Practice Problems: Amplifiers

Table of Contents
1. Question 1
2. Question 2
3. Question 3
4. Question 4
5. Question 5
6. Question 6
7. Question 7
8. Question 8
9. Question 9
10. Question 10
11. Question 11
12. Question 12
13. Question 13
14. Question 14
15. Question 15
16. Question 16
17. Question 17
18. Question 18
19. Question 19
20. Question 20
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Question 1

An audio systems engineer is designing a common-emitter BJT amplifier for a public address system. The amplifier circuit has the following parameters:
Base current (I_B) = 25 μA
Collector current (I_C) = 2.5 mA
Collector-emitter voltage (V_CE) = 8 V
What is the DC current gain (β) of the transistor?
(a) 50
(b) 75
(c) 100
(d) 125

Question 2

A communications engineer is analyzing a common-source MOSFET amplifier with the following specifications:
Drain current (I_D) = 4 mA
Gate-source voltage (V_GS) = 2.5 V
Threshold voltage (V_TH) = 1.0 V
Device transconductance parameter (k_n) = 2 mA/V²
If the amplifier operates in saturation, what is the width-to-length ratio (W/L) of the MOSFET?
(a) 2.67
(b) 3.56
(c) 4.44
(d) 5.33

Question 3

A design engineer is working on a voltage amplifier with a non-inverting op-amp configuration. The circuit specifications are:
Feedback resistor (R_f) = 100 kΩ
Input resistor (R_in) = 10 kΩ
Input signal voltage (V_in) = 0.5 V
What is the output voltage (V_out) of this amplifier?
(a) 5.0 V
(b) 5.5 V
(c) 6.0 V
(d) 6.5 V

Question 4

An RF engineer is designing a common-emitter amplifier stage with the following circuit parameters:
Collector resistor (R_C) = 3.3 kΩ
Emitter resistor (R_E) = 1.0 kΩ
AC short circuit across R_E (bypass capacitor present)
Transconductance (g_m) = 40 mS
What is the voltage gain (A_v) of this amplifier?
(a) -66
(b) -99
(c) -132
(d) -165

Question 5

A power amplifier designer is evaluating a Class A power amplifier with these operating conditions:
DC supply voltage (V_CC) = 24 V
Quiescent collector current (I_CQ) = 500 mA
Output power delivered to load (P_out) = 3.6 W
What is the efficiency (η) of this Class A amplifier?
(a) 15%
(b) 20%
(c) 25%
(d) 30%

Question 6

An instrumentation engineer is designing an inverting op-amp amplifier for a sensor interface circuit. The design requirements are:
Desired voltage gain (A_v) = -15
Input resistor (R_in) = 20 kΩ
Input voltage (V_in) = -0.3 V
What feedback resistor (R_f) value is needed to achieve this gain?
(a) 200 kΩ
(b) 250 kΩ
(c) 300 kΩ
(d) 350 kΩ

Question 7

A circuit designer is analyzing a common-drain (source follower) MOSFET amplifier with the following parameters:
Transconductance (g_m) = 5 mS
Source resistor (R_S) = 2.2 kΩ
Output resistance (r_o) = 50 kΩ
What is the voltage gain (A_v) of this source follower circuit?
(a) 0.846
(b) 0.892
(c) 0.917
(d) 0.956

Question 8

A test engineer is characterizing a two-stage cascaded amplifier system where:
First stage voltage gain (A_v1) = 25
Second stage voltage gain (A_v2) = 40
Input signal (V_in) = 10 mV
What is the overall voltage gain in decibels (dB)?
(a) 54 dB
(b) 60 dB
(c) 66 dB
(d) 72 dB

Question 9

A broadband amplifier engineer needs to determine the input impedance of a common-emitter amplifier with the following characteristics:
Base biasing resistors: R₁ = 100 kΩ, R₂ = 50 kΩ
Current gain (β) = 150
Emitter resistor (R_E) = 1.5 kΩ
AC thermal resistance (r_e) = 26 Ω
What is the approximate input impedance (Z_in) seen from the base terminal (ignoring biasing resistors)?
(a) 3.90 kΩ
(b) 228.9 kΩ
(c) 3.9 kΩ
(d) 229 kΩ

