State type of feedback used for oscillator circuit. Explain Barkhausen criteria.
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Type of feedback used for oscillator circuit : Positive feedback 

Barkhausen's criterion is a necessary condition for oscillation:  

It states that if A is the gain of the amplifying element in the circuit and β(jω) is the transfer function of the feedback path, so βA is the loop gain around the feedback loop of the circuit, the circuit will sustain steady-state oscillations only at frequencies for which: 

1. The loop gain is equal to unity in absolute magnitude, that is, βA | = 1 and 

2. The phase shift around the loop is zero or an integer multiple of 2π.

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Concept of feedback: Feedback is process to fed back part of output to the input. Feed back concept is introduced in amplifier to improve the parameters of amplifier like voltage gain, current gain, stability, bandwidth. There are two types of feedback depending upon whether output increases or decreases. There are four different types of feedback topologies based on type of output signal and feedback signal (voltage or current signal). Voltage feedback is taken in series with the load and current feedback is taken in shunt with the load. They are 

a) Voltage-series: Output signal is voltage signal, feedback signal is voltage signal. 

b) Current series: Output signal is current signal, feedback signal is voltage signal. 

c) Current shunt: Output signal is current signal, feedback signal is current signal 

d) Voltage shunt: Output signal is voltage signal, feedback signal is current signal. 

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