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.22Hard
In a Common Emitter amplifier, if the input impedance is 1 kΩ and the output impedance is 50 kΩ, what is the approximate voltage gain if β = 100 and gm = 0.04 S?
Answer: B
Voltage gain = gm × Rc (where Rc ≈ output impedance) = 0.04 × 50k = 2000 for a CE amplifier configuration.
Q.23Easy
An inverting amplifier using op-amp has Rf = 100 kΩ and Rin = 10 kΩ. What is the voltage gain magnitude?
Answer: A
For inverting amplifier, Gain = -Rf/Rin = -100k/10k = -10. The negative sign indicates 180° phase shift.
Q.24Medium
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.25Easy
In a Hartley oscillator, the frequency of oscillation is determined by which circuit element combination?
Answer: B
Hartley oscillator uses two inductors and one capacitor in the tank circuit. Frequency f = 1/(2π√L_eq×C) where L_eq is equivalent inductance.
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Q.26Medium
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.27Medium
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.
Q.28Hard
In a differential amplifier, Common Mode Rejection Ratio (CMRR) is measured as 80 dB. What is the numerical ratio of differential gain to common mode gain?
Which type of feedback is applied in a non-inverting amplifier configuration?
Answer: B
Non-inverting amplifier uses negative feedback from output to inverting input, which reduces gain but improves linearity, bandwidth, and input impedance.
Q.30Easy
For an ideal op-amp, the input offset voltage is:
Answer: B
An ideal op-amp has zero input offset voltage, zero input bias current, infinite gain, infinite input impedance, and zero output impedance.
Q.31Medium
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.32Hard
A feedback amplifier with closed-loop gain Acl = 10 has feedback fraction β = 0.1. What is the open-loop gain (A)?
Answer: B
From Acl = A/(1 + Aβ), we get 10 = A/(1 + A×0.1). Solving: 10(1 + 0.1A) = A → 10 = 0.9A → A ≈ 1000 (for high gain).
Q.33Medium
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.34Medium
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.35Easy
A transistor amplifier has input signal of 10 mV and output signal of 5 V. What is the voltage gain in dB?
Answer: A
Gain = Vout/Vin = 5V/10mV = 500. Gain (dB) = 20 log₁₀(500) ≈ 54 dB.
Q.36Medium
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.37Medium
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.38Easy
A power amplifier delivers 100 W to a 8 Ω load. What is the RMS output voltage? (Assuming pure resistive load)
Answer: A
P = V²/R → V = √(P×R) = √(100×8) = √800 ≈ 28.3 V RMS.
Q.39Hard
In a two-stage amplifier with individual gains A₁ = 50 and A₂ = 100, if negative feedback β = 0.01 is applied across both stages, what is the approximate closed-loop gain?
Answer: B
Overall open-loop gain = 50×100 = 5000. Closed-loop gain = A/(1+Aβ) = 5000/(1+5000×0.01) = 515000 ≈ 98 ≈ 100. With feedback applied, typical result is ~990.
Q.40Medium
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.
