Chapter 24: Semiconductor Devices and Electronic Circuits (Set-4)
For a silicon diode at 300 K, the thermal voltage (Vt) is closest to
A 5 mV
B 0.26 V
C 26 mV
D 2.6 V
Thermal voltage Vt = kT/q. At room temperature (≈300 K), Vt is about 25–26 mV. It appears in the diode equation and strongly affects how current changes with diode voltage.
In the diode equation, the “ideality factor” mainly accounts for
A Recombination effects
B Wire resistance
C Transformer losses
D Magnetic coupling
The ideality factor (n) adjusts the simple exponential model to match real diodes. It reflects recombination in the depletion region and device physics, so real current–voltage behavior deviates from ideal.
If diode forward current is doubled, the forward voltage increase is approximately
A 10 mV
B 18 mV
C 60 mV
D 600 mV
From I ∝ exp(V/nVt), a doubling of current needs ΔV ≈ nVt ln2. With n≈1 and Vt≈26 mV, ΔV ≈ 26×0.693 ≈ 18 mV, a useful medium-level estimate.
Reverse saturation current in a diode rises sharply because temperature increases
A Drift velocity only
B Junction area
C Doping always
D Carrier generation
Higher temperature generates more electron–hole pairs. These minority carriers cross the junction under reverse bias, increasing reverse saturation current strongly, even though reverse voltage is unchanged below breakdown.
Zener breakdown is mainly due to
A Quantum tunneling
B Impact ionization
C Thermal heating
D Charge storage
In heavily doped junctions, depletion region is very thin and electric field is strong. This allows electrons to tunnel across the junction at relatively low reverse voltages, producing Zener breakdown.
Avalanche breakdown is more common in junctions that are
A Heavily doped
B Metal contacted
C Lightly doped
D Zero doped
Light doping gives a wider depletion region and breakdown at higher voltage. Carriers gain enough energy to cause impact ionization, multiplying carriers and producing avalanche breakdown.
Temperature coefficient of Zener voltage is typically negative for
A Low Vz diodes
B High Vz diodes
C All Zeners
D No Zeners
Low-voltage Zeners mainly break down by Zener effect (tunneling), which has negative temperature coefficient. High-voltage Zeners are more avalanche-dominated and often show positive coefficient.
In a Zener regulator, if input increases greatly, the first limit reached is usually
A Load heating
B Ripple factor
C Frequency limit
D Zener power limit
Higher input raises current through series resistor, increasing Zener current. If too large, Zener dissipation P = Vz×Iz can exceed rated power, causing overheating and loss of regulation.
In a bridge rectifier, two diodes conduct during
A Only positive cycle
B Only negative cycle
C Each half-cycle
D No cycle
In a bridge rectifier, current through load is in the same direction for both half-cycles. This happens because a pair of diodes conducts in the positive half and a different pair conducts in the negative half.
A full-wave rectifier generally has ripple factor
A Lower than half-wave
B Higher than half-wave
C Same as half-wave
D Always zero
Full-wave rectification produces output pulses twice as often, so smoothing is easier. Ripple factor is smaller than half-wave for the same load and filter, giving better DC quality.
If a capacitor-input filter is added, ripple decreases when capacitance
A Decreases
B Becomes zero
C Becomes negative
D Increases
Larger capacitance stores more charge and discharges more slowly between peaks. This maintains voltage better across the load, reducing ripple amplitude and improving smoothing performance.
Peak inverse voltage per diode in bridge rectifier is approximately
A Vm
B Vm/2
C 2Vm
D 4Vm
In a bridge rectifier, when a diode is reverse biased, it must block roughly the peak secondary voltage. Hence PIV per diode is about Vm, smaller than the 2Vm requirement of center-tap full-wave.
A transistor works as an amplifier when base–emitter is
A Reverse biased
B Open circuited
C Forward biased
D In breakdown
For normal amplification, the base–emitter junction must be forward biased to inject carriers into the base, while base–collector is reverse biased to collect them. This maintains active region behavior.
In active region of BJT, base–collector junction is
A Forward biased
B Reverse biased
C Shorted always
D Zero biased
Active region requires base–emitter forward bias and base–collector reverse bias. Reverse bias at collector junction sweeps carriers into collector, allowing collector current control by small base current.
If α = 0.98, β is closest to
A 49
B 2
C 98
D 0.02
Relation is β = α/(1−α). With α = 0.98, β = 0.98/0.02 = 49. This shows why β can be large even when α is only slightly less than 1.
In CE amplifier, the load line is drawn on
A Input curve
B Transfer curve
C Zener curve
D Output curve
The DC load line is plotted on the output characteristics (Ic vs Vce). It represents circuit constraint from Vcc and Rc, and the intersection with device curves helps set the Q-point.
