One of the criteria for oscillations to occur in an amplifier is that:

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RRB ALP Mechanic Radio & TV 23 Jan 2019 Official Paper (Shift 3)
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  1. the amplifier must have a variable power supply
  2. the loop gain must be slightly greater than unity
  3. the amplifier must be in the cut-off state
  4. the amplifier output impedance must be zero

Answer (Detailed Solution Below)

Option 2 : the loop gain must be slightly greater than unity
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The correct option is 2.

Concept:

  • The output of the oscillator will not depend upon input voltage because there is no input voltage for an oscillator
  • An oscillator is an amplifier circuit with positive feedback in which a part of the output is feedback to the input via a feedback circuit
  • It should satisfy Barkhausen criteria |Aβ|  \(\geq\) 1, where A is the amplifier gain and β is the feedback gain 

 

Barkhausen Criterion 'or' Conditions for Oscillation:

The circuit will oscillate when two conditions, called Barkhausen’s criteria are met. These two conditions are:

  • The loop gain must be unity or greater.
  • The feedback signal feeding back at the input must be phase-shifted by 360° (which is the same as zero degrees). In most of the circuits, an inverting amplifier is used to produce 180° phase-shift and an additional 180° phase shift is provided by the feedback network.
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