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.2Easy
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.3Easy
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.4Medium
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.5Medium
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.
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Q.6Easy
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.7Medium
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.8Medium
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.9Medium
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.10Hard
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.11Medium
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.12Hard
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.13Medium
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.14Hard
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.15Medium
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.16Easy
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.17Easy
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.18Medium
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.19Medium
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.20Hard
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.
