NTPC MCQs

Explanation: gm=IC/VT=1 mA/25 mV=0.04 S=40 mS.

Explanation: rπ=beta/gm=100/0.04=2500 ohm=2.5 kOhm.

Explanation: Channel-length modulation shortens the effective channel as VDS rises, giving the saturation-region output characteristic a finite positive slope.

Explanation: The current ratio is exp(ΔV/VT)=exp(50/25)=e^2≈7.4, assuming the exponential term dominates.

Explanation: After forward conduction, stored minority-carrier charge must be removed before the diode fully blocks reverse current, producing reverse recovery time.

Explanation: Schottky conduction mainly involves majority carriers, so there is little minority-carrier storage and reverse-recovery delay is small.

Explanation: A PIN diode has a P-I-N structure. The intrinsic region gives useful RF switching, attenuation, and photodetection characteristics.

Explanation: Af=A/(1+Aβ)=1000/(1+9)=100.

Explanation: Negative feedback desensitizes the gain; fractional gain variation is reduced approximately by the desensitivity factor 1+Aβ.

Explanation: At the oscillation frequency, positive regenerative feedback requires |Aβ|≈1 and net loop phase 0° modulo 360°.

Explanation: The Wien bridge provides frequency-selective positive feedback and is widely used for low-distortion sine-wave generation.

Explanation: For equal arms, the lead-lag network has zero phase shift at f0=1/(2πRC).

Explanation: Quartz resonators have very high Q, producing a narrow and stable resonance that gives excellent oscillator frequency stability.

Explanation: Stored charge in the intrinsic region makes a forward-biased PIN diode behave approximately as a controllable low RF resistance, useful in RF switches and attenuators.

Explanation: Heavy doping creates a very narrow junction that permits quantum tunneling. Over part of its characteristic, current decreases as voltage increases, producing negative differential resistance.

Explanation: Light-generated electron-hole pairs separated by the built-in junction field produce a voltage and current, which is the photovoltaic effect.

Explanation: An optocoupler uses a light emitter and photosensitive receiver to transfer information across an electrically isolated barrier.

Explanation: Photogenerated carriers provide an effective base drive, and transistor action amplifies the resulting photocurrent.

Explanation: Voltage gain across a feedback capacitance can multiply its apparent input effect, increasing effective input capacitance and reducing high-frequency bandwidth.

Explanation: At low frequencies, coupling and emitter/source bypass capacitors have larger reactance, creating high-pass poles that determine the low-frequency response.

Explanation: At a first-order -3 dB cutoff, amplitude falls to 1/sqrt(2) of midband value: 100/√2≈70.7.

Explanation: Series mixing at the input tends to increase the input resistance of a voltage-series feedback amplifier.

Explanation: Voltage sampling with negative feedback tends to reduce output resistance, improving voltage-source behavior.

Explanation: An ideal differential amplifier responds only to the difference between its inputs and rejects signals common to both, so ideal common-mode gain is zero.

Explanation: CMRR=Ad/Ac=10000/1=10000.

Explanation: CMRR(dB)=20 log10(10000)=20×4=80 dB.

Explanation: A differential pair steers an approximately fixed tail current between its two devices according to the differential input voltage.

Explanation: Matched transistors arranged as a current mirror provide approximately replicated reference currents and are widely used for IC biasing and active loads.

Explanation: Transistor active loads can present a high small-signal resistance while occupying little IC area, which increases amplifier voltage gain.

Explanation: A voltage follower directly feeds the output to the inverting input and applies the signal to the non-inverting input, producing approximately unity closed-loop gain.

Explanation: Because V+ exceeds V-, the large open-loop gain drives the output toward the positive saturation rail.

Explanation: Positive feedback creates separate upper and lower switching thresholds, improving noise immunity around the transition level.

Explanation: For a single-pole compensated op-amp, closed-loop bandwidth is approximately GBW divided by noise gain: 1 MHz/100 = 10 kHz.

