Wrong output signal in Colpitts oscillator

Thread Starter

SurrogateDrone

Joined May 29, 2022
3
Hello, everyone,

I have mounted a Colpitt oscillator on my breadboard and in order to have a comparison I tried to draw it on CircuitJS1 as well.
However I come up with different results because the output signal is completely different.
Here is the link to the circuit I tried to draw: https://tinyurl.com/2exsm9un
I attached the diagram and the waveform that I see with the oscilloscope with respect to the circuit mounted on the breadboard.
The signal in this case is distorted and this is due to the value of R3 as expected, but this is not a problem, at least for now.

Thank you in advance for your help.

P.s Sorry, I can't edit the title, I know it's too generic :(
 

Attachments

Last edited:

michael8

Joined Jan 11, 2015
295
The circut LC has L = 100uH and C = 1nF (ignoring the 10nF in series and other stray things).

1/(2*pi*sqrt(100e-6*1e-9)) -> 503292 or about 500 KHz and the scope picture has a period of
about 1.8 to 1.9 uS so thats in the 500 KHz range. So the real circuit frequency matches the LC
resonant frequency.

Reality looks fine -- what's the problem? (I'd use LTspice for simulations)
 

Thread Starter

SurrogateDrone

Joined May 29, 2022
3
Yeah, the circuit on the breadboard looks fine to me and I previously checked the frequency as well, but the simulation on falstad.com is wrong and I don't really know why, I feel like I'm missing something stupid.
We usually use LTspice, but I usually share a falstad link to a friend of mine who isn't really into electronics and those animations are easier to read for a complete beginner and I didn't expect this.
Also, I don't really know why the value of R3 is critical for the distortion of the waveform, I guess it's because it's the amplifier output resistance but we just started to study oscillators this week so I'm not sure!
 

michael8

Joined Jan 11, 2015
295
R3 is the emitter resistor. The base has a roughly fixed bias voltage so R3 sets the transistor collector/emitter current.
The base is at 5*(3.3e3/(3.3e3+10e3)) -> 1.24 volts and the emitter should be about .6 volts less so 0.64 volts.
0.64volts / 1K -> 640 uA emitter current...

The transistor gain is related to the transistor current.

Try increasing R3 in the real circuit. Likely there is a value where it still oscillates but with lower amplitude.
 

Thread Starter

SurrogateDrone

Joined May 29, 2022
3
I tried to add a 3k resistor ( 2k and 1k connected in series) and it has a lower amplitude and it looks better than the first one. I tried with a 10k resistor and the waveform has a much lower amplitude and no distorsion.
Also, the frequency should be 528 kHz, so if we take into account the instrumental errors and the measurement the value that I got it's good enough.
Is this correct? Also, I'd like to thank you, you've been truly helpful
 

Attachments

Top