Important Problems in Amplifiers

SOLVED PROBLEMS

Problem 2.1

An audio frequency amplifier is to be designed for operating over a range of 20 HZ and 20 KHz. Calculate the value of input coupling capacitor C₁ if the total series resistance is 10 KΩ

Given:

To Find:

Coupling Capacitor C₁

Solution:

Problem 2.2

For a typical voltage amplifier, the voltage gain = 100 at the cut-off frequencies. Find the maximum voltage gain.

Given:

Voltage gain = 100

To Find:

Maximum voltage gain

Solution:

Problem 2.3

At I_C = 1 mA and V_CE = 10 V, a transistor have C_c = C_b'c = 3 pF, h_fe = 200 and ω_T = 500 µ rad/sec. Calculate g_m, r_b'e, C_e and ω_β.

Given:

To find:

g_m, r_b'e, C_e, ω_β.

Solution:

Problem 2.4

The short circuit CE gain of a transistor at 2 MHz is 20. The value of f_B is 150 KHz. Calculate 1.f_T 2. h_fe 3. Current gain at f = 5 MHz and 50 MHz.

Given:

f = 2 MHz

A₁ = 20

f_B = 150 KHz

To find:

Solution

Problem 2.5

For the FET amplifier, the component values are R_D= 2.2 KΩ R_L = 4.7 ΚΩ, R_G = 10 ΜΩ R_S = 600 Ω, C_gd = 2 pF, C_gs = 4 pF, g_m = 4 mS. Determine the high frequency response of this amplifier.

Given:

To Find:

Frequency response of amplifier

Solution:

Cut off frequency of the input RC network

Problem 2.6

A CE amplifier is driven by a voltage source of internal resistance R_S = 800 Ω and the load impedance is a resistance R_L = 1000 Ω The h-parameters are h_ie = 1 KΩ, h_re = 2 × 10^-4, h_fe = 50, h_oe = 25 μA/V. Compute the current gain A₁, input impedance R_i, voltage gain A_V and output impedance R_o.

Solution:

 

Problem 2.7

A voltage source of internal resistance R_S = 900 Ω drives a CC amplifier using load resistance R_L = 2000Ω. The CE h-parameters are h_ie = 1200 Ω, h_re = 2 x10^-4, h_fe = 60 and h_oe = 25 μA/V. Compute the current gain, input impedance, voltage gain and output impedance.

Solution:

Current gain, A₁ = 1 + h_fe = 1 + 60 = 61

Input impedance,

Problem 2.8

Find the input impedance, output impedance, voltage gain and current gain for the CB circuit shown in Fig.2.35. Assume V_BE = 0.6 V

Solution

Problem 2.9

An emitter follower has circuit parameter R_S = 500Ω, R₁ = R₂ = 50 KΩ, R_L = 2 KΩ, h_fe = 100, h_ie = 1.1 KΩ Determine the input impedance, output impedance, ΚΩ, current gain and voltage gain.

Solution:

1. Input Impedance

2. Output Impedance

3. Current Gain

4. Voltage Gain

AV = 0.9946

Problem 2.10

The hybrid parameters for a transistor in CE configuration are h_ie = 1000 Ω, h_fe = 150, h_re = 1.2 x 10^-4, h_oe = 25 x 10^-6 Ω, R_L= 10 KΩ, RS = 5 KΩ. Compute the values of input impedance, output impedance, current gain and voltage gain.

Solution:

1. Input Impedance

2. Output Impedance

3. Current Gain

4. Voltage Gain

Problem 2.11

The common collector amplifier shown in Fig.2.36 has V_CC = 10 V, R_B = 470 KΩ, R_E = 3.3 KΩ, β is 100. Find the input impedance of the amplifier and overall voltage gain.

Solution:

From Fig.2.36,

Problem 2.12

Calculate the current gain, voltage gain, input and output impedance for the CC amplifier shown in Fig.2.37, h_ic = 1.4 KΩ, h_fe = 100, h_rc = 20 µA/V, h_oc = 20 × 10^-6.

Solution:

Current gain

Input Impedance

Voltage gain

Output Impedance