Explanation: The input-referred offset is amplified by the circuit's noise gain, giving about 2 mV × 50 = 100 mV.

Explanation: Practical input transistors require small DC bias currents. Input bias current is commonly defined as the average magnitude of the two input currents.

Explanation: For matched transistors with equal base-emitter voltage, the ideal mirror reproduces the reference current, so the output is approximately 2 mA.

Explanation: Emitter degeneration in one mirror transistor allows the Widlar circuit to produce small bias currents using practical resistor values.

Explanation: The cascode arrangement reduces the effect of output-voltage variation on mirrored current, substantially increasing output resistance.

Explanation: A high-resistance tail source makes the total tail current less sensitive to common-mode input changes, thereby improving CMRR.

Explanation: With matched devices and equal input voltages, the ideal 2 mA tail current divides equally, giving approximately 1 mA in each transistor.

Explanation: Higher junction temperature can increase leakage and alter VBE-related current, increasing dissipation and temperature further unless bias stabilization limits the process.

Explanation: If current rises, the emitter voltage rises across RE, reducing effective base-emitter drive and opposing the current increase.

Explanation: A heat sink improves heat transfer away from the device, lowering junction temperature for a specified dissipated power and ambient condition.

Explanation: Temperature rise is P×theta = 2×30 = 60 degrees C. Adding 25 degrees C ambient gives an estimated junction temperature of 85 degrees C.

Explanation: Input offset current quantifies mismatch between the two input bias currents and is commonly specified as their difference in magnitude.

Explanation: Vp = SR/(2πf) = 0.5×10^6/(2π×10^4) ≈ 7.96 V, so about 8 V.

Explanation: Instrumentation amplifiers are designed to amplify small differential signals while loading sources minimally and rejecting large common-mode components.

Explanation: Bridge sensors often produce a small differential output superimposed on a common-mode level; an instrumentation amplifier provides gain with strong common-mode rejection.

Explanation: Near zero input, neither transistor may conduct until its base-emitter threshold is reached, producing a dead zone and crossover distortion.

Explanation: Class-AB bias keeps both output devices slightly conducting around zero crossing, reducing the dead zone responsible for crossover distortion.

Explanation: For an ideal class-B push-pull stage delivering a sinusoid at maximum undistorted swing, the theoretical maximum efficiency is π/4 ≈ 78.5%.

Explanation: Class-A operation keeps the active device conducting throughout the complete 360-degree signal cycle.

Explanation: Class-C bias allows device current for less than half of each cycle. Tuned circuits are commonly used to recover a sinusoidal output in RF applications.

Explanation: Direct-band-gap semiconductors allow efficient radiative electron-hole recombination without requiring a momentum-changing phonon.

Explanation: Photon energy is E=hf=hc/lambda, so shorter wavelengths correspond to higher photon energies.

Explanation: E≈1240/620=2 eV.

Explanation: A negative gate voltage repels electrons from the n-channel, depleting it and reducing drain current.

Explanation: The electron term q n mu_n increases with electron concentration, so total conductivity increases under the stated assumptions.

Explanation: Electrical resistivity rho and conductivity sigma are related by rho=1/sigma.

Explanation: In the low-field region, drift velocity v_d = mu E, so mobility is v_d/E.

Explanation: v_d=mu E=1200×10=12000 cm/s.

Explanation: The characteristic diffusion length is L=sqrt(D tau), describing the average scale over which excess minority carriers diffuse before recombination.

Explanation: For nondegenerate n-type material, a Fermi level closer to the conduction band corresponds to a larger equilibrium electron concentration.

Explanation: An ohmic contact is engineered so current can flow readily in both directions with approximately linear current-voltage behavior over its intended range.

Explanation: A Schottky diode uses a rectifying metal-semiconductor junction rather than a conventional PN junction.

Explanation: Schottky junction conduction is a majority-carrier process and typically gives a lower forward drop than an ordinary silicon PN diode at comparable current.

