# Simple Question about Common Collector Colpitt Oscillator

#### silv3r.m00n

Joined Apr 15, 2010
68
Consider a simple circuit like this, its a common collector colpitts oscillator.

Which of the following statement is true ?

1. The "emitter drives the tank" and base receives feedback.
or
2. The "base drives the tank" and the emitter receives feedback.

The way most texts explain it, the "emitter drives the tank" and base receives feedback, but in that case *****
the feedback goes through a loop and arrives back (via base-emitter junction) at the driving point, which seems really weird ....
am i missing something ?
there must be some sort of "isolation" or de-coupling between the input and the output.

or

on the other hand, i tried to speculate that "base drives the tank" and emitter receives feedback, but in that case *****
there seems to be no gain of any kind, so how can oscillations sustain ....

@Danko
@crutschow
@Bordodynov

#### atferrari

Joined Jan 6, 2004
4,166
Are you sure there is a C like the one shown with no ID?

#### silv3r.m00n

Joined Apr 15, 2010
68
Are you sure there is a C like the one shown with no ID?
sure, the middle C protects the base DC from shorting via the inductor.
but acts as a short for the AC signal.

#### Bordodynov

Joined May 20, 2015
2,731
The oscillating resonant circuit converts (transforms) the signal.

#### silv3r.m00n

Joined Apr 15, 2010
68

#### Bordodynov

Joined May 20, 2015
2,731
I have shown that the voltage at the oscillating circuit (voltage at point B) is greater than at the source (at point B). In the real circuit the signal source is an emitter repeater, the effective output impedance increases as the signal increases.

#### LvW

Joined Jun 13, 2013
1,160
Consider a simple circuit like this, its a common collector colpitts oscillator.
......................
Which of the following statement is true ?

1. The "emitter drives the tank" and base receives feedback.
or
2. The "base drives the tank" and the emitter receives feedback.
The way most texts explain it, the "emitter drives the tank" and base receives feedback, but in that case *****
the feedback goes through a loop and arrives back (via base-emitter junction) at the driving point, which seems really weird ....
am i missing something ?
there must be some sort of "isolation" or de-coupling between the input and the output.
o
on the other hand, i tried to speculate that "base drives the tank" and emitter receives feedback, but in that case *****
there seems to be no gain of any kind, so how can oscillations sustain ....
1) In a common collector circuit (emitter follower) the base node (B) is the input and the emitter node (E) is the output.
Hence, there is a feedback loop between E and B (the emiter drives the base through a RC network)
2) Why do you expect "some sort of isolation" between input and output.? In contrary - each oscillator requires such a feedback loop which couples back a sufficient part of the output back to the input (with correct phase for one single frequency).
3.) The gain within the complete feedback loop ("loop gain") must be > 0 dB for one single frequency. The gain for an emitter follower is always below "1" (0 dB), but the resonant effect of the feedback path allows a transfer > 1 at the resonant frequency see the contributon from Bordodynov). Hence the oscillation condition (loop gain>1) can be fulfilled.

#### sparky 1

Joined Nov 3, 2018
401