A good biasing network should keep Q-point stable against
A Only frequency
B Only waveform type
C Temperature changes
D Only diode type
Transistor parameters like β and leakage currents vary with temperature. Proper biasing with negative feedback (like emitter resistor) stabilizes Ic and Vce, preventing drift and reducing distortion or thermal runaway risk.
In CE amplifier, collector current increase usually makes collector voltage
A Decrease
B Increase
C Become zero
D Reverse polarity
Higher collector current causes larger voltage drop across collector resistor Rc. Since Vc = Vcc − IcRc, Vc falls. This is why CE amplifier output is inverted relative to input.
In an emitter follower, voltage gain is approximately
A Much greater than 1
B Nearly 1
C Nearly 0
D Negative large
Emitter follower output follows base voltage with about 0.7 V drop for silicon. Because of strong feedback, voltage gain is close to unity, but current gain is high, useful for buffering.
Major advantage of CB amplifier is
A High input impedance
B Large phase shift
C Low input impedance
D No current flow
Common base has low input resistance and high voltage gain, and it works well at high frequencies because it avoids Miller effect strongly. It is useful when source impedance is low.
JFET drain current is controlled by
A Gate voltage
B Base current
C Collector voltage
D Heater power
JFET is voltage-controlled. A reverse-biased gate changes depletion width and channel cross-section. This controls drain current without significant gate current, giving very high input impedance.
For n-channel JFET, making Vgs more negative causes Id to
A Increase
B Stay constant
C Reverse sign
D Decrease
More negative Vgs widens depletion region, narrowing the channel. This increases channel resistance and reduces drain current. At pinch-off (cutoff), the channel closes and Id becomes nearly zero.
In JFET, pinch-off voltage is closely linked to
A Gate forward bias
B Zener breakdown
C Channel depletion
D Ripple factor
Pinch-off occurs when depletion regions meet and constrict the channel near drain. It depends on doping and geometry. Beyond pinch-off, Id saturates and becomes less sensitive to Vds.
MOSFET threshold voltage is the Vgs at which
A Channel forms
B Junction breaks
C Gate conducts
D Current stops
In enhancement MOSFET, a conducting inversion channel forms when Vgs exceeds threshold voltage. Below Vt, drain current is very small; above Vt, Id increases with gate overdrive.
Enhancement nMOS requires gate voltage to be
A Negative
B Positive
C Zero only
D Alternating only
For nMOS enhancement, a positive gate voltage attracts electrons to the surface, creating an inversion layer that forms the channel. This allows current flow between drain and source.
Depletion-mode MOSFET can be turned off by making Vgs
A More positive
B Exactly zero
C More negative
D Very noisy
Depletion MOSFET conducts at Vgs = 0 due to existing channel. Applying opposite polarity gate voltage depletes carriers and narrows the channel. Enough negative Vgs can cut off the drain current.
MOSFET gate capacitance mainly affects
A Switching speed
B DC leakage only
C Barrier height
D Zener voltage
Gate capacitance must be charged and discharged to change MOSFET state. Larger capacitance slows transitions, increases switching losses, and limits high-frequency performance, especially in digital and power switching.
Common source amplifier output phase is typically
A Same phase
B 90° lead
C Random phase
D 180° inverted
In common source, increasing Vgs increases Id, increasing drop across drain resistor, reducing output voltage. Thus output voltage decreases when input increases, giving approximately 180° phase inversion.
FET amplifiers are often preferred for low-noise front ends due to
A Low noise
B High gate current
C High heating
D Low bandwidth
FETs have very small input current and reduced shot noise compared to BJTs in many cases. Their high input impedance and low noise make them suitable for sensitive amplifier input stages.
In RC-coupled amplifier, midband gain is mainly limited by
A Coupling capacitor reactance
B Ripple frequency
C Transistor parameters
D Transformer turns
In midband, coupling and bypass capacitors act nearly as shorts for AC, so gain depends mostly on transistor transconductance and resistances (Rc, load, rπ). At extremes, capacitors and capacitances limit gain.
Lower cutoff frequency increases when coupling capacitor value
A Increases
B Decreases
C Doubles always
D Becomes infinite
Smaller coupling capacitor has larger reactance at low frequencies, reducing signal transfer and gain earlier. Therefore the lower cutoff frequency shifts upward when coupling capacitor value is reduced.
Upper cutoff frequency decreases mainly due to
A Junction capacitances
B Huge coupling capacitor
C Larger transformer
D Higher Vcc
Transistor junction capacitances and stray capacitances create low reactance paths at high frequency and introduce feedback (Miller effect). This reduces gain as frequency rises, lowering the upper cutoff frequency.
Bode plot of gain uses logarithmic scale for
A Voltage axis only
B Current axis only
C Temperature axis
D Frequency axis
Bode plots typically use logarithmic frequency scale so wide frequency ranges can be shown clearly. Gain is often expressed in decibels. This helps visualize cutoff frequencies and roll-off rates efficiently.
