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Colpitts Oscillator simulation Help

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John_li

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Hi all,
I make a colpitts oscillator simulation with Pspice.But unfortunately it can not get reasonable result.It can not oscillate stably.I set C1,C2,C3,and L1 initial condition is 1V.Attach schematics and result FYI.Anyone kindly help me why not oscillate stabily?And How can i tune it?Thanks!

 

arsenal

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Reduce the transient step!
Set initial current to the inductor!
Try increasing the transient time!

Just curious! So in the gain stage is not shunted to the crystal?
 

    John_li

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John_li

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I set run time is 5ms/step time:10us,L1 initial current:1~3mA.but still fail.
 

Sadegh.j

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the loop gain is less than 1
 

    John_li

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John_li

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How to caculate the loop gain?
 

Sadegh.j

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Hi

The loop gain in colpittes oscillator is=(n Gm(x) jw/(c))/((jw)^2+jw Gt /C+1/ CL )

where
Gm : nonlinear characteristic of the transistor
n=c1/(c1+c2)
c1: The cap between the collector and the emitter
c2: The cap between the the emitter and ground
Gt=G+GL = n^2 (Ge + Gm(x)) +GL
Ge=q/Re where Re is between emitter and ground
Gl= 1/ RL, where RL is between collector and ground (load)

Now set the phase of the loop gain =0 and the amplitude of it to 1.

Basically to find the loop gain , you break the loop from one point and calculate the voltages on each side of the broken point. Then you set them equal. Tell me if it does not help and I will right it down on a piece of paper properly.

Cheers
 

    John_li

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John_li

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Dear Sadegh.j,
Thanks for your reply,Can you kindly give me an exmple to caculate it?Thanks
 

Sadegh.j

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You got to give me some, since I have to write it down and it is a bit long.
 

    John_li

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John_li

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Thanks at first.If i want to let it oscillate in 500KHz(LF) or 900MHz(HF),How can i caculate,and any formula to evaluate the Zin for negtive impedence and C1,C2?My E-Mail:John_li3000@sohu.com,look forwards to your reply.
 

Sadegh.j

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From the previous loop function, if you set the phase equal to zero then:

w= 1/((LC)^(1/2))

Where C= C1C2/(C1+C2)

You just choose value for L and C and you have the right frequency. In fact I made the attached circuit a while ago on a board and the equation is quite accurate.

I did not really understand your second question. sorry
 

    John_li

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John_li

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Thanks.Whether q=1.602E-19(C)in formula:Ge=q/Re,and how can i get the Gm(X) value?
 

Sadegh.j

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hi

Check the attached file. As I promise you, it is the detailed calculation of the formulas given above.

I tried to write every detail. I hope this makes everything clear. If it does not, don't hesitate to tell me.

Cheers
 

    John_li

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John_li

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Dear Sadegh.j,
It is so clear and easy to understand,and it is helpful for me.Thanks for your always help.
 

Sadegh.j

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It is a pleasure
 

    John_li

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