Oscillator NPN

Auto start (bottom circuit as trigger to accelerate the oscillation, don't touch) If you want to use this signal, remember to use a high input impedance circuit. You can perfectly see the 180 degrees out of phase. Oscillator uses three RC cells and one transistor as common emitter amplifier. Re and Ce are necessary if you think to build this circuit because gain (hfe c.a. or Beta c.c.) is very sensible to temperature variations of Base-Emitter junction. Re stabilizes the Q point. The total loop gain must be greater than one (Gain > 1) As we are using a common emitter amplifier, the gain of this will be always negative. That's the reason why we use three RC cells. Every cell signal 60 degrees out of phase for a total of 180. This is the same as saying it is a phase inverter. The oscillation happens only at certain frequency where the transfer of energy from the input to the output of the RC group is, exactly, 180 degrees out of phase. Any other oscillations can't be possible. As you can see, Ic (green) is 180 degrees out of phase to collector Vout (blue). I don't need to say that Ic (green) is on phase with two Vresistors on base transistor.

published 12 years ago