SECTION A – Short Answer Type Questions (2 Marks each)
(a) Explain the Role of Capacitor Used in IC 741.
In the IC 741 operational amplifier, capacitors play an essential role in frequency compensation and stabilization.
Functions:
Miller Capacitor (Compensation Capacitor):
A small capacitor (≈30 pF) is connected between the collector and base of the second-stage transistor.
It introduces a dominant pole, ensuring the amplifier’s gain decreases at high frequencies, thereby enhancing stability.
Bypass Capacitors:
Filter unwanted AC components from the DC supply, improving signal integrity.
Result: The capacitor ensures phase margin improvement, preventing oscillations and maintaining stable operation of the op-amp.
(b) Define Slew Rate and Write the Value of Slew Rate for IC 741.
Slew Rate (SR):
The maximum rate of change of output voltage per unit time in an op-amp is called the Slew Rate.
SR=dVoutdtSR = \frac{dV_{out}}{dt}SR=dtdVout
It represents how fast the output can respond to a rapid change in input signal.
For IC 741:
SR=0.5 V/μsSR = 0.5 \, V/\mu sSR=0.5V/μs
Significance:
Affects high-frequency performance and distortion.
Higher slew rate → better response to fast signals (important in pulse amplifiers and waveform generators).
SECTION B – Long Answer Type Questions (10 Marks each)
(a) Explain the Working of PLL (Phase-Locked Loop) with Its Block Diagram and Applications.
Definition:
A Phase-Locked Loop (PLL) is a feedback control system that synchronizes the output frequency and phase of a voltage-controlled oscillator (VCO) with an input reference signal.
Block Diagram Components:
Phase Detector (PD): Compares input signal phase with VCO output and generates an error voltage.
Low Pass Filter (LPF): Removes high-frequency components from the error signal.
Voltage Controlled Oscillator (VCO): Adjusts its output frequency according to the error voltage.
Feedback Path: Feeds VCO output back to PD.
Working:
If the phase or frequency differs, PD generates an error voltage.
LPF smooths this voltage, which controls the VCO to adjust output frequency until both are locked in phase.
Applications:
Frequency synthesizers. FM demodulation.
Motor speed control. Data communication synchronization.
Equation:
fout=fin±nfreff_{out} = f_{in} \pm n f_{ref}fout=fin±nfref
where nnn = integer multiple in frequency multiplication.
(b) Discuss the Temperature Compensation of Logarithmic Amplifier.
A logarithmic amplifier (log amp) produces an output voltage proportional to the logarithm of the input voltage. However, it is highly sensitive to temperature variations due to the thermal dependence of semiconductor junctions.
Temperature Compensation Method:
Matched Diode Compensation:
Two diodes or transistors are used with identical temperature characteristics.
One diode is in the feedback loop, and the other forms a compensating network.
Since both experience the same temperature change, their voltage variations cancel out.
Mathematical Relation:
Vout=Klog(VinIsR)V_{out} = K \log\left(\frac{V_{in}}{I_s R}\right)Vout=Klog(IsRVin)
where IsI_sIs (saturation current) changes with temperature.
Applications:
Audio level measurement Radar and communication systems.
Signal compression.
SECTION C – Very Long Answer Type Questions (10 Marks each)
(a) Explain the Operation of Monostable Multivibrator Using 555 Timer and Design a Pulse Generator.
A monostable multivibrator using a 555 timer has one stable state and one quasi-stable state.
Circuit Description:
The timer is configured in monostable mode by connecting a resistor (R) and capacitor (C) externally.
A negative trigger pulse applied to pin 2 sets the output high for a time period TTT, after which it returns to its stable (low) state.
Time Duration of Output Pulse:
T=1.1 R CT = 1.1 \, R \, CT=1.1RC
Working Steps:
Initially, output = LOW and capacitor = discharged.
A negative trigger pulse (<1/3 Vcc) triggers output HIGH.
Capacitor starts charging through R.
When capacitor voltage reaches 2/3 Vcc, output returns LOW.
Applications:
Pulse width modulation (PWM).
Timer circuits.
Missing pulse detectors.
(b) Explain the Working of Voltage Controlled Oscillator (VCO).
A Voltage Controlled Oscillator (VCO) generates an output signal whose frequency is directly proportional to an applied control voltage.
Basic Principle:
As control voltage increases, output frequency also increases.
Implemented using 555 timer, varactor diodes, or op-amps.
Block Diagram Components:
Integrator: Converts control voltage into a linearly varying voltage.
Schmitt Trigger: Generates square wave output when threshold levels are reached.
Feedback Network: Maintains oscillation continuity.
Mathematical Expression:
fout=12RC(Vcontrol)f_{out} = \frac{1}{2 R C (V_{control})}fout=2RC(Vcontrol)1
Applications:
Phase-Locked Loops (PLLs). Function generators. FM modulation.
