Previous Year Questions- Operational Amplifiers - 2 | Analog and Digital Electronics - Electrical Engineering (EE) PDF Download

Q16: The operational amplifier shown in the figure is ideal. The input voltage (in Volt) is V_i = 2sin(2π × 2000t). The amplitude of the output voltage V_o (in Volt) is _______. (SET-2  (2015))

(a) 0.14
(b) 1.24
(c) 2.25
(d) 2.44
Ans: (b)
Sol: V_i = 2sin(2π × 2000t)
The transfer function of the system is
The input is 2sin⁡(2π × 2000t)
ω = 4000π
V₀ = 2∣H(jω)∣_{ω = 4000π} × sin(4000πt − ϕ)  
Output = 1.245sin(4000πt − 51.46°)
So, amplitude of output is 1.245.

Q17: The op-amp shown in the figure has a finite gain A = 1000 and an infinite input resistance. A step-voltage V_i = 1mV is applied at the input at time t = 0 as shown. Assuming that the operational amplifier is not saturated, the time constant (in millisecond) of the output voltage  V_o is (SET-1 (2015))

(a) 1001
(b) 101
(c) 11
(d) 1
Ans: (a)
Sol: The circuit is
Now, the gain of op-amp is 1000
So, V_o = 1000(V₊ − V₋)
Since V₊ = 0 (grounded)
V_- = V_A
The above circuit can be redrawn as
So, V_o = 1000VA
Now, KCL at node 'A'
V_A − V_i = (1001) × 1000V_AsC
V_A − V_i = s(1001000 × 10⁻⁶)V_A
V_A − s1.001V_A = V_i
Since, V_o = −1000V_A
Since, pole is at (1/1.001), so time constant is approax '1001'.

Q18: Consider the circuit shown in the figure. In this circuit R = 1kΩ, and C = 1μF. The input voltage is sinusoidal with a frequency of 50 Hz, represented as a phasor with magnitude V_i and phase angle 0 radian as shown in the figure. The output voltage is represented as a phasor with magnitude V_o and phase angle δ radian. What is the value of the output phase angle δ (in radian) relative to the phase angle of the input voltage? (SET-1 (2015))
(a) 0
(b) π
(c) $\pi /2$π/2
(d) −π/2
Ans: (d)
Sol: The circuit is
Since circuit has negative feedback, so with help of virtual short V_A = V_B
So, KVL in the loop from A to B given,
So, V_A = −IR
and V_A = V_B
So,

Now,

So, V_o = V_B − IR

Q20: The transfer characteristic of the Op-amp circuit shown in figure is  (SET-3(2014))
(a) (b) (c) (d) Ans:

Q21: An operational amplifier circuit is shown in the figure.
The output of the circuit for a given input v_i is (SET-3 (2014))
(a)

(b)

(c)

(d) +V_sat or −V_sat

Q23: In the figure shown, assume the op-amp to be ideal. Which of the alternatives gives the correct Bode plote for the transfer function  (SET-1 (2014))
(a) (b) (c) (d) Ans:

Q24: Given that the op-amps in the figure are ideal, the output voltage V_o is (SET-1 (2014))
(a) (V₁ − V₂)
(b) 2(V₁ − V₂)
(c) (V₁ − V₂)/2
(d) $(V1+V2)$(V₁ + V₂)
Ans: 

Q25: In the circuit shown below the op-amps are ideal. Then, V_out in Volts is (2013)
(a) 4
(b) 6
(c) 8
(d) 10
Ans: 

Q26: In the circuit shown below what is the output voltage (V_out) if a silicon transistor Q and an ideal op-amp are used? (2013)
(a) -15 V
(b) -0.7V
(c) +0.7V
(d) +15V
Ans:

Q27: The circuit shown is a (2012)
(a) low pass filter with f_3dB =

(b) high pass filter with f_3dB =
(c) low pass filter with f_3dB =
(d) high pass filter with f_3dB =
Ans: (b)
Sol: It is a high pass filter with

Q28: For the circuit shown below,
The CORRECT transfer characteristic is (2011)
(a) (b) (c) (d) Ans:

Q30: Given that the op-amp is ideal, the output voltage V₀ is (2010)
(a) 4V
(b) 6V
(c) 7.5V
(d) 12.12V
Ans: