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

Q1: A current controlled current source (CCCS) has an input impedance of 10 Ω and output impedance of 100 kΩ. When this CCCS is used in a negative feedback closedloop with a loop gain of 9, the closed loop output impedance is (2019)
(a) 10 Ω
(b) 100 Ω
(c) 100 kΩ
(d) 1000 kΩ
Ans: (d)
Sol: "CCCS" (Current controlled current source amplifier)
Given, Z₀ = 100kΩ
Loop gain, Aβ = 9
Z_0F = Z₀[1 + Aβ] (High impedance CS)
= 100kΩ[1 + 9]
= 100kΩ × 10
= 1000kΩ

Q2: A hysteresis type TTL inverter is used to realize an oscillator in the circuit.
If the lower and upper trigger level voltages are 0.9 V and 1.7 V, the period (in ms), for which output is LOW, is _____. (SET-3(2014))
(a) 0.32
(b) 0.63
(c) 0.82
(d) 0.98
Ans: (b)
Sol: Discharging curve,
V_c(t) = 0 − (0−1.7)e^−t/RC
At, t = T₂
V_c(t) = 0.9V
0.9 = 1.7e^−t/RC
0.63 = T₂
T₂ = 0.63ms

Q3: An oscillator circuit using ideal op-amp and diodes is shown in the figure.
The time duration for +ve part of the cycle is Δt₁ and for -ve part is Δt₂. The value of   will be ______. (SET-2 (2014))
(a) 0.6
(b) 0.8
(c) 2
(d) 2.4
Ans: (b)
Sol: This circuit is a stable multivibrator (or) free running oscillator.
When V₀ = +V_sat
V_c = V_final + (V_inital − V_final)e^−t/RC
In time t = Δt₁
V_c = V_UTP;  V_initial = V_LTP; V_final = +V_sat
e^Δt1/RC = 2....(i)
In time t = Δt₂
Vc = V_LTP; V_initial = V_UTP; V_final = −V_sat
From equation (i) and (ii),

Q4: In the Wien Bridge oscillator circuit shown in figure, the bridge is balanced when (SET-1(2014))
(a)

(b)
(c)
(d)
Ans: (c)
Sol: 

Q5: In the feedback network shown below, if the feedback factore k is increased, then the (2013)
(a) The input impedance increases and output impedance decreases
(b) The input impedance increases and output impedance also increases
(c) The input impedance decreases and output impedance also decreases
(d) The input impedance decreases and output impedance increases
Ans:

Q6: The typical frequency response of a two-stage direct coupled voltage amplifier is as shown in figure (2005)
(a) (b) (c) (d) Ans: (b)

Q7: The feedback used in the circuit shown in figure can be classified as (2004)
(a) shunt-series feedback
(b) shunt-shunt feedback
(c) series-shunt feedback
(d) series-series feedback
Ans: 

Q8: The output voltage (V₀) of the Schmitt trigger shown in figure swings between +15V and -15V. Assume that the operational amplifier is ideal. The output will change from +15V to -15V when the instantaneous value of the input sine wave is (2002)
(a) 5 V in the positive slope only
(b) 5 V in the negative slope only
(c) 5 V in the positive and negative slopes
(d) 3 V in the positive and negative slopes
Ans: 

Q9: For the oscillator circuit shown in figure, the expression for the time period of oscillation can be given by (where τ = RC) (2001)
(a) τ ln 3
(b) 2τ ln 3
(c) $\tau$τ ln 2
(d) 2τ ln 2
An