colpitt oscillator problem

[rameshrai]

hi,

i am trying to simulate colpitt oscillator in Proteus. the circuit is supposed to work for 100MHz. following is the schematic diagram. it is not working, i see only decreasing oscillating signal at the output.





i also doubt the value of C1 and C6 capacitor values, how are the value of these capacitors calculated?

 

[LvW]

Where are tha parts values coming from? Your calculation?
Does the circuit have the desired DC operational point?

 

[rameshrai]

the transistor was biased to give voltage gain of 10. Ic was taken as 10mA. running simulation shows no detectable current Ic and dc voltage across Vc point.

 

[LvW]

the transistor was biased to give voltage gain of 10. Ic was taken as 10mA. running simulation shows no detectable current Ic and dc voltage across Vc point.

If there is no currrent Ic and no voltage at the collector there is the possibility that something is wrong with your calculation, don`t you think so?

 

[FvM]

SPICE simulator operation with Proteus may be a bit strange, but apart from this point the circuit is impossible to oscillate with the given LC values. They must be scaled by at least two orders of magnitude to get a reasonable resonator characteristic impedance Z0 = √L/C

 

[rameshrai]

i think something is wrong with calculation but this is what i wanted to ask here

 

[LvW]

the transistor was biased to give voltage gain of 10. Ic was taken as 10mA. .

If you present your calculation we can possibly detect some errors.

 

[chuckey]

C2 and C3 seem a big big, try 22PF and 220PF. I can't read your inductor values.
Frank

 

[rameshrai]

i think the circuit just does not work for such high frequency

 

[Audioguru]

i think the circuit just does not work for such high frequency

Most of your parts values are for a low frequency because your calculations are wrong.

My FM transmitter uses the same transistor as you and works perfectly at 100MHz and higher. It is a modified Colpitts circuit.

 

[FvM]

I believe the circuit will work with the capacitor values suggested by chuckey and respectively adjusted inductor.