Positive Feedback

The use of positive feedback is useful for producing oscillators. The condition for positive feedback is that a portion of the output is combined in phase with the input. For an amplifer with positive feedback the gain is given by the expression below.

The large open loop gain of an op-amp makes it inevitable that the condition will be reached, and the gain expression becomes infinite.

Op-Amp Oscillators

Colpitts Oscillator

The Colpitts oscillator achieves positive feedback by using an inverting amplifier plus the 180° phase shift across a parallel resonant circuit. The series capacitors combine to produce so the resonant frequency is

Calculation notes: If component values are changed, the new frequency will be calculated. If a frequency is entered, then you must click on the active component name of the component you wish to calculate to force the frequency to match the entered value.