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Problems with Colpitts oscillator

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Newbie level 6
Dec 28, 2005
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colpitts oscilator multisim

I have a problem in simulating the circuit below with pspice.I've took it directly from the book by Sedra & Smith but the oscillations don't start.Where is the problem?


Not all programs simulate osc circuits well. I don't know Orcad, but I had the same experiences before with Protel 99SE. Try the student version of ANsoft designer or Multisim. You can also check for proper open loop performance with your program to make sure about phase-shift and gain margin.

collpits oscillator in multisim

i think knowing the reason for which your simulation don't start!!!
Obviously we start from the point that the circuit gets the Barkausen conditions....
u should verify these conditions opening the loop in a good point and verify that they are fulfilled.
Then u must put a time varying discontinue in your circuits...
for example u put a power supply given by a step at the time when start your simulation...
if your simulation time is long enough u will see all the transient until u reach the steady state...
then if u change the start condition of the reactive elements u will see a different transient..
for some intial conditions of the reactive elements
u will not see the transient...
enjoy yourself with these tests...

assuming that the design of oscillator has no problem, then for transient simulation, you need to perturb one node of the circuit by short duration current pulse or in the pspice you can set the node voltage. Set one of node voltages different from it's dc point.

Re: colpitts oscilator multisim

I have the problem to simulate a ring VCO too, even with the initial condition set to 0. However, when I plug a function generator with the gate, the output seems to work. The blue color is the original input with inverse output in red color.However, I don't think it is right because the VCO should be self-oscillating. I can't upload my file so I can't show u all the detail.

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or you can go to the web address above.

Increase the size of transistors till you get osicllation. ( i assume that you already know how to simulate an oscillator.) you may wonder why you need to increase the size of transistors to achieve an oscillation. It is needed to increase the oscillation period to be much larger than the rise and fall time of each node.

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