# 27MHz colpitts oscillator qestion

Discussion in 'General Electronics Discussion' started by samy555, Apr 26, 2012.

1. ### samy555

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0
May 11, 2010
This circuit was intended as 27MHz colpitts oscillator:

and whem simulated using multisim10 sofware I get:

I have some questions:
Any osci. Needs a FB to sustain oscillations, I think that this FB from collector to base path through the parallel two caps; the internal transistor capacitance b/w collector and Emitter, and the C4(=4.7p) cap.
From the Fairchild datasheets of the 2n3904 transistor I get:

Also, there is a curve related the capacitance with REVERSE BIAS VOLTAGE (V)
My questions:
(1) What is the approxi. value of the internal transistor capacitance b/w collector and Emitter at 27MHz?
(2) How the phase shifted from 180 degree at the collector to 0 or 360 degree at the base? Please give me a step by step illustration.
(3) The freq of the osci. was 25.4MHz while my calculaed value of the resonant freq was 27MHz? what factors affect this?
thanks

Last edited: Apr 26, 2012
2. ### duke37

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769
Jan 9, 2011
I have not used Multisim but would like to make the following comments.
1. The supply is not bypassed, this will not be a trouble with an ideal battery.
2. The tuned circuit has a very low impedance, I would expect the reactances to be 10 or 20 times the values that you used.
3. The base is bypassed with a large capacitance and is not a part of the oscillator circuit.
4. Feedback is through C4.

The frequency will be wrong if you have not taken the transitor's capacitances into account, just tune to the right frequency.

3. ### samy555

63
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May 11, 2010
I can not understand you!
What is the best value in your opinion?
Thank you

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Bigger L, Smaller C

5. ### samy555

63
0
May 11, 2010
But I read somewhere that the best is to choose a small coil and large capacitor to improve the quality factor.

6. ### duke37

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Jan 9, 2011
It must be 50 years since I played with 27MHz equipment so I cannot be definitive but I think I used a 3 to 30pF trimmer for C3 and about 10 turns of 0.25 inch diameter for the inductance. What would your 50nH inductance consist of?

C1 is there to hold the base at a constant voltage. The value of the capacitor is not important as long as it does its job. 10nF would appear to be overkill and in real life, perhaps a 1nF disc ceramic would be better (lower inductance).

Simulating a circuit such as this is fine to get an understanding of what is happening but in the real world, capacitances have inductance and inductors have capacitance, these are hard to simulate. For example, a 700pF capacitor on 27MHz may be running above its resonant frequency (depending on its construction) and so appear inductive, this makes the simulation using ideal components rubbish.