Positive Feedback Amplifier — Oscillator


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.

Positive Feedback Amplifier — Oscillator





Fig-1 shows a transistor amplifier with positive feedback.





Positive Feedback Amplifier
Positive Feedback Amplifier




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.





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.





Fig-2




(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.





Our another posts link:





Engineering: Types of Electron Emission ,Solar cellsolar plant for 100 kW Installation.





Education: How to be successful in life,Student and Social Service compositionImportance of Reading Newspaper.





Biographies of famous peopleSTEVE JOBSProphet Muhammad (Sm),Biography of Bill GatesAPJ Abdul Kal.





BCS Preliminary: All Bcs Question, Answer and Solve.





IT solutions: Google Adsense Approval পাওয়ার করণীয়কম্পিউটারের  Speed  বাড়ানোর কিছু উপায়





To visit our Job related website Click Here


Next Post Previous Post
No Comment
Add Comment
comment url