Colpitts oscillator in multisim

[okbro]

i am trying to simulate the colpitts oscillator circuit as shown but i don't get any oscillator output signal, anyone can explain why?

 

[danadak]

R2 shows at 4.7 ohms, not 4.7K ohms ?


Regards, Dana.

 

[Papabravo]

I have not examined your circuit in detail, but this is not uncommon when simulating oscillators with ideal components. Usually they require something to "kickstart" the process like an initial condition, a noise source, or parasitic elements. It seems like some circuits want to oscillate and others require more encouragement.

 

[okbro]

R2 shows at 4.7 ohms, not 4.7K ohms ?


Regards, Dana.

yes i changed to 4.7kOhm with 5% tolerance but still no output waveform.

 

[okbro]

I have not examined your circuit in detail, but this is not uncommon when simulating oscillators with ideal components. Usually they require something to "kickstart" the process like an initial condition, a noise source, or parasitic elements. It seems like some circuits want to oscillate and others require more encouragement.

i have all components tolerance set to 5% even the 4.7KOhm(which is incorrectly 4.7Ohm, corrected it) and still there is no output. I have done many oscillator simulation and all have so far worked but not this one.

 

[Lightium]

Whats the resistance on the inductor?

 

[rjenkinsgb]

i am trying to simulate the colpitts oscillator circuit as shown

That's a rather dodgy circuit.

It should definitely have a decoupling cap from base to ground, and, strictly, a cap across the emitter resistor to fit the definition of a Colpitts circuit: Feedback via a capacitive divider.

eg. This is the fundamental config for a common base Colpitts:

 

[okbro]

Whats the resistance on the inductor?

the resistance of the inductor is not set, but i used the same inductor for many oscillator and they worked. i also added small resistance in series to the inductor but the circuit is not oscillating.

 

[okbro]

That's a rather dodgy circuit.

It should definitely have a decoupling cap from base to ground, and, strictly, a cap across the emitter resistor to fit the definition of a Colpitts circuit: Feedback via a capacitive divider.

eg. This is the fundamental config for a common base Colpitts:

yes, this is the form of Colpitts oscillator I know, but the one I showed should also work, isn't it? I have seem them in FM transmitter circuit and was wondering whether this is actually the oscillator or not.

 

[Lightium]

If you want a 1MHz osc may I suggest this one:

 

[okbro]

If you want a 1MHz osc may I suggest this one:
View attachment 139949

thank you, but i want to know whether the circuit i posted works or not

 

[Nigel Goodwin]

yes, this is the form of Colpitts oscillator I know, but the one I showed should also work, isn't it? I have seem them in FM transmitter circuit and was wondering whether this is actually the oscillator or not.

At the very least you've left out the capacitor decoupling the base.

 

[okbro]

At the very least you've left out the capacitor decoupling the base.

i tried the decoupling capacitor at the base but does not work, used various values, is the circuit i posted an oscillator at all?

 

[Lightium]

Here try this:

 

[okbro]

Here try this:
View attachment 139950

yes, this works. thank you. but the resonant frequency should be near 100KHz but it is showing 44.7KHz.

 

[Lightium]

yes, this works. thank you. but the resonant frequency should be near 100KHz but it is showing 44.7KHz.

Yes, I calculated it has 110KHz, but yes it's only 44KHz, I don't know the reason for the discrepancy.

 

[rjenkinsgb]

4mH with 2.2nF should be resonant at 53.65 KHz. That does not allow for the other circuit capacitance.

 

[okbro]

4mH with 2.2nF should be resonant at 53.65 KHz. That does not allow for the other circuit capacitance.

resonance frequency with 4.88mH and 2.2nF is 48.60 KHz so does that mean the capacitor 680pF(C1) has no effect?so that means this is not a Colpitts oscillator. what is the function of the capacitor between collector and emitter lik the capacitor C3 we see in many one transistor FM circuits shown below?

 

[rjenkinsgb]

The 680pF is what provides the positive feedback, with the transistor acting as a common-base amp.

If that was also in direct parallel with the inductor, it would bring the frequency down to 46.89 KHz
The 48.6KHz is presumably due partly to that, and partly to the transistor and other component capacitance.

Adding caps from emitter to ground as well as base to ground would make it a "true" Colpitts common base type oscillator.

There is presumably enough stray capacitance in the FM TX to make up for not having a separate emitter cap. It would not need much with it only having 6.8pF for feedback.

 

[danadak]

thank you, but i want to know whether the circuit i posted works or not

I simed in Simetrix, the osc starts and dies out. Tried various feedback
values, emitter R......

I found that if I used 70 ohms for the inductor, that killed it quickly.
Which made me think de-Qing the L needs more gain in circuit, and
Remitter (unbypassed) not helping......

But could not get sustained oscillations.


Regards, Dana.