Electronics > RF, Microwave, Ham Radio

Troubles with Colpitts Oscillator

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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

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