• When some part of the output signal is fed back to input signal then feedback is said to be done.

• Depending on the relative polarity of the signal one may have positive or negative feedback. When the input signal and feedback signal have same polarity means they are in phase then the feedback is said to be positive otherwise the feedback is negative.

• Positive feedback drives the amplifier circuit into oscillation as in various types of oscillator circuit. But for an amplifier to be work as an oscillator it required to satisfy the some conditions that conditions are known as barkhausen’s criteria.

• The conditions are as follows:

1. The total phase shift around a loop, as the signal is proceeds from input through amplifier, feedback network and back to input again is must be 0 or 360 degree.

2. The magnitude of product of open loop gain A and feedback factor β must be equal to 1 or unity.

                                           |Aβ=1|
   

If the |Aβ<1| means less than unity then oscillations are exponentially decaying.

If the |Aβ>1| means greater than unity then oscillations are exponentially increasing without any limit. It is important to know that barkausen criteria is necessary condition for oscillation.