Chapter 24: Semiconductor Devices and Electronic Circuits (Set-3)
When forward current in a diode increases, its dynamic resistance generally
A Increases sharply
B Becomes infinite
C Stays constant
D Decreases
Dynamic resistance is the slope dV/dI of the diode V–I curve at an operating point. At higher forward