Hi all,

I have designed a simple Colpitts oscillator as shown below:

In LTSpice it works great! I have built this in real life with all values the same. The only difference is that the transistor used is a BC548 instead of what is shown (This is because I know that the 2N3904 and the BC548 are interchangable).

When I power the circuit I DO get an oscillation however it is very small in amplitude (less than 1vPP) and there is a large DC offset (about half the supply typically).

What is going on?
It should be noted that the circuit is built on breadboard so will parasitic capacitance affect anything?

All the best,
Robin

 

...It should be noted that the circuit is built on breadboard so will parasitic capacitance affect anything?

Yes, especially at those frequencies. Breadboards are horrible above about 100kHz...

 

Hello,

The coil L1 is extremely small, try a value of about 100 X larger eq. 1 μH.
The capacitors C2 and C3 can be reduced to about 200 pF.

Also a breadboard will have parasitic capacitance.

Bertus

 

Take the output via a cap if you want to get rid of the DC offset.

 

This version has worked well for me.
All the way up to 100 MHz on a bread board!

http://www.learnabout-electronics.org/Oscillators/osc24.php

 

This version has worked well for me.
All the way up to 100 MHz on a bread board!

http://www.learnabout-electronics.org/Oscillators/osc24.php

Interesting site.

 

This version has worked well for me.
All the way up to 100 MHz on a bread board!

http://www.learnabout-electronics.org/Oscillators/osc24.php

What values did you use to get 100MHz?
And any pictures?

Many thanks !

 

What values did you use to get 100MHz?
And any pictures?

Many thanks !

I think it was 1 nF capacitors and 5nH coil. Coil was made by wrapping standard 22 gauge insulated solid hookup wire around a pencil 6 or 7 times (maybe it was 9 times?). There are calculators on line. I was able to pick up the silence on an fm radio. Then stretch the coil a bit (just spring-like, not deforming permanently) so you can hear the fm receiver catch and lose the signal. The signal was quite strong - up to 2x supply voltage.

 

I'll rebuild it. I really should have a notebook.

 

I'll rebuild it. I really should have a notebook.

So I built it on stripboard, breadboard and LTSpice. None of them oscillate. Instead they have a decayed oscillation that dies out after 45uS

 

I think you need a smaller capacitor connecting the tank to the emitter if that happens (from memory).

 

100nf tank capacitors and 50nF inductor

Also, the load on the output must be very low or you will lose your signal.

My output was the 10M ohm cap (I need to measure voltage across a component on my ipad simulator - no meter available).

The output across the 10M resistor is 6.5 v peak to peak. 100 MHz.

Also, you should expect 20 to 30 pF across each tank cap from board capacitance so use 68 or 82pF caps for prototyping.

Coil could be 5 turns, 0.25" about 0.7" long.
Calculate your own coil here. This site will post as 5.00E-002 microhenries
http://www.66pacific.com/calculators/coil_calc.aspx


Cheers

 

Ok so I built this too. Nothing.

I have used the BC548 as the active component (and tried multiple devices), and I am using a 20MHz oscilloscope (which is analoge).
I used all the values you suggested but for some reason there is no oscillation

 

You will not see a 100 MHz signal on a 20MHz scope.

 

You will not see a 100 MHz signal on a 20MHz scope.

I know that but its analog so would the signal just look like a block? And how would I get for example a 1MHz sine wave from the circuit you showed?

Many thanks
Robin

 

Increase the size of the components in the tank.
The must stay somewhat "balanced" (as you go up in size). Not linear but make sure your simulator sows a signal just above your supply voltage. Also, not all simulators will work with self-oscillating circuits without an initials perturbation (enter a starting value).

Here is a simulation of a 1MHz oscillator. Note that the slower you go, the more difficult it is to keep it going. Use the fattest wire (lowest resistance) coils possible and lowest ESR capacitors possible since the simulators do not have resistance in the ideal components.