Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Colpitts oscillator in multisim

okbro

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

ckt.png
 

Papabravo

Well-Known Member
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

Member
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.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
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:

130px-Cb_colp.svg.png
 

okbro

Member
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:

130px-Cb_colp.svg.png
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.
 

okbro

Member
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?
index.jpg
 

rjenkinsgb

Well-Known Member
Most Helpful Member
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

Well-Known Member
Most Helpful Member
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.
 

Latest threads

New Articles From Microcontroller Tips

Top