Explanation: Photoconductive-mode photodiodes are reverse biased, which widens the depletion region and generally improves response speed.

Explanation: In photovoltaic mode, the photodiode is used with approximately zero applied bias and develops a light-dependent voltage/current, as in solar-cell operation.

Explanation: Spectral response specifies detector responsivity or sensitivity as a function of optical wavelength.

Explanation: MOSFET transconductance is the incremental derivative gm=∂ID/∂VGS at the operating point.

Explanation: gm=2×2 mA/0.2 V=20 mS.

Explanation: The common-drain MOSFET stage, or source follower, provides non-inverting voltage buffering similar to a BJT emitter follower/common-collector stage.

Explanation: With the gate at AC ground and input applied at the source, the common-gate topology largely avoids the Miller multiplication associated with common-source feedback capacitance.

Explanation: Increasing gate voltage raises drain current, increasing the drain-resistor drop and lowering drain voltage, so the stage is inverting.

Explanation: If drain/source current rises, the source voltage rises, reducing VGS and opposing the increase. This source degeneration provides negative feedback.

Explanation: SOA curves define permissible current-voltage operating combinations while accounting for limits such as power dissipation, breakdown, and device-specific failure mechanisms.

Explanation: Power BJTs can suffer localized current crowding and thermal instability called secondary breakdown, which restricts their safe operating area.

Explanation: The MOSFET gate is capacitive. A series gate resistor limits charging/discharging current and helps control switching edge rate, ringing, and driver stress.

Explanation: Emitter degeneration introduces local negative feedback, reducing gain but improving linearity and bias stability.

Explanation: The capacitor provides a low-impedance AC path around the emitter resistor while leaving the resistor available for DC bias stabilization.

Explanation: RC coupling transfers the signal while isolating the DC operating points of adjacent amplifier stages.

Explanation: Cascaded voltage gains multiply: 20×15=300.

Explanation: For cascaded stages, gains expressed in decibels add, so total gain is 20+30=50 dB.

Explanation: The cascode suppresses large voltage variation at the input device's output node, reducing Miller effect while retaining useful gain.

Explanation: The passband bandwidth is the difference between the upper and lower cutoff frequencies: BW=fH-fL.

Explanation: BW=fH-fL=100000-100=99900 Hz=99.9 kHz.

Explanation: A single pole contributes approximately -20 dB/decade, equivalent to about -6 dB/octave.

Explanation: Each pole contributes about -20 dB/decade, so two coincident poles give approximately -40 dB/decade.

Explanation: Af=A/(1+A beta)=200/(1+4)=40.

Explanation: gm=IC/VT=2 mA/25 mV=0.08 S=80 mS.

Explanation: r_pi=beta/gm=120/0.06=2000 ohm=2 kOhm.

Explanation: For the simplified CE stage, Av≈-gmRC=-0.04×2000=-80.

Explanation: Negative feedback holds the inverting node near the grounded non-inverting input voltage, although there is no direct physical ground connection at that node.

Explanation: Vp,max=SR/(2 pi f)=10^6/(2 pi×10^5)≈1.59 V.

Explanation: Output offset due to input offset is approximately noise gain times Vos: 101×1 mV=101 mV.

Explanation: Placing the diode within an op-amp feedback arrangement allows accurate rectification of signals much smaller than a normal diode's forward drop.

Explanation: The parallel resistor provides finite DC feedback, preventing tiny offsets and bias currents from integrating indefinitely and driving the output into saturation.

Explanation: The ideal differentiator's gain rises with frequency, making it highly sensitive to high-frequency noise and stability problems; practical designs limit this gain.

Explanation: The desensitivity factor is 1+A beta=1+4=5.

Explanation: Negative feedback commonly trades gain for bandwidth; under the constant gain-bandwidth approximation, a tenfold gain reduction gives about tenfold bandwidth increase.