Electronics and Communication questions reward anyone who is comfortable moving between the time domain and the frequency domain. This set covers network theory, analog and digital circuits, signals and systems, control systems, communication systems, and electromagnetics. Numerical solutions keep the units visible at every step, since a dropped factor is the most common reason a correct method still produces a wrong option.
In a common emitter amplifier, the voltage gain is maximum when the load resistance is
Answer: C
Voltage gain AV = gm × RL. Maximum gain occurs when load resistance is much larger than the output impedance of the transistor, limited only by practical considerations.
Q.202Easy
The input impedance of a common base amplifier is
Answer: B
Common base configuration has very low input impedance (typically 20-100 Ω) because the input is at the emitter which is forward biased.
Q.203Easy
For maximum power transfer in an amplifier circuit, the load impedance should be
Answer: B
Maximum power transfer theorem states that maximum power is delivered when load impedance is the complex conjugate of source impedance (ZL = ZS*).
Q.204Medium
The bandwidth of a common emitter amplifier can be increased by
Answer: D
Emitter degeneration without bypass capacitor increases input impedance and reduces gain but significantly increases bandwidth due to negative feedback.
Q.205Medium
In a differential amplifier, the common mode rejection ratio (CMRR) is defined as
Answer: A
CMRR = Ad/Ac, where Ad is differential mode gain and Ac is common mode gain. High CMRR (>80dB typical) is desired to reject common mode noise.
Q.206Easy
Which configuration provides the highest voltage gain among the three BJT amplifier configurations?
Answer: C
Common emitter provides both voltage and current gain with gain values typically 100-1000. CB provides only current gain, CC provides only voltage gain close to 1.
Q.207Medium
The input impedance of a voltage follower (common collector) amplifier is
Answer: B
CC amplifier has very high input impedance (Zin ≈ β·re) making it suitable as a buffer stage between high impedance sources and low impedance loads.
Q.208Medium
At what frequency does the gain of a RC coupled amplifier reduce to 70.7% of its mid-band gain?
Answer: B
The -3dB cutoff frequency is where the magnitude of gain reduces to 1/√2 (≈ 70.7%) of maximum gain. This defines the bandwidth of the amplifier.
Q.209Medium
In an RC coupled amplifier, the lower cutoff frequency is primarily determined by
Answer: A
Lower cutoff frequency fL ≈ 1/(2π·CC·Rin) where CC is coupling capacitor and Rin is input impedance. Larger CC gives lower fL.
Q.210Hard
The Miller effect in a common emitter amplifier causes
Answer: D
Miller effect refers to the multiplication of base-collector capacitance by (1+Av) at the input, increasing input capacitance, reducing input impedance, and decreasing bandwidth.
Q.211Medium
For a feedback amplifier with feedback fraction β, the loop gain is defined as
Answer: A
Loop gain = Aβ, where A is open-loop gain and β is feedback fraction. For stability, loop gain should be <1 for phase margin requirements.
Q.212Hard
Negative feedback in an amplifier primarily results in
Answer: B
Negative feedback reduces gain by factor (1+Aβ) but provides benefits: improved linearity, reduced distortion, increased input impedance (series feedback), decreased output impedance (shunt feedback).
Q.213Medium
In a Class A amplifier, the maximum theoretical efficiency is
Answer: A
Maximum theoretical efficiency of Class A = π/4 ≈ 78.5% when maximum swing is used, but practically 25-50% is achieved due to quiescent current and losses.
Q.214Hard
The Barkhausen criterion for oscillation states that
Answer: B
Barkhausen criterion: For sustained oscillations, |Aβ| = 1 (unity gain) AND total phase shift = 0° (or 360°). Both conditions must be satisfied simultaneously.
Q.215Medium
In a Colpitts oscillator, the frequency of oscillation is determined by
Answer: C
Colpitts oscillator frequency f = 1/(2π√(L·Ceq)) where Ceq = (C1·C2)/(C1+C2). Voltage divider is formed by C1 and C2 for feedback.
Q.216Easy
An ideal operational amplifier has the following characteristics EXCEPT
Answer: B
Ideal op-amp has: infinite input impedance (Zin→∞), infinite open-loop gain (A→∞), zero output impedance (Zout→0), infinite bandwidth (BW→∞).
Q.217Easy
In a non-inverting amplifier configuration with feedback resistors RF and Ri, the voltage gain is
Answer: A
Non-inverting amplifier gain Av = 1 + (RF/Ri). With RF=0 (short circuit), gain=1 (unity gain buffer). As RF increases, gain increases.
Q.218Medium
The gain-bandwidth product (GBP) of an op-amp is approximately constant. For a 741 op-amp with GBP ≈ 1 MHz, the open-loop gain at 100 kHz is approximately
Answer: A
GBP = Aol × f. For 741: 1 MHz = Aol × 100 kHz, therefore Aol = 1 MHz / 100 kHz = 10 V/V. GBP is approximately constant over frequency.
Q.219Medium
In a saturated BJT transistor used as a switch, the voltage drop VCE(sat) is typically
Answer: B
In saturation, VCE(sat) ≈ 0.1-0.3 V for silicon BJT (not zero due to residual resistance). VBE(on) ≈ 0.7V in saturation. This is used in switching applications.
Q.220Hard
Which statement about thermal stability in BJT amplifiers is CORRECT?
Answer: C
Thermal stability improved by: (1) RE without bypass for DC stabilization, (2) lower supply voltage, (3) smaller β transistor, (4) heat sinking. VBE decreases with temperature (negative TC), increasing base current and ICO, causing thermal runaway.
