Positive feedback

The feedback can be in phase or out of phase. For the circuit to oscillate, the feedback should be in phase, or positive. Negative feedback simply reduces the gain.The output of a common emitter or common source amplifier is out of phase from the input. If you couple the collector to the base through a capacitor, you will not get oscillation.

The output of a common base or common gate amplifier is in phase with input. But these circuits have limited gain. It is difficult to make them oscillate, even with the positive feedback.Common-collector and common drain circuits do not have enough gain to generate oscillators.