Explanation of Barkhausen Criterion
Barkhausen criterion is that in order to produce continuous undamped oscillations at the output of an
amplifier, the positive feedback should be such that :
mv Av = 1
Once this condition is set in the positive feedback amplifier, continuous undamped oscillations
can be obtained at the output immediately after connecting the necessary power supplies.
(i) Mathematical explanation.
The voltage gain of a positive feedback amplifier is give by;
If mv Av = 1, then Avf → ∞.
We know that we cannot achieve infinite gain in an amplifier. So what does this result infer in
physical terms ?
(Explanation of Barkhausen Criterion) It means that a vanishing small input voltage would give rise to finite (i.e., a definite
amount of) output voltage even when the input signal is zero. Thus once the circuit receives the input
trigger, it would become an oscillator, generating oscillations with no external signal source.
(ii) Graphical Explanation
Let us discuss the condition mν Aν = 1graphically. Suppose the
voltage gain of the amplifier without positive feedback is 100. In order to produce continuous
undamped oscillations, mv Av = 1 or mv × 100 = 1 or mv = 0.01. This is illustrate in Fig-1.
Since the condition mνAν = 1 is met in the circuit shown in Fig-1, it will produce sustained oscillations.
Suppose the initial triggering voltage is 0.1V peak.
Starting with this value, circuit (Aν = 100 ;
mν = 0.01) will progress as follows.
The same thing will repeat for 3rd, 4th cycles and so on. Note that during each cycle, Vf=0.1Vpk and Vout = 10 Vpk. Clearly, the oscillator is producing continuous undamped oscillations.
Note. The relation mν Aν = 1 holds good for true ideal circuits.
However, practical circuits need an mν Aν product that is slightly greater than 1. This is to compensate for power loss (e.g., in resistors) in the circuit.
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