JEE Main 2022 · 26 June · evening · Single correct · +4 / −1
The positive feedback is required by an amplifier to act an oscillator. The feedback here means :
- (a) External input is necessary to sustain ac signal in output.
- (b) A portion of the output power is returned back to the input.
- (c) Feedback can be achieved by LR network.
- (d) The base-collector junction must be forward biased.
Answer and solution
Answer (b) A portion of the output power is returned back to the input.
Solution
Physics sir · 4 steps
Feedback just means sending some of the output back into the input, in phase, so the oscillation feeds itself.
- What feedback is. In any amplifier, feedback means a part of the output is taken and returned to the input. It is called positive feedback when the returned signal is in phase with the input, so it adds to it.
- Why oscillators need it. An oscillator gives an ac output with no external input. The in-phase signal fed back from the output replaces the input. Once the loop gain reaches 1, each cycle drives the next and the oscillation keeps going on its own.
- Check the rest. Option (a) contradicts what an oscillator is. Option (c) names a circuit, not the meaning of feedback. In practice the frequency is set by an LC tank, with feedback through inductive coupling or an RC network. Option (d) is wrong because the transistor works in the active region, where the base-collector junction is reverse biased.
- So. Feedback means a portion of the output power is returned back to the input, which is option (b).
The idea to keep. An oscillator is an amplifier that supplies its own input: part of the output is fed back in phase, and with enough gain the oscillation sustains itself with no external signal.
Answer from the NTA final answer key. Solution by PrepWiser.