Negative Feedback in amplifiers : (a) improves signal to noise ratio at the output (b) increases distortion (c) reduces input offset voltage (d) increases bandwidth Options :
(a) and (d) are correct
Statement (b) is the giveaway — it is exactly backwards. Negative feedback reduces distortion; that is one of the principal reasons for using it. With a feedback factor \(D=1+A\beta\), any distortion generated inside the loop is divided by D:
\(D_f=\dfrac{D_{ol}}{1+A\beta}\)
Because the output is compared with the input, any departure from a faithful copy appears in the error signal and is corrected. Statement (b) being false eliminates options 3 and 4 at once.
Statement (d) — correct. Negative feedback widens the bandwidth by the same factor that it reduces the gain, leaving their product fixed:
\(A_f=\dfrac{A}{1+A\beta}, \qquad f_{H,f}=f_H\left(1+A\beta\right)\)
\(A_f\times f_{H,f}=A\times f_H=\text{constant}\)
This is the gain-bandwidth product: a 741 with an open-loop gain of 200,000 and a 5 Hz corner delivers 1 MHz of bandwidth at unity gain, or 100 kHz at a closed-loop gain of ten. Gain is traded for bandwidth on a one-for-one basis.
Statement (a) — correct in the standard sense. Noise generated within the amplifier is attenuated by the same factor \(1+A\beta\) as the distortion, so relative to the signal it falls and the output signal-to-noise ratio improves. (Noise arriving at the input alongside the signal is amplified equally with it and cannot be improved by any amount of feedback — the improvement applies only to noise originating inside the loop.)
Statement (c) — wrong. Negative feedback does not reduce the input offset voltage. Offset is indistinguishable from a genuine input signal, so it is amplified by the full closed-loop gain and appears at the output as
\(V_{out,offset}=V_{os}\left(1+\dfrac{R_f}{R_1}\right)\)
Reducing offset needs trimming, chopper stabilisation or a better-matched input pair — not feedback.
Assemble. (a) and (d) survive, which is option 2.
The complete ledger of what feedback does, every entry scaling by the same factor \(1+A\beta\): gain down, bandwidth up, distortion down, noise (internal) down, gain stability improved, and input and output impedances raised or lowered depending on the topology.
Hence, the correct statements are (a) and (d).
Similar Questions
Match the following lists :
List – I List – II a. Voltage series feedback i. Trans resistance amplifier b. Current series feedback ii. Current shunt feedback c. Current amplifier iii. Trans-conductance d. Voltage shunt feedback iv. Voltage amplifier Codes :
Match the following :
List – I List – II a. voltage shunt negative feedback i. increase of CMRR b. constant current source differential amplifier ii. O/P voltage attenuated by a factor 1/29 c. Phase shift oscillator iii. FSK decoder d. PLL iv. decrease of O/P impedance Codes :
Negative feedback in amplifier results in
1. reduced voltage gain
2. reduced bandwidth
3. increased S/N ratio
4. reduced distortionAn amplifier has open-loop voltage gain of 40. 10 % of negative feedback is effected. What will be the gain with feedback ?
Match List - I with List - II.
List - I (Feedback connection type) List - II (Input/output impedance) (A) Voltage series feedback (I) \(Z_{of}=\dfrac{Z_{o}}{1+\beta A}\) (B) Voltage shunt feedback (II) \(Z_{of}=Z_{o}\left(1+\beta A\right)\) (C) Current series feedback (III) \(Z_{if}=\dfrac{Z_{i}}{1+\beta A}\) (D) Current shunt feedback (IV) \(Z_{if}=Z_{i}\left(1+\beta A\right)\) Choose the correct answer from the options given below :
Read the following statements :
(a) Whatever the mode of feedback be, the gain after negative feedback is \(\dfrac{A}{1+A\beta}\)
(b) A negative feedback reduces the bandwidth of the amplifier.
(c) A negative feedback increases the output impedance
(d) A negative feedback enhances the stability of operationWhich of the above statements are correct ?
Given below are two statements :
Statement I : For negative feedback systems, the open loop gain decreases by a certain factor.
Statement II : Negative feedback systems are better in terms of system stability
In the light of the above statements, choose the correct answer from the options given below :
Important Questions from Feedback Amplifier
Arrange the following amplifiers in the order of maximum achievable -3dB frequency. Assume the transconductance ($g_m$) of the driver transistor (M1) same in all the circuits.
Choose the correct answer from the options given below:
Arrange the following amplifiers in the order of maximum achievable voltage gain. Assume the geometry and overdrive voltage is same for the transistor M1 in all the circuits and $R_D < <r_{o1}$.
choose the correct answer from the options given below:
The voltage gain of a certain amplifier without feedback is 10. With feedback its voltage gain reduces to 5. Determine its feedback factor.
Match the following lists :
List – I List – II a. Voltage series feedback i. Trans resistance amplifier b. Current series feedback ii. Current shunt feedback c. Current amplifier iii. Trans-conductance d. Voltage shunt feedback iv. Voltage amplifier Codes :
Match the following :
List – I List – II a. voltage shunt negative feedback i. increase of CMRR b. constant current source differential amplifier ii. O/P voltage attenuated by a factor 1/29 c. Phase shift oscillator iii. FSK decoder d. PLL iv. decrease of O/P impedance Codes :
