# Linear Integrated Circuit Questions and Answers – Operational Amplifier Internal Circuit – 3

This set of Linear Integrated Circuit Questions and Answers for Freshers focuses on “Operational Amplifier Internal Circuit – 3”.

1. Determine the early voltage, if the output resistance is 2.5×2kΩ and input current is 2mA
a) 9.8v
b) 5.6v
c) 7.8v
d) 10v

Explanation: Output resistance, Ro=VA/Iref
⇒ VA = Ro×Iref =2.5×2kΩ×2mA =10v.

2. In practical application of current mirror, early voltage is assumed to be
a) Infinite
b) Zero
c) Unity
d) None of the mentioned

Explanation: Early voltage is assumed to be infinity, so that output resistance tend to infinity and the output current is constant.

3. A widlar current source is used
a) to get low value of current
b) to get high value of CMRR
c) to get low voltage of gain
d) to get high value of Output

Explanation: In the widlar current source Re is added to emitter lead of transistor, which consequently results in smaller output current value.

4. What will be the value of emitter resistance in widlar current source for output current 10mA, having Iref=2.7A
a) 67/(1+1/β)Ω
b) 13/(1+1/β)Ω
c) 14/(1+1/β)Ω
d) 1.36/(1+1/β)Ω

Explanation: Emitter resistor, RE=VT/(1+1/β) Iref×ln(Io/Iref)
⇒ RE= 0.025/(1+1/β)10mA × ln(12.7A/10mA) = 14/(1+1/β)Ω.

5. A current repeater having identical transistor has collector current, IC1 =0.39mA. Find IC2,IC4& IC6
a) 0.39mA, 0.39mA, 0.78mA
b) 0.78mA, 0.39mA, 0.39mA
c) 0.39mA, 0.78mA, 0.39mA
d) None of the mentioned

Explanation: In current repeater, the current IC = IC1 =IC2 =⋯= IC N≅ Iref . Where, N – Number of transistors used in current repeater circuit.
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6. If the reference and collector current are 0.539mA and 0.49mA respectively, how many transistors are used in current repeater circuit? (Assume β =150)
a) 11
b) 14
c) 10
d) 8

Explanation: The current equation is given as IC= Iref×β/(β+1+N)
⇒ 0.49mA= 0.539mA×150/(150+1+N)
⇒ N=14.

7. For the current repeater shown in the circuit, determine IC4 value, Where β = 75. a) 0.035mA
b) 0.028mA
c) 0.04mA
d) 0.052mA

Explanation: The reference current, Iref = VCC-VBE/R1 =(15v-0.7v)/39kΩ = 0.366mA.
⇒ Iref=IC+4×IB
=IC(1+1/β)
∴ IC= Iref×(1+1/β)
=Iref×(1+1/β) = 0.366mA×(1+1/75) =0.347mA
⇒ IC1=IC2=IC3=0.347mA
To determine IC4,
RE=VT/(1+1/β)×IC4×ln(C3/IC4)
⇒ 1.62kΩ = 25mv/(1+1/75)×IC4×ln(0.347mA/IC4)
⇒ IC4=0.035mA (find using trial and error method).

8. The requirements for a good current source is the one in which, (Take Output current – IO and Output resistance – rO )
a) IO independent upon current gain and should be low
b) rO should be very high
c) IO in the circuit should be low
d) IO independent upon current gain and rO should be very high

Explanation: The need for high output resistance current source can be seen because the common mode gain of the differential amplifier can only be reduced by using high resistance current sources.

9. Which current source exhibits a very high output resistance?
a) Simple current mirror
b) Wilson current mirror
c) Widlar current mirror
d) All of the mentioned

Explanation: The output resistance of Wilson current mirror is substantially greater than ≅(β× Output resistance)/2 than Simple or Widlar current source.

10. What will be the overall gain in Darlington circuit, if the individual transistor gain is 200?
a) 10000
b) 40000
c) 8000
d) 1000

Explanation: Overall current gain, β= β1×β2 (Multiplication of current gain of individual transistor)
⇒ β=200×200=40000

11. To increase the input resistance in differential amplifier, replace the transistor by
a) Current mirror
b) Current repeater
c) Darlington pair
d) All of the mentioned

Explanation: Higher value of input resistance can be obtained by using Darlington pair in place of transistor.

12. What is the drawback in using Darlington pair in differential amplifier?
a) Large current gain
b) Output current in milli ampere
c) Gain is proportional to load resistor
d) High offset voltage

Explanation: Due to cascaded stage, Darlington differential amplifier offers higher offset voltage which is two times larger than ordinary two transistor used in differential amplifier.

13. Determine the amount of shift happens in level shifter?
a) Vcc + 0.7v
b) Vcc – 0.7v
c) -0.7v
d) +0.7v

Explanation: Level shifter is basically a simple type emitter follower. Hence, level shifter also act as a buffer to isolate high gain stages from the output stage. Therefore, the amount of shift obtained is
VO – Vi = -VBE = -0.7v.

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