colpitts oscilator problem

[ant17]

hi guys i have tried many versions of the colpitts oscillate but i can not get it to oscillate its a npn bc337 transistor look at the schematic
https://upload.wikimedia.org/wikipedia/commons/f/f6/NPN_Colpitts_oscillator_collector_coil.png i have tried a number different circuits but can ot get it to oscillate i finding it strange many people have tried to get it to work and had success but i don't understand why my only thought is i might be using the wrong pin out but i have tried other transistors if anybody had the same trouble or no what to suggest i would be greatful

 

[ted]

Hi,

I think some resistor values are bit off from what I would personally choose, and therefore closed loop gain is not sufficient for oscillations.

I can see, that exactly this circuit drawing you had problems with, including same component values, is circulating around in the web.

Try something like R1=820, R2=R3=100k. That might wake the circuit up. However, the initial amplitude is high, and falls to a small value, when C3 charges up. Therefore C3=1nF gives faster the steady-state oscillations (of low amplitude). But the start-up still behaves the same way, only settling much faster.

Personally I believe this version of Colpitts oscillator is not the best configuration. One on for example page https://www.electronics-tutorials.ws/oscillator/colpitts.html works well, and I have used that basic topology even in a real product.

-ted

Added note: I tried it with BC337-25 and -40 on ltspice IV, and it did not work too well. Also, with 2N2222 the circuit is too tricky to tweak and likely never would be robust and reliable. Therefore I strongly recommend you to consider the configuration where the tuned circuit is on base "side". (See the link above)

 

[godfreyl]

These sorts of oscillator work well too:
http://www.newcircuits.com/circuit.php?id=rfr002
http://www.buildcircuit.com/simple-steps-for-making-fm-transmitter/

They have the main tuning capacitor in parallel with the coil, and a small feedback capacitor from collector to emitter. The supply bypass capacitor is important too, for any high frequency circuit.

There's lots of good info and circuits for transmitters and receivers here: http://www.sm0vpo.com/

 

[ted]

I simulated the topology I linked into, and added emitter bias resistor, too. It seems to work pretty well.

 

[chuckey]

The reason the first oscillator doe not work is that the input impedance at the emitter is in the order of tens of ohms, as this is across the 100PF, this impedance is reflected across the whole tuned circuit. Try making the lower cap 470/1000 PF
Frank

 

[ant17]

thanks guys will try the first circuit then the second simulated one

 

[ant17]

thanks guys will try the first circuit then the second simulated one

hi guys i tried and tried to get the simulated one working but no luck

so tried the one on the tutorial link with a radio frequency choke and lc tank circuit which to my amazement worked i got a semi sinusoidal that almost like a camel which was about 10vpp its works but strange waveform any ideas on what might be going on with the wave form

 

[BradtheRad]

The schematic in post #1 has the power supply in the oscillating loop. This means the supply cannot have much internal resistance.

It also creates spikes on the output waveform, each time the bias capacitor sends a pulse to the transistor.

It does not get going too easily, unless it gets an instigator (or shock), such as a disconnecting a component momentarily, then reconnecting it.

Below is a link to my animated simulation.
Clicking it will (a) open the falstad.com/circuit website, (b) load the schematic, and (c) run it on your computer.

You can observe the same things I did.

https://tinyurl.com/n27tsta