Important Solved Problems of Feedback Amplifiers and Oscillators

SOLVED PROBLEMS

Problem 4.1

An amplifier has an open-loop gain of 1000 and a feedback ratio of 0.04. If the open loop gain changes by 10% due to temperature, find the percentage change in gain of the amplifier with feedback.

Solution:

Given:

A = 1000, β = 0.04 and dA/A = 10

The percentage change in gain of the amplifier with feedback is

Problem 4.2

An amplifier has a voltage gain of 400, f₁ = 50Hz, f₂ = 200 KHz and a distortion of 10% without feedback. Determine the amplifier voltage gain f_1f, f_2f and D_f when a negative feedback is applied with feedback ratio of 0.01.

Solution:

Given:

A = 400, f₁ = 50 Hz, f₂ = 200 Hz, D = 10% and β = 0.01

Voltage gain with feedback is

New lower 3 dB frequency

New upper 3 dB frequency

Distortion with feedback

Problem 4.3

A voltage-series negative feedback amplifier has a voltage gain without feedback of A = 500, input resistance R_i = 3 KΩ, output resistance R_o = 20 KΩ and feedback ratio β = 0.01. Calculate the voltage gain A_f, input resistance R_if and output resistance R_of of the amplifier with feedback.

Solution:

Given:

A=500, R_i = 3 KΩ, R_o = 20 KΩ and β = 0.01

Voltage gain, A_f 

Input resistance R_if 

Output resistance R_of 

 

Problem 4.4

An emitter follower has R_S = 600Ω, R_L = 2 KΩ, h_fe = 80 and h_ie = 5 KΩ Calculate A_i, R_i, A_V, R_o and R_of.

Solution:

Problem 4.5

CE amplifier with voltage-shunt feedback, has the following values of circuit parameters, R_S = 600Ω, h_ie = 5 KΩ, h_fe = 80, R_L = 2 KΩ, R_B = 40 ΚΩ Calculate A_V, R_if, A_Vf, R_of and R'_of

Solution:

Problem 4.6

The current series feedback type of transistor has the following parameters: R₁ = 20 KΩ, R₂ = 20 KΩ, h_ie = 2 KΩ, R_L = 1 KΩ, R_e = 100 Ω and h_ie = 80. Calculate A, β, R_if, A_f and the loop gain in dB.

Solution:

Problem 4.7

In a Hartley oscillator L₂ = 0.4 mH and C = 0.004 μF. If the frequency of the oscillator is 120 KHz, find the value of L₁. Neglect the mutual inductance.

Solution:

The frequency of Hartley oscillator is given by

Problem 4.8

In a Hartley oscillator, the value of the capacitor in the tuned circuit is 500PF and the two sections of coil have inductances 38 µH and 12 µH. Find the frequency of oscillations and the feedback factor β.

Solution:

Problem 4.9

In a Colpitts oscillator, C₁ = 0.2 μF and C₂ = 0.02 μF. If the frequency of the oscillator is 10 KHz, find the value of the inductor. Also find the required gain for oscillation.

Solution:

The frequency of the Colpitts oscillator is given by

The voltage gain required to produce oscillation is

Problem 4.10

In an RC phase shift oscillator, if R₁ = R₂ = R₃ is 200 KΩ and C₁ and C₂ and C₃ = 100 PF. Find the frequency of oscillator.

Solution:

Problem 4.11

Determine the frequency of oscillations when a RC phase shift oscillator has R = 10 KΩ, C = 0.01 μF and RC = 2.2 KΩ. Also find the minimum current gain needed for this purpose.

Solution:

The frequency of oscillation is

For sustained oscillations the minimum value of current gain or forward current gain ratio h_fe is

 

Problem 4.12

In a Wien bridge oscillation, if the value of R is 100 KΩ, and frequency of oscillation is 10 KHz, find the value of capacitor C.

Solution:

The operating frequency of a Wien-bridge oscillator is given by

Problem 4.13

A crystal has the following parameters L = 0.5 H, C_S = 0.06 PF, C_P = 1 PF and R = 5 KΩ Find the series and parallel resonant frequencies and a factor of the crystal.

Solution:

(a) The series resonant frequency of the crystal is

(b) The parallel resonant frequency of the crystal