The feedback in the amplifier section of an oscillator must be POSITIVE FEEDBACK. This is the condition where a fraction of the amplifier's output signal is fed back to be in phase with the input, and by adding together the feedback and input signals, the amplitude of the input signal is increased. For example, a common emitter amplifier creates a phase change of 180° between its input and output, the positive feedback loop must therefore also produce a 180° phase change in the signal fed back from output to input for positive feedback to occur.