250+ TOP MCQs on Operational Amplifier Internal Circuit

Linear Integrated Circuit Multiple Choice Questions on “Operational Amplifier Internal Circuit – 1”.

1. Which is not the internal circuit of operational amplifier?
A. Differential amplifier
B. Level translator
C. Output driver
D. Clamper

Answer: D
Clarification: Clamper is an external circuit connected at the output of Operational amplifier, which clamp the output to desire DC level.

2. The purpose of level shifter in Op-amp internal circuit is to
A. Adjust DC voltage
B. Increase impedance
C. Provide high gain
D. Decrease input resistance

Answer: A
Clarification: The gain stages in Op-amp are direct coupled. So, level shifter is used for adjustment of DC level.

3. How a symmetrical swing is obtained at the output of Op-amp
A. Providing amplifier with negative supply voltage
B. Providing amplifier with positive voltage
C. Providing amplifier with positive& negative voltage
D. None of the mentioned

Answer: C
Clarification: For example, consider a single voltage supply +15v. During positive half cycle the output will be +5v and -10v during negative half cycle.
Therefore, the maximum peak to peak output swing, -5v (-10v) = -15v (Asymmetrical swing).
So, to get symmetrical swing both positive and negative supply voltage with bias point fixed suitably is required.

4. What is the purpose of differential amplifier stage in internal circuit of Op-amp?
A. Low gain to differential mode signal
B. Cancel difference mode signal
C. Low gain to common mode signal
D. Cancel common mode signal

Answer: D
Clarification: Any undesired noise, common to both of the input terminal is suppressed by differential amplifier.

5. Which of the following is not preferred for input stage of Op-amp?
A. Dual Input Balanced Output
B. Differential Input Single ended Output
C. Cascaded DC amplifier
D. Single Input Differential Output

Answer: C
Clarification: Cascaded DC amplifier suffers from major problem of drift of the operating point, due to temperature dependency of the transistor.

6. What will be the emitter current in a differential amplifier, where both the transistor are biased and matched? (Assume current to be I_Q)
A. I_E = I_Q/2
B. I_E = I_Q
C. I_E = (I_Q)²/2
D. I_E = (I_Q)²

Answer: A
Clarification: Due to symmetry of differential amplifier circuit, current I_Q divides equally through both transistors.

7. From the circuit, determine the output voltage (Assume α_F=1)

A. V_O1=3.9v , V_O2=12v
B. V_O1=12v , V_O2=3.9v
C. V_O1=12v , V_O2=0v
D. V_O1=3.9v , V_O2=-3.9v

Answer: B
Clarification: The voltage at the common emitter ‘E’ will be -0.7v, which make Q₁ off and the entire current will flow through Q₂.
⇒ V_O1 = VCC V_O2= VCC-α_F×I_Q×RC,
⇒ V_O1 = 12v , V_O2=12v-1×3mA×2.7k = 3.9v.

8. At what condition differential amplifier function as a switch
A. 4V_Td T
B. -2V_T ≤ V_d ≤ 2V_T
C. 0 ≤ V_dT
D. 0 ≤ V_d ≤ 2V_T

Answer: A
Clarification: For V_d > 4V_T, the output voltage are V_O1 = VCC, V_O2= VCC-α_F I_QRC. Therefore, a transistor Q₁ will be ON and Q₂ will be OFF. Similarly for V_d> -4V_T, both transistors Q₂ & Q₁ will be ON.

9. For V_d > ±4V_T, the function of differential amplifier will be
A. Switch
B. Limiter
C. Automatic gain control
D. Linear Amplifier

Answer: B
Clarification: At this condition, input voltage of the amplifier is greater than ±100mv and thus acts as a limiter.

10. Change in value of common mode input signal in differential pair amplifier make
A. Change in voltage across collector
B. Slight change in collector voltage
C. Collector voltage decreases to zero
D. None of the mentioned

Answer: A
Clarification: In differential amplifier due to symmetry, both transistors are biased and matched. Therefore, Voltage at each collector will be same.

11. Find collector current I_C2, given input voltages are V₁=2.078v & V₂=2.06v and total current I_Q=2.4mA. (Assume α=1)

A. 0.8mA
B. 1.6mA
C. 0.08mA
D. 0.16mA

Answer: A
Clarification: Collector current, I_C2=α_F×I_Q/(1+e^V_d⁄V_T),
V_T = Volts equivalent of temperature = 25mv,
⇒ V_d = V₁-V₂ =2.078v-2.06v=0.018v (equ1)
Substituting equation 1,
⇒ V_d/V_T = 0.018v/25mv = 0.72v (equ2)
Substituting equation 2,
⇒ I_C2= 1×2.4mA/(1+e^0.72) = 2.4mA/(1+2.05) = 0.8mA.

12. A differential amplifier has a transistor with β₀= 100, is biased at I_CQ = 0.48mA. Determine the value of CMRR and A_CM, if RE =7.89kΩ and RC = 5kΩ.
A. 49.54 db
B. 49.65 d
C. 49.77 db
D. 49.60 db

Answer: B
Clarification: Differential mode gain, A_DM= -g_mRC and Common mode gain,
⇒ A_CM= -(g_mRC./(1+2g_mRE)
(for β₀≫1).
Substituting the values,
⇒ g_m= I_CQ/V_T = 0.48mA/25mv=19.2×10^-3Ω^-1
⇒ A_DM= -g_m×RC= -19.2×10^-3Ω^-1×5kΩ= -96
⇒ A_CM= -(g_mRC./(1+2g_mRE)= -(19.2×10^-3Ω^-1×5kΩ) /(1+2×-⇒ 19.2×10^-3Ω^-1×7.89kΩ) = -0.3158
CMRR = -96/-0.3158= 303.976
=20log⁡303.976
=49.65db