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Using an oscillator, I want to provide a square wave input to the second opamp circuit shown in this TI application note (but using IC741 instead.) The problem lies in the s domain:

Sinewave generator by Texas Instruments

I get a voltage input of \$\dfrac{1}{(s^s+as+c)}\$ at the non-inverting port, which is a decaying signal and I want an attenuated signal.

Also, I cannot seem to understand what C2 is used for, it just seems to contribute to KCL?

rdtsc
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Mr. Johnny Doe
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2 Answers2

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The circuit is a square-wave oscillator followed by a Sallen-Key LPF tuned to attenuate harmonics above the fundamental. Since the 3rd harmonic of a square wave is 1/3 of the fundamental, the second-order Sallen-Key filter is not going to to do a great job, but it should look more-or-less like a sine wave at the output (once it has been operating for a few cycles).

If you are seeing something else, re-calculate your values and make sure the parts you are using match the theoretical values closely enough.

The oscillator would be about as bad (actually a bit more stable) if you used the 555 followed by the low-pass filter (LPF).

Spehro Pefhany
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The second op-amp circuit is a Sallen-Key lowpass filter, that can have significant gain peaking. The transfer function is second-order of the form you have shown.

You can read about the topology here.

You are correct that the output would decay for a single unit step input, but if you are inputting a square wave at the peak frequency the filter would remove most of the harmonics leaving just the fundamental sinusoid at the output. The repeated energy supplied by the square wave keeps the output from decaying.

Also I agree with Jim and Andy about using the 741. It will just make things that much more difficult. Much better amplifiers are readily available almost everywhere.

John D
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  • So essentially the given filter prevents exponential decay by acting as a filter and remove harmonics?? The issue is our teacher INSISTS on only a 741, I hope that she realises how hard my project is and lets me off. Or else I need some feedback circuit. Don't want exponential decay to ruin a good project. – Mr. Johnny Doe Apr 05 '18 at 15:17
  • So using an IC 741, I can build those three resistors, and plug that in as my A2?? – Mr. Johnny Doe Apr 05 '18 at 15:19
  • I don't know what you mean by "build those three resistors". You need to build the Sallen-Key filter as shown in the diagram. It has 2 resistors and 2 caps. You can certainly use the 741 if you have to, just respect the datasheet limits on supply, input and output ranges, etc. It does not sound like a hard project, and your teacher shouldn't "let you off" because then you won't learn anything from it. – John D Apr 05 '18 at 15:28
  • I think I understand. By build, I meant connect, I somehow didn't catch it while typing it. I'm trying to build a signal generator to obtain and realise every major signal(square, triangle, step, ramp, sine etc), not sure what "cap" means in this context – Mr. Johnny Doe Apr 05 '18 at 16:40
  • Also, If I want to shift the DC level of a square wave, would I rather use a clamper or a adder circuit? – Mr. Johnny Doe Apr 05 '18 at 17:15
  • "cap" in the above context means "capacitor". As to the clamp or adder question that's up to you, I don't know which you would rather use and both are possible. – John D Apr 05 '18 at 17:18