I have designed an op-amp based Colpitts oscillator in multisim. When I press run button it shows- "timestep too smaall error". I've googled and tried everything but it still shows the error.

enter image description here

The link to the Multisim (.ms) file -

http://www.4shared.com/file/bqbr2V_i/Design1.html

Can anyone advise me as to what I'm overlooking and how to make this circuit oscillate?

  • Not sure where you got that design, but as drawn your C-L-C pi circuit is grounded (by an ideal ground, for purposes of simulation) at both ends, so it will have absolutely no effect on the op-amp circuit. – The Photon Apr 17 '13 at 19:17
  • It also looks like the op-amp output is tied directly to ground, but I am not sure I can tell the difference between a connection and a jump in your image. – The Photon Apr 17 '13 at 19:18

The op-amp output is tied to ground and, there is no positive feedback anyway so it wouldn't work. Even if you switched the LC arrangement to give positive feedback there is no non-linear circuit to give gain control - the circuit, when "wired correctly" will alternate between full clipping and hardly anything.

Maybe you were trying to do this one: -

enter image description here

It works because there is positive feedback - the amp inverts (180º shift) and the L and C conspire to produce another 180º at resonance. I can't vouch for it working and I don't think the capacitor C2 will do anything except make it appear like a colpitts oscillator. I found it by googling "op-amp colpitts".

This is a better circuit and you can swap the crystal and series 75pF for an inductor methinks: -

enter image description here

It works better because there are diodes that limit the amplitude and make the circuit stable. It's basically a small variation on the standard emitter follower colpitts circuit.

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