Solutions to Analog Electronics Question Paper
PART-A: Short-Answer Type Questions
Q1. Define CMRR. Why is it significant in amplifiers?
- CMRR (Common-Mode Rejection Ratio):
It is the ratio of differential gain (Ad) to common-mode gain (Acm).
CMRR = 20 log10 (Ad / Acm) (in dB).
- Significance:
A high CMRR indicates the amplifier can reject noise or interference that appears
equally on both input lines, ensuring accurate differential signal amplification.
Q2. List the characteristics of an ideal Op-Amp.
1. Infinite open-loop gain (Av).
2. Infinite input impedance (Zin).
3. Zero output impedance (Zout).
4. Infinite bandwidth.
5. Infinite Common-Mode Rejection Ratio (CMRR).
6. Zero offset voltage (output is zero when inputs are equal).
Q3. Why is MOSFET called a voltage-controlled device?
- In MOSFETs, the current flowing between the drain and source terminals (ID) is
controlled by the voltage applied at the gate terminal (VGS).
- The gate forms a capacitor with the channel, and there is no significant gate current,
making it a voltage-controlled device.
PART-B: Analytical/Problem-Solving Questions
Solutions to Analog Electronics Question Paper
Q4. Explain the Instrumentation amplifier using an Op-Amp and derive the expression
for its output voltage.
1. Working Principle:
An instrumentation amplifier consists of three Op-Amps (two in the input stage and
one in the output stage) and is used for precise, low-noise differential signal
amplification.
2. Circuit:
Input stage amplifies difference between V1 and V2.
Output stage provides further amplification and high input impedance.
3. Derivation:
Voltage across Rg: Vg = (V2 - V1).
Gain of input stage: A = 1 + (2R1 / Rg).
Output voltage: Vout = A * (V2 - V1).
Q5. Explain the RC phase shift oscillator and derive the expression for its frequency.
1. Working Principle:
Uses a combination of an amplifier and an RC feedback network to generate
sinusoidal oscillations.
The RC network produces a phase shift of 180°, and the inverting amplifier adds
another 180°, ensuring a total phase shift of 360°.
2. Frequency of Oscillation:
f = 1 / (2πRC√6)
R and C are the resistance and capacitance in the feedback network.
Q6. Explain the working of a Class 'A' power amplifier with a circuit diagram.
Solutions to Analog Electronics Question Paper
1. Operation:
A Class 'A' amplifier operates in the linear region for the entire input cycle.
It provides high fidelity but is less efficient.
2. Efficiency:
Theoretical maximum efficiency is 25% for resistive load and 50% for
transformer-coupled load.
3. Circuit Diagram:
Consists of a transistor with a biasing circuit, load resistor, and input signal source.
PART-C: Descriptive/Design Questions
Q7. Explain the construction and working of an N-channel enhancement MOSFET. Plot
transfer and output characteristics.
1. Construction:
The MOSFET has a source, gate, and drain terminal.
An N-type channel is formed between the source and drain when a positive voltage is
applied to the gate.
2. Working:
At VGS < Vth, no current flows (OFF state).
At VGS ≥ Vth, current ID increases with VGS.
Saturation occurs when VDS > (VGS - Vth).
3. Characteristics:
Transfer characteristics: ID vs VGS.
Output characteristics: ID vs VDS for different VGS.
Q8. Describe the successive approximation ADC and compare it with other ADC
Solutions to Analog Electronics Question Paper
techniques.
1. Successive Approximation ADC:
Uses a DAC and a successive approximation register (SAR) to find the digital
equivalent of the analog input.
The SAR sets each bit based on the comparison of the DAC output and the input
signal.
2. Comparison:
Advantages: High speed, good resolution.
Disadvantages: Requires precision components, complexity increases with resolution.
Compared to Flash ADC: Slower but uses fewer components.
Compared to Dual-Slope ADC: Faster but less noise immune.
