Positive Feedback Amplifier — Oscillator
A transistor amplifier with proper positive feedback can act as an oscillator i.e., it can generate oscillations without any external signal source.
Fig-1 shows a transistor amplifier with positive feedback.
Remember that a positive feedback amplifier is one that produces a feedback voltage (Vf) that is in phase with the original input signal.
As you can see, this condition is meet in the circuit show in Fig-1.
A phase shift of 180° is produce by the amplifier and a further phase shift of 180° is introduce by feedback network.
Consequently, the signal is shift by 360° and fed to the input i.e., feedback voltage is in phase with the input signal.
(i) We note that the circuit shown in Fig-1 is producing oscillations in the output. However, this circuit has an input signal.
This is inconsistent with our definition of an oscillator i.e., an oscillator is a
circuit that produces oscillations without any external signal source.
(ii) When we open the switch S of Fig-1, we get the circuit show in Fig-2. This means the input signal (Vin) is remove.
However, Vf (which is in phase with the original signal) is still apply to the input signal.
The amplifier will respond to this signal in the same way that it did to Vin i.e., Vf will be amplified and sent to the output.
The feedback network sends a portion of the output back to the input.
Therefore, the amplifier receives another input cycle and another output cycle is produce.
This process will continue so long as the amplifier is turn on.
Therefore, the amplifer will produce sinusoidal output with no external signal source.
The following points may be note carefully :
(a) A transistor amplifer with proper positive feedback will work as an oscillator.
(b) The circuit needs only a quick trigger signal to start the oscillations.
Once the oscillations have started, no external signal source is need.
(c) In order to get continuous undamped output from the circuit, the following condition must be meet :
mvAv = 1
where Av = voltage gain of amplifer without feedback
mv = feedback fraction
This relation is call Barkhausen criterion.
This condition will be explain in the Art-2.
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