Summary of Effect of Negative Feedback on Amplifier

Summary of Effect of Negative Feedback on Amplifier

• Table 9.10.1 summarizes the effect of negative feedback on amplifier.

 

Ex. 9.10.1 Determine the gain with feedback for the amplifier with open loop gain of 300 and feedback factor of 0.1.

AU : ECE : Dec.-09, Marks 2

Sol.

 

Ex. 9.10.2 The gain and distortion of an amplifier are 100 and 4 % respectively. If a negative feedback with P = 0.3 is applied, find the new distortion in the system.

AU : ECE : Dec.-04, Marks 5

Sol.:  D = 1 + β A_v = 1 + 0.3 × 100= 31

Distortion =         4% / 31 =  0.129 %

 

Ex. 9.10.3 The distortion in an amplifier is found to be 3 %, when the feedback ratio of negative feedback amplifier is 0.04. When the feedback is removed, the distortion becomes 15 %. Find the open loop and closed loop gain.

AU : ECE : May-05, Marks 2

Sol. :

Distortion with feedback = Distortion without feedback / D

3 % = 15% / D

D = 5

we have, D = 1 + A_v  β

5 = 1 + A_v × 004

A_v  = 100

A_vf = A_v / 1 + A_v β = A_v / D = 100 / 5 = 20

 

Ex. 9.10.4 A voltage series feedback amplifier has a voltage gain with feedback as 83.33 and feedback ratio as 0.01. Calculate the voltage gain of the amplifier without feedback.

AU : ECE : Dec.-06, Marks 2

Sol. : Given : A_vf = 83.33 β = 0.01

We know that,

A_vf  = A / 1 + A β

83.33 = A / 1 + 0.01 A

83.33 + 0.8333 A = A

A = 500

The voltage gain of the amplifier without feedback is 500.

 

Ex. 9.10.5 An amplifier has a midband gain of 125 and a bandwidth 250 kHz.

1) If 4 % negative feedback is introduced, find the new bandwidth and gain.

2) If the bandwidth is to be restricted to 1 MHz find the feedback ratio.

AU : ECE : Dec.-06, 11, Marks 6

Sol.

 

Ex. 9.10.6 An amplifier has a voltage gain of 1000. With negative feedback, the voltage gain reduces to 10. Calculate the fraction of the output, that is feedback to the input.

AU : ECE : May-07, Marks 2

Sol. : Given : A_v = 1000, A_vf = 10

A_vf = A / 1+ Aβ

10 = 1000/ 1 + 1000 × β

β = 0.099

 

Ex. 9.10.7 An amplifier with negative feedback give an output of 12.5 V with an input of 1.5 V. When feedback is removed, it requires 0.25 V input for the same output. Find

1) The value of voltage gain without feedback

2) Value of feedback β if the input and output are in phase and β is real.

Sol. :

Ex. 9.10.8 An amplifier, without feedback, has a voltage gain of 400, lower cut-off frequency fi = 50 Hz, upper cut-off frequency f2 = 200 kHz and a distortion of 10 %. Determine the amplifier voltage, gain, lower cut-off frequency and upper cut-off frequency and distortion, when a negative feedback is applied with feedback ratio of

Sol. :

 

Ex. 9.10.9 An amplifier has a mid frequency gain of 100 and a bandwidth of 200 kHz.

1. What will be the new bandwidth and gain, if 5 % negative feedback is introduced ?

2. What should be the amount of feedback, if the bandwidth is to be restricted to 1 MHz ?

Sol :

 

Ex. 9.10.10 An amplifier has a voltage gain of 4000. It's input impedance is 2 K and output impedance is 60 K. Calculate the voltage gain, input and output impedance of the circuit is 5 % of the feedback is fed in the form of series negative voltage feedback.

Sol. : The voltage gain with feedback can be given as

In a voltage series feedback input resistance with feedback is given as

In a voltage series feedback output resistance with feedback is given as

 

Ex. 9.10.11 An amplifier is required with a voltage gain of 100 which does not very by more that 1 %. If it is to use negative feedback with a basic amplifier the voltage gain of which can vary by 20 %, find the minimum voltage gain required and the feedback factor.

Sol. : Given A_vf = 100

 

Ex. 9.10.12 A single stage RC coupled amplifier has a midhand gain of 1000 is made into a negative feedback amplifier by feeding 10 % of the output voltage in series with input opposing.

i) What is the ratio of half power frequencies with feedback to those without feedback ?

ii) If f_L = 20 Hz and f_H = 50 kHz for the amplifier without feedback, find the corresponding values after feedback is incorporated.

Sol. : A_v = 1000 and β = 0.1

 

Ex. 9.10.13 An amplifier with an open loop voltage gain of 1,000 delivers 10 W of output power at 10 percent second harmonic distortion when the input single is 10 mV. If 40 dB negative voltage series feedback is applied and the output power is to remain at 10 W, determine : i) The required input signal ii) The percent harmonic distortion. Sol. : i) 40 dB negative feedback means,

Sol .:

When the amplifier delivers 10 W, its output voltage is

V_o = A_vV_s = 1000 × (10 × l0^-3) = 10 V.

If the output power is to remain at 10 W, the output voltage also must remain at 10 V. Hence, the input signal required when feedback is applied will be,

V_s = V_o / A_f = 10 V / 10 = 1V

ii) The distortion of the amplifier with feedback will be reduced by feedback factor, i.e. by |1 + Aβ| = 100. Hence,

D_f = 10% / 100 = 0.1 %

 

Ex. 9.10.14 An amplifier has an open loop gain of 100, and its lower and upper cut-off frequency of 100 Hz and 100 kHz respectively. A feedback network with feedback factor of 0.99 is connected to the amplifier. Calculate the new lower and upper cutoff frequencies.

Sol. :

Examples for Practice

Ex. 9.10.15 A feedback amplifier has an open loop gain of 600 and feedback factor β = 0.01. Find the closed loop gain with negative feedback.

AU : ECE : Dec.-04, Marks 2

[Ans. : 85.714]

Ex. 9.10.16 An amplifier has mid-band voltage gain ( frimid) °f 1000 withff = 50 Hz and = 50 kHz, if 5 % feedback is applied then calculate gain ff, and ff[ with feedback

[Ans. : f_Lf = 0.98 Hz, f_Hf = 2.55 MHz]

Ex. 9.10.17 An amplifier with open loop gain of A = 2000 ± 150 is available. It is necessary to have the amplifier whose voltage gain varies by not more than ± 0.2 %. Calculate P and Af. 

[Ans. : β = 0.01825, A_f = 53.33]

Ex. 9.10.18 An RC coupled amplifier has a voltage gain of 1000, ff = 50 Hz, f2 = 200 kHz and a distortion of 5 %. With feedback find Ayf, BWf when P = 0.01 is applied.

AU ; ECE : May-08, Marks 5

[Ans. : A_Vf = 90.9, BW_f = 2.2 MHz, Distortion = 0.4545 %]

Review Questions

1. Compare the four types of feedback topologies with respect to basic amplifier characteristics.

AU : ECE : Hay-06, Harks 6

2. Discuss the effect of negative feedback with respect to closed loop gain, bandwidth and distortion.

AU : ECE : Dec.-06, Marks 5