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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
Z0F = 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,
Vc(t) = 0 − (0−1.7)e−t/RC
At, t = T2
Vc(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.
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)The time duration for +ve part of the cycle is Δt1 and for -ve part is Δt2. The value of  Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE) 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 = +Vsat
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)Vc = Vfinal + (Vinital − Vfinal)e−t/RC
In time t = Δt1
V= VUTP;  Vinitial = VLTP; Vfinal = +Vsat
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)eΔt1/RC = 2....(i)
In time t = Δt2
Vc = VLTP; Vinitial = VUTP; Vfinal = −Vsat
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)From equation (i) and (ii),
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)
Q4: In the Wien Bridge oscillator circuit shown in figure, the bridge is balanced when (SET-1(2014))
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(a) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)

(b) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)
(c) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)
(d) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)
Ans: (c)
Sol: Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)

Q5: In the feedback network shown below, if the feedback factore k is increased, then the (2013)
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(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:
(a)
Sol: The given configuration is a voltage-series feedback configuration.
So, the input impedance increases
Rif = Ri(1 + Aok)
So, the output impedance decreases
RofPrevious Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)


Q6: The typical frequency response of a two-stage direct coupled voltage amplifier is as shown in figure (2005)
(a) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(b) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(c) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(d) Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)Ans:
(b)


Q7: The feedback used in the circuit shown in figure can be classified as (2004)
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(a) shunt-series feedback
(b) shunt-shunt feedback
(c) series-shunt feedback
(d) series-series feedback
Ans: 
(b)
Sol: Equivalent circuit can be drawn with input voltage comparison and current feedback. It is shunt-shunt feedback.

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

Q9: For the oscillator circuit shown in figure, the expression for the time period of oscillation can be given by (where τ = RC) (2001)
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)(a) τ ln 3
(b) 2τ ln 3
(c) ττ ln 2
(d) 2τ ln 2
An
s: (b)
Sol: T = 2RC ln 3 = 2τ ln 3

Q10: An op-amp has an open-loop gain of 105 and an open-loop upper cutoff frequency of 10 Hz. If this op-amp is connected as an amplifier with a closed-loop gain of 100, then the new upper cutoff frequency is (2001)
(a) 10 Hz
(b) 100 Hz
(c) 10 kHz
(d) 100 kHz
Ans: 
(c)
Sol: AOL = 105
f = 10Hz
Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE)f'2 = f2(1 + βAOL)
= 10 × 103Hz
= 10kHz 
The document Previous Year Questions- Oscillators and Feedback Amplifiers | Analog and Digital Electronics - Electrical Engineering (EE) is a part of the Electrical Engineering (EE) Course Analog and Digital Electronics.
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FAQs on Previous Year Questions- Oscillators and Feedback Amplifiers - Analog and Digital Electronics - Electrical Engineering (EE)

1. What are the key differences between an oscillator and an amplifier in electrical engineering?
Ans. An oscillator is a circuit that generates a repetitive waveform, typically a sine or square wave, without requiring an external input signal, while an amplifier increases the amplitude of an input signal. Oscillators are used in applications like clock generation, whereas amplifiers are used to boost weak signals for further processing.
2. How do negative feedback and positive feedback affect the stability of an oscillator?
Ans. Negative feedback enhances the stability of an oscillator by reducing gain and minimizing distortion, leading to a more stable output frequency. In contrast, positive feedback can increase gain, which may lead to instability and oscillation, potentially causing the circuit to oscillate at unintended frequencies.
3. What are the common types of oscillators used in electrical engineering?
Ans. Common types of oscillators include relaxation oscillators (like astable multivibrators), sinusoidal oscillators (such as Colpitts and Hartley), and crystal oscillators. Each type has distinct characteristics and applications, such as generating clock signals or providing stable frequency references.
4. How does the phase shift in a feedback amplifier influence its performance?
Ans. The phase shift in a feedback amplifier is crucial for determining its stability and frequency response. A phase shift of 180 degrees at the feedback point can lead to positive feedback, causing oscillation, while a phase shift of less than 180 degrees ensures negative feedback, stabilizing the amplifier's output.
5. What role do operational amplifiers (op-amps) play in feedback amplifier designs?
Ans. Operational amplifiers are widely used in feedback amplifier designs due to their high gain, high input impedance, and low output impedance. They can be configured for various feedback types (positive or negative) to achieve desired gain, stability, and frequency response in amplifier circuits.
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