Question 10

A communications system engineer is working on a differential amplifier with the following specifications:
Differential mode gain (A_d) = 120
Common mode gain (A_c) = 0.3
Input differential voltage (V_d) = 5 mV
What is the Common Mode Rejection Ratio (CMRR) in decibels?
(a) 48 dB
(b) 52 dB
(c) 56 dB
(d) 60 dB

Question 11

A power electronics engineer is designing a Class B push-pull amplifier with these parameters:
DC supply voltage (V_CC) = 30 V
Load resistance (R_L) = 8 Ω
Peak output voltage (V_om) = 24 V
What is the maximum theoretical efficiency of this Class B amplifier?
(a) 68.5%
(b) 72.0%
(c) 75.5%
(d) 78.5%

Question 12

An analog IC designer is calculating the small-signal transconductance of a bipolar transistor operating at:
Collector current (I_C) = 2.6 mA
Temperature (T) = 300 K
Thermal voltage (V_T) = 26 mV at 300 K
What is the transconductance (g_m) of this transistor?
(a) 50 mS
(b) 75 mS
(c) 100 mS
(d) 125 mS

Question 13

A video amplifier designer is evaluating a common-base BJT amplifier with:
Collector resistor (R_C) = 4.7 kΩ
Emitter resistance (r_e) = 25 Ω
Current gain (α) = 0.99
Load resistance (R_L) = 10 kΩ
What is the voltage gain (A_v) of this amplifier (considering R_C || R_L)?
(a) 126
(b) 128
(c) 130
(d) 132

Question 14

A control systems engineer is designing a summing amplifier using an op-amp with:
Input resistor R₁ = 10 kΩ with V₁ = 1.0 V
Input resistor R₂ = 20 kΩ with V₂ = 0.8 V
Input resistor R₃ = 40 kΩ with V₃ = 1.2 V
Feedback resistor R_f = 20 kΩ
What is the output voltage (V_out)?
(a) -3.4 V
(b) -3.6 V
(c) -3.8 V
(d) -4.0 V

Question 15

An audio engineer is analyzing a JFET common-source amplifier with the following parameters:
Drain resistor (R_D) = 2.2 kΩ
Source resistor (R_S) = 680 Ω (bypassed)
Transconductance (g_m) = 4.5 mS
Drain resistance (r_d) = 80 kΩ
What is the voltage gain (A_v) of the amplifier?
(a) -8.8
(b) -9.4
(c) -9.9
(d) -10.5

Question 16

A medical device engineer is designing a precision instrumentation amplifier with three op-amps. The input stage specifications are:
Gain resistor (R_G) = 1 kΩ
Input stage resistors (R₁) = 50 kΩ each
Output stage: R₂ = 100 kΩ, R₃ = 100 kΩ
What is the overall voltage gain of this instrumentation amplifier?
(a) 99
(b) 101
(c) 199
(d) 201

Question 17

A wireless systems engineer is determining the 3-dB bandwidth of an amplifier with the following frequency response:
Lower cutoff frequency (f_L) = 50 Hz
Upper cutoff frequency (f_H) = 2.5 MHz
Midband gain (A_vm) = 200
What is the bandwidth (BW) of this amplifier?
(a) 2.45 MHz
(b) 2.50 MHz
(c) 2.55 MHz
(d) 2.60 MHz

Question 18

A microwave engineer is analyzing a cascode amplifier configuration with:
Lower transistor transconductance (g_m1) = 50 mS
Upper transistor transconductance (g_m2) = 48 mS
Load resistor (R_L) = 1.8 kΩ
Lower transistor output resistance (r_o1) = 40 kΩ
What is the approximate voltage gain (A_v) assuming high output resistance for the upper transistor?
(a) -84
(b) -88
(c) -90
(d) -94

Question 19

A sensor interface designer is configuring a transimpedance amplifier (current-to-voltage converter) with:
Feedback resistor (R_f) = 500 kΩ
Input current from photodiode (I_in) = 12 μA
Op-amp offset voltage negligible
What is the output voltage (V_out)?
(a) -4.8 V
(b) -5.4 V
(c) -6.0 V
(d) -6.6 V

Question 20

An automotive electronics engineer is evaluating a Darlington pair amplifier configuration with:
First transistor current gain (β₁) = 100
Second transistor current gain (β₂) = 80
Base current of first transistor (I_B1) = 5 μA
Collector resistor (R_C) = 2.7 kΩ
What is the total collector current (I_C) in the Darlington configuration?
(a) 38 mA
(b) 40 mA
(c) 42 mA
(d) 44 mA