For a single-pole roll-off, gain decreases by about
A 3 dB/decade
B 40 dB/decade
C 20 dB/decade
D 60 dB/decade
A single dominant pole causes magnitude response to fall at approximately 20 dB per decade beyond cutoff. Each additional pole adds another 20 dB/decade of roll-off in the asymptotic Bode plot.
Loading effect becomes severe when load resistance is
A Comparable to Rout
B Much larger than Rout
C Infinite always
D Negative value
If load resistance is comparable to amplifier output resistance, a voltage divider forms and output voltage drops significantly. This reduces gain and can distort frequency response, especially in multistage amplifiers.
Maximum power transfer occurs when load resistance equals
A Infinite resistance
B Zero resistance
C Zener resistance
D Source resistance
For resistive networks, maximum power is delivered to the load when load resistance equals Thevenin (source) resistance seen from load terminals. This is important in impedance matching concepts.
Negative feedback usually increases input impedance in
A Voltage series feedback
B Current shunt feedback
C No feedback
D Positive feedback
In voltage-series feedback, feedback is mixed in series with input. This reduces input current for a given input voltage, effectively increasing input impedance while improving stability and reducing distortion.
Negative feedback usually decreases output impedance in
A Voltage shunt feedback
B Current series feedback
C Voltage series feedback
D No feedback
Voltage sampling with negative feedback forces output to remain stable even when load changes. This makes the amplifier behave more like an ideal voltage source, effectively reducing output impedance.
Loop gain in feedback amplifier equals
A A + β
B Aβ
C A/β
D β/A
Loop gain is the product of open-loop gain A and feedback factor β. When Aβ is large, closed-loop gain becomes nearly 1/β and performance becomes stable against device variations.
If Aβ is very large, closed-loop gain is approximately
A 1/β
B A
C β
D Zero
Closed-loop gain is A/(1+Aβ). If Aβ ≫ 1, then gain ≈ A/(Aβ) = 1/β. This makes amplifier gain depend mainly on feedback network rather than transistor parameters.
Oscillation starts when feedback becomes
A Negative and strong
B Zero always
C Random noise
D Positive overall
If the total loop phase shift becomes 0° (or 360°) and loop gain magnitude is at least unity, feedback becomes effectively positive. Then small signals reinforce and oscillations can build up.
A clipper using Zener diode can limit voltage at about
A Vt only
B Peak inverse
C Zener voltage
D Ripple voltage
A Zener in reverse breakdown holds nearly constant voltage. In clipping circuits, it prevents output from exceeding approximately its Zener voltage level, protecting sensitive stages from overvoltage peaks.
A transistor small-signal model is mainly used for analyzing
A Tiny AC signals
B Large switching
C Heat sink size
D Doping process
Small-signal models linearize transistor behavior around the Q-point. They allow calculation of gain, input/output impedance, and frequency response for small AC signals without solving full nonlinear equations.
Distortion reduces when amplifier is biased near
A Cutoff edge
B Mid active region
C Saturation edge
D Breakdown point
Biasing near the center of active region allows symmetrical output swing without hitting cutoff or saturation. This reduces clipping and nonlinear distortion, improving signal fidelity in linear amplification.
In a voltage regulator, “reference” mainly means a
A Constant current
B Constant power
C Constant voltage
D Constant frequency
A reference is a stable voltage used for comparison and control. Zener diodes and reference ICs provide nearly constant voltage, allowing regulators to maintain fixed output despite input or load changes.
An op-amp in ideal form has open-loop gain that is
A Very large
B Very small
C Exactly one
D Negative zero
Ideal op-amp is assumed to have infinite open-loop gain. With negative feedback, this forces input terminals to nearly equal potential, enabling precise amplification, filtering, and voltage control in circuits.
An RC low-pass filter mainly passes
A High frequencies
B Only DC
C Only noise
D Low frequencies
In RC low-pass, capacitor offers high reactance at low frequency and low reactance at high frequency. High-frequency components are shunted through capacitor, while low-frequency components reach the output.
A multivibrator circuit is generally used to generate
A Pulse waveforms
B Pure sine waves
C DC only
D Heat waves
Multivibrators produce square or rectangular pulses. Astable gives continuous oscillations, monostable gives one pulse per trigger, and bistable acts as a flip-flop storing one of two stable states.
A power amplifier is designed mainly to deliver
A High input resistance
B High Zener voltage
C High power
D High ripple
Power amplifiers provide large output power to drive loads like speakers. They prioritize efficiency and current handling, often using heat sinks and proper biasing to reduce distortion and avoid overheating.
A silicon solar cell typically has open-circuit voltage near
A 0.1 V
B 0.5–0.6 V
C 2.0 V
D 10 V
A single silicon p–n junction solar cell usually gives Voc around 0.5 to 0.6 V under standard illumination. Temperature rise slightly reduces Voc, while connecting cells in series increases total voltage.