Troubles with Colpitts Oscillator

[fnpfar]

Hello. I´m trying to build a colpitts oscillator to use as a local osc. for various projects.
I build this schematic, but when powered, it doesnt work, only the transistor heats up, and the voltage on the base remains near 0v, despite I have added a potentiometer to add bias.



What is wrong with it? I tested it with various combinations of coils and capacitors, but It doesnt oscillate.

Thank you for your help!!

[Tom45]

Shouldn't you have an inductor where you have the 1K resistor?

[fnpfar]

But without this limiting resistor current would flow through the transistor burning it.. i think (?)

[Tom45]

The inductor is more or less a dead short only at DC. Compute the impedance of the inductor at the oscillator's frequency.

[orbanp]

Hi fnpfar,

That is no way to set the bias for a transistor!
Add a 1k resistor form the emitter to the ground and bypass it with a 100n cap.
Then you can set the bias comfortably!

The oscillator in that configuration runs in the simulator with the bias (emitter) current set to about 1.8mA.

Good luck, Peter

[rfbroadband]

download a free spice simulator and start by simulating and verifying the bias. Next do transient sim....

[uncle_bob]

Hi

Where did you get that schematic from and why do you call it a Colpitts?

What is your objective (just a sine wave out)? Why not use a crystal? For 20 cents that would solve a lot of issues.

To do it correctly:

Keep the emitter hooked to the junction of the two caps.

Move the ground to the bottom of the inductor (L1/C1 junction).

Put a 1K or so from emitter to ground

Put a 10K from 9V to the base.

Put a 10K from base to ground.

Run the collector to 9V through a 330 ohm resistor.

Start with C1 = C2

Start with the impedance of C1 and C2 in the 10 to 100 j Ohm range at your desired frequency.

Start with an inductor with a Q > 100 and an impedance of 2x that of C1 or C2 (it resonates C1 in series with C2).

The circuit should come up without the coil in the circuit at:

4.5V on the base
3.8V on the emitter
8V on the collector

Connect the coil and you should see several volts p-p on the 330 ohm resistor. This is with a high impedance probe running into your oscilloscope.

Bob

[fnpfar]

Thanks a lot for your questions. I tried some other configurations (common base and common collector) and they worked well. Now I understand why that circuit didnt work.

Thank you!.

Fran

[tatus1969]

don't know if I overlooked it, but what is the value of L?