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.102Medium
A BJT amplifier exhibits thermal runaway due to increased leakage current. Which design modification can best prevent this phenomenon?
Answer: B
Voltage divider bias with low impedance provides stiff bias independent of transistor parameters, preventing thermal runaway by maintaining stable base voltage despite temperature changes.
Q.103Medium
Which configuration provides the highest input impedance and lowest output impedance among op-amp circuits?
Answer: C
Voltage follower has Zin ≈ infinity (determined by op-amp input impedance) and Zout ≈ 0, making it ideal for impedance transformation.
Q.104Medium
For the oscillator to sustain oscillations, the loop gain must satisfy which condition (assuming phase shift is 360° or 0°)?
Answer: B
Barkhausen criterion requires loop gain ≥ 1 (or |A×β| ≥ 1) for sustained oscillations; less than 1 causes damping, more than 1 causes amplitude growth until saturation.
Q.105Medium
A Class B amplifier has two transistors operating in push-pull configuration. What is the maximum theoretical efficiency?
Answer: B
Class B has maximum efficiency of π/4 ≈ 78.5%. Each transistor conducts 180° of the signal cycle, eliminating idle current waste compared to Class A.
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Q.106Medium
In an RC phase shift oscillator, how many RC sections are typically used to achieve 180° phase shift?
Answer: C
Three RC sections are used, each providing 60° phase shift. Total = 3 × 60° = 180°, which combined with 180° from inverting amp gives 360° (0°) for sustained oscillation.
Q.107Medium
The Miller effect in a Common Emitter amplifier results in:
Answer: C
Miller effect causes effective input capacitance to increase by factor (1 + |Av|), reducing input impedance and bandwidth. This limits high-frequency response.
Q.108Medium
In a Wein bridge oscillator, the frequency of oscillation is given by f = 1/(2πRC). At what condition does oscillation start?
Answer: B
Wein bridge oscillator requires loop gain ≥ 3 to overcome the attenuation of the bridge network (which has 31 attenuation at oscillation frequency).
Q.109Medium
In a transimpedance amplifier (TIA), which parameter determines the transimpedance gain?
Answer: B
Transimpedance gain = Vout/Iin = -Rf for inverting configuration. The feedback resistor directly sets the input-to-output conversion.
Q.110Medium
Which oscillator topology has the poorest frequency stability due to dependency on passive component tolerances?
Answer: B
RC phase shift oscillator depends entirely on RC values which have poor tolerance and temperature coefficients, resulting in frequency drift.
Q.111Medium
For maximum power transfer from a source to load through an amplifier, what should be the relationship between source and load impedance?
Answer: B
Maximum power transfer theorem states that maximum power is delivered when load impedance equals complex conjugate of source impedance, especially in AC circuits.
Q.112Medium
An op-amp has an open-loop gain of 100,000 and a feedback resistor ratio of Rf/Rin = 10. What is the closed-loop gain?
Answer: A
For a non-inverting amplifier, Acl ≈ 1 + (Rf/Rin) = 1 + 10 = 11. The feedback dominates over the open-loop gain.
Q.113Medium
In a Colpitts oscillator, the feedback network consists of:
Answer: A
Colpitts oscillator uses capacitive voltage divider with two capacitors in series across an inductor for frequency-determining feedback.
Q.114Medium
A Class B power amplifier has two transistors. What is the primary advantage over Class A?
Answer: A
Class B achieves ~78.5% efficiency by conducting only 180° of the cycle compared to Class A's ~25% efficiency, though it introduces crossover distortion.
Q.115Medium
In a push-pull amplifier configuration, what is the phase relationship between the two transistor inputs?
Answer: A
Push-pull amplifiers use complementary transistors driven 180° out of phase to handle positive and negative half-cycles separately.
Q.116Medium
An inverting amplifier has Rin = 10 kΩ and Rf = 100 kΩ. If the input signal is 100 mV, what is the output voltage?
Answer: A
For inverting amplifier, Vo = -(Rf/Rin) × Vin = -(100k/10k) × 0.1 = -10 × 0.1 = -1 V.
Q.117Medium
Which parameter primarily determines the frequency stability of an RC oscillator?
Answer: A
RC oscillators derive their frequency from RC time constants. The frequency f = 1/(2πRC) approximately, making RC the dominant parameter.
Q.118Medium
A Class AB amplifier operates with a quiescent current (ICQ) that is:
Answer: A
Class AB uses a small quiescent current to bias transistors slightly into conduction, eliminating crossover distortion while maintaining better efficiency than Class A.
Q.119Medium
In a differential amplifier using matched transistors, the Common Mode Rejection Ratio (CMRR) is primarily limited by:
Answer: A
CMRR depends on the matching of transistor parameters and passive components. Mismatches in emitter resistances and transistor characteristics directly degrade CMRR.
Q.120Medium
For a Source Follower (Common Drain FET amplifier), what is the typical voltage gain range?
Answer: B
Source follower has voltage gain Av = gm·Rl/(1 + gm·Rl) ≈ 1 for practical cases, always less than 1. It provides high input impedance and low output impedance.
