Match List - I with List - II. Choose the correct answer from the options given below :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) \(Z_{if}=Z_{i}\left(1+\beta A\right)\)
(A)-(IV), (B)-(I), (C)-(II), (D)-(III)
Two rules settle every entry, and they are entirely independent of one another :
| Half of the name | Refers to | Effect |
|---|---|---|
| Series mixing | Input | Raises Zi by (1 + βA) |
| Shunt mixing | Input | Lowers Zi by (1 + βA) |
| Voltage sampling | Output | Lowers Zo by (1 + βA) |
| Current sampling | Output | Raises Zo by (1 + βA) |
Applying them.
(A) Voltage series — series mixing raises the input impedance, so \(Z_{if}=Z_{i}\left(1+\beta A\right)\), which is (IV).
(B) Voltage shunt — voltage sampling lowers the output impedance, so \(Z_{of}=Z_{o}/\left(1+\beta A\right)\), which is (I).
(C) Current series — current sampling raises the output impedance, so \(Z_{of}=Z_{o}\left(1+\beta A\right)\), which is (II).
(D) Current shunt — shunt mixing lowers the input impedance, so \(Z_{if}=Z_{i}/\left(1+\beta A\right)\), which is (III).
— option 4.
Why series raises and shunt lowers the input impedance. With series mixing the feedback voltage is subtracted in series with the source, so the source must supply a larger voltage to force the same input current — and a larger voltage for the same current is a higher impedance. With shunt mixing the feedback current is subtracted at the input node, so the source must supply more current at the same voltage — a lower impedance.
Why voltage sampling lowers and current sampling raises the output impedance. Voltage sampling makes the amplifier defend its output voltage against changes of load, which is what a low output impedance means. Current sampling makes it defend its output current, which is what a high output impedance means. In each case the feedback holds constant whatever it senses.
This is why the topology is chosen, not accepted. A voltage amplifier wants high input and low output impedance, so it uses voltage-series feedback; a current amplifier wants the opposite and uses current-shunt. The four topologies exist precisely because the two sampling and two mixing choices can be combined independently.
Hence, the correct code is (A)-(IV), (B)-(I), (C)-(II), (D)-(III).
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 ?
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 :
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 :
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 ?
Important Questions from Feedback Amplifier
The effect of negative feedback is to increase the __________ of a series voltage negative feedback amplifier by a factor of (1 + A vβ).
Which of the following improvement is obtained in negative feedback amplifier?
Feedback in an amplifier always helps to ________
What is the effect of current shunt feedback in an amplifier?
- The output impedance of a voltage series feedback is 10 Ω, If the gain of the basic amplifier is 100 and feedback fraction is 0.01, what is the output impedance without feedback?
