Q. What is Barkhausen criterion? Why is the value of Ab kept slightly greater than 1?

The condition |Ab| = 1 is called the Barkhausen criterion. It implies that when |Ab| = 1, there exists an output voltage even in the absence of any externally applied signal. If |Ab| = 1, the feedback voltage is sufficient for producing self sustained oscillations. If Ab = 1 precisely, then after sometime it is found that Ab will either become lower or higher than unity. This is due to the change in the characteristics of the active devices with voltage, temperature or age. If Ab < 1, the removal of input signal source will result in dying out of oscillations. If Ab > 1, then the amplitude of the oscillations will continue to increase until clipping occurs because of saturation and cut-off causing the voltage gain to reduce.

Hence, Ab is kept slightly larger than unity so that any variation in circuit parameters may not cause Ab to become less than unity.