have it – analog circuits also need to be debugged, as you can see!
Update – this issue has now been resolved, see the comments on yesterday’s weblog post.
Yes, a poor connection was the likely culprit – note that the ‘scope is refusing to evaluate most ‘averaged’ parameters, suggesting that the waveform was significantly unstable.
Hi
Wouldn’t it be easier to create a 50 hZ square wave with a 80 hz low pass filter behind ?
Cheers Rubi
See the comments on tomorrow’s weblog post. The values you mention would require an extremely steep filter to get a clean sine wave (i.e. little harmonic content).