Operational Amplifier Internal Circuit Interview Questions and Answers

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Operational Amplifier Internal Circuit – 2”.

1. How are the arbitrary signal represented, that are applied to the input of transistor? (Assume common mode signal and differential mode signal to be V_CM & V_DM respectively).
a) Sum of V_CM & V_DM
b) Difference of V_CM & V_DM
c) Sum and Difference of V_CM & V_DM
d) None of the mentioned
View Answer

Answer: c
Explanation: In practical situation, arbitrary signal are signal are represented as Sum and Difference of common mode signal and differential mode signal.

2. How the differential mode gain is expressed using ‘h’ parameter for a single ended output?
a) – h_feRC/h_ie
b) 1/2×(h_feRC)/h_ie
c) – 1/2×h_feRC
d) None of the mentioned
View Answer

Answer: b
Explanation: Formula for differential mode gain using ‘h’ parameter model for a single ended output.

3. Find Common Mode Rejection Ration, given g_m =16MΩ^-1, RE=25kΩ
a) 58 db
b) 40 db
c) 63 db
d) 89 db
View Answer

Answer: a
Explanation: Formula for Common Mode Rejection Ration, CMRR= 1+2g_mRE,
⇒ CMRR = 1+(2×16MΩ^-1×25kΩ)
= 801 = 20log⁡801 = 58.07 db.

4. In differential amplifier the input are given as V₁=30sin⁡Π(50t)+10sin⁡Π(25t) , V₂=30sin⁡Π(50t)-10 sin⁡Π(25t), β₀ =200,RE =1kΩ and RC = 15kΩ. Find the output voltages V₀₁, V₀₂ & g_m=4MΩ^-1
a) V₀₁=-60[10 sin⁡Π(25t) ]-6.637[30sin⁡Π(50t) ], V₀₂=60[10 sin⁡Π(25t) ]-6.637[30sin⁡Π(50t) ].
b) V₀₁=-6.637[10 sin⁡Π(25t) ]-60[30sin⁡Π(50t) ], V₀₂=6.637[10 sin⁡Π(25t) ]-60[30sin⁡Π(50t) ].
c) V₀₁=-60[30 sin⁡Π(50t) ]-6.637[10sin⁡Π(25t) ], V₀₂=60[30 sin⁡Π(50t) ]-6.637[10sin⁡Π(25t) ].
d) V₀₁=-6.637[30 sin⁡Π(50t) ]-60[10sin⁡Π(25t) ], V₀₂=6.637[30 sin⁡Π(50t) ]-60[10sin⁡Π(25t) ].
View Answer

Answer: a
Explanation: Differential mode gain, A_DM = -g_m RC,
⇒ A_DM = -4MΩ^-1×15kΩ = 60
⇒ r_Π=β₀/g_m =200/4MΩ^-1 =50kΩ
Common mode gain, A_CM=-β_o×RC/r_Π+(β_O+1)×RE
⇒ A_CM =-200×15kΩ/50kΩ+2(1+200)×1kΩ=-6.637
Common mode signal, V_CM=(V₁+V₂)/2= 30sin⁡Π(50t)
Differential mode signal, V_DM=(V₁-V₂)/2= 10 sin⁡Π(25t)
Output voltages are given as
⇒ V₀₁=A_DM)× V_DM)+ A_CM× V_CM
= -60[10 sin⁡Π(25t)]-6.637[30sin⁡Π(50t)],
⇒ V₀₂=-A_DM× V_DM+ A_CM× V_CM
= 60[10 sin⁡Π(25t)]-6.637[30sin⁡Π(50t)].

5. If the value of Common Mode Rejection Ratio and Common Mode Gain are 40db and -0.12 respectively, then determine the value of differential mode gain
a) 0.036
b) -1.2
c) 4.8
d) 12
View Answer

Answer: d
Explanation: Common mode rejection ratio, CMRR =log^-1×(40/20) = 100
⇒ CMRR =(∣A_DM∣/ ∣A_CM∣)
⇒ ∣A_DM∣ =100×0.12 = 12.

6. To increase the value of CMRR, which circuit is used to replace the emitter resistance Re in differential amplifier?
a) Constant current bias
b) Resistor in parallel with Re
c) Resistor in series with Re
d) Diode in parallel with Re
View Answer

Answer: a
Explanation: Constant current bias offers extremely large resistor under AC condition and thus provide high CMRR value.

7. What is the purpose of diode in differential amplifier with constant current circuit?
a) Total current independent on temperature
b) Diode is dependent of temperature
c) Transistor is depend on temperature
d) None of the mentioned
View Answer

Answer: a
Explanation: The base emitter voltage of transistor (VBE) in constant current circuit by 2.5mv/^oc, thus diode also has same temperature. Hence two variations cancel each other and total current I_Q become in depend of temperature.

8. How to improve CMRR value
a) Increase common mode gain
b) Decrease common mode gain
c) Increase Differential mode gain
d) Decrease differential mode gain
View Answer

Answer: b
Explanation: For a large CMRR value, A_CM should be small as possible.

9. Define total current (I_Q) equation in differential amplifier with constant current bias current
a) I_Q=1/R₃×(VEE/R₁+R₂)
b) I_Q =(VEE×R₂)/(R₁+R₂)
c) I_Q=1/R₃×(VEE×R₂/R₁+R₂)
d) I_Q)=R₃×(VEE/R₁+R₂)
View Answer

Answer : c
Explanation: The equation for total current is obtained by applying Kirchhoff’s Voltage Law to constant current circuit in differential amplifier.

10. Constant current source in differential amplifier is also called as
a) Current Mirror
b) Current Source
c) Current Repeaters
d) All of the mentioned
View Answer

Answer: a
Explanation: The output current is reflection or mirror of the reference input current. Therefore, the constant current source circuit referred as Current Mirror.

11. When will be the mirror effect valid
a) β≫1
b) β=1
c) β<1
d) β≠1
View Answer

Answer: a
Explanation: If value of β is used in the equation, IC=β/(β+2)×I_ref. It almost become unity and the output current become equal to reference current.

12. Calculate the value of reference current and input resistor for current mirror with IC=1.2μA & VCC=12v. Assume β=50.
a) 1.248mA, 9kΩ
b) 1.248mA, 9.6kΩ
c) 1.248mA, 9.2kΩ
d) 1.2mA, 9.6kΩ
View Answer

Answer: a
Explanation: We know that collector current, IC=β/(β+2)×I_ref,
⇒ I_ref=(β+2)/β×IC= (50+2)/50× 1.2μA = 1.248mA
⇒ I_ref=(VCC-VBE)/R₁
⇒ R₁=(12v-07v)/1.248mA = 9.05kΩ.

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