Understanding BJT oscillator tank circuit

In a oscillator circuit like this one:



How does the bjt put energy back into the tank circuit? What is the theory behind this bjt putting energy back into the tank circuit? is there a website that explains this bjt feedback amplifier? I can't seem to find anything on Google about this, I am obviously searching the wrong things.

Thank you for your help.

The transistor is a "common-base" amplifier. Its collector feeds positive feedback to its emitter through C2.
Some websites call the oscillator a "Colpitts" type.

If the emitter voltage increases, the base to emitter voltage DECREASES, so the current through the transistor reduces, so the voltage at the collector increases. So if enough of the collector voltage is fed back to the emitter, the transistor oscillates.
Frank

chuckey said:

If the emitter voltage increases, the base to emitter voltage DECREASES, so the current through the transistor reduces, so the voltage at the collector increases. So if enough of the collector voltage is fed back to the emitter, the transistor oscillates.
Frank

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Thank you that is very helpful! and the C to E is in phase and the B to E is out of phase so it would be pushing positive energy back into the system.

b4bb4ge said:

Thank you that is very helpful! and the C to E is in phase and the B to E is out of phase so it would be pushing positive energy back into the system.

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No.
B to E is also in phase but is not used in an oscillator. C to B is out of phase and can be used for negative feedback.

But it seems from simulation that the Collector and Emitter are in phase and if that is the case and as you say the Collector to Base are out of phase then shouldn't the Base to Emitter be out of phase? thanks for all your help!

If the base of an NPN transistor goes more positive then its collector to emitter current increases causing the emitter voltage to increase (if the emitter has a series resistor to ground) and the collector voltage to decrease. The emitter goes more positive like the base so they are in phase. The collector goes in the opposite direction to the base and emitter voltages so it is out-of-phase with them.

How does the bjt put energy back into the tank circuit? What is the theory behind this bjt putting energy back into the tank circuit? is there a website that explains this bjt feedback amplifier? I can't seem to find anything on Google about this, I am obviously searching the wrong things.

Thank you for your help.

Click to expand...

Hi b4bb4ge
What you've showed , is a simple colpitts oscillator which is based on a CB amplifier . the capacitor which is in parallel with CE is playing role of positive feedback through the collector into the emitter .
The tank circuit , in collector will select BW and center frequency of the out put signal . i think for more information you can refer to the google or many of the text books like micro electronics or control circuit analysis books . . for searching in google try to find out these keywords : positive feedback oscillators . or colpitts oscillator or ... etc .

Best Wishes
Goldsmith