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Problem with Simulation of Colpitts Oscillator by Pspice

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pyth2004

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pspice oscillator

According to some old posts, I have set the initial value of capacitors (C1, C2 and C4) to be zero and capacitor C3 to be 9v which is same as the power supply , but I dunno why I got this odd simulation result.

For my design, the oscillation frequency is chosen to be 100kHz.


Please help. Thanks :(
 

colpitts oscillator pspice

Hi! pyth2004, here are some advice for you.

1) Are you sure your transistor can operate under 3A? This bias current is unusually large. Normally a 100kHz oscillator dont even need 20mA for its bias. With a good transistor, you need less than 1mA to design a colpitts.

2) R2 and R3, won't these 2 resistors be too small?? These two resistors may degrade the tank's Q value.

3) The resonant freq. is 1 / (LC)^2 / 2 / pi , right? the L = 2.87uH , C = 15nF / 2 (serial), will these gives the osc. freq. at 100kHz??
 

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    pyth2004

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colpitts oscillator spice

dsjomo said:
Hi! pyth2004, here are some advice for you.

1) Are you sure your transistor can operate under 3A? This bias current is unusually large. Normally a 100kHz oscillator dont even need 20mA for its bias. With a good transistor, you need less than 1mA to design a colpitts.

2) R2 and R3, won't these 2 resistors be too small?? These two resistors may degrade the tank's Q value.

3) The resonant freq. is 1 / (LC)^2 / 2 / pi , right? the L = 2.87uH , C = 15nF / 2 (serial), will these gives the osc. freq. at 100kHz??
Click to expand...

Thanks for your reply.

I just want to make it at least oscillating first (it is approximate to 100kHz)... I dunno why the biasing voltage of the transistor is always negative by the simulation.
 

colpitts oscillator simulation

It is not shown in the schematics but I would guess that the battery is reversed and the -9V is connected where the +9V should have been, as the voltage at the collector shows to be approx. -9V.
Another question: are the voltages on the schematic the instantaneous voltage at turn on?
Its better to do only DC operating point first and to post the results.
My way to initiate oscillations is first to check DC operating point (In AC analysis), and then (In transient analysis) to put initial conditions only on the inductor to a low current like 10uA, and see the oscillations settle.
 

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    pyth2004

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pspice simulation colpitts oscillator

Hi,

C3 is not at all necessary here in simulation.
C4 is bypass capacitor its value is too less 1nf ???
set initial voltage on c1 = 1v
Do simulation and let me know
 

pspice simulation of colpitts oscillator

This should work. Try this one in your PSPICE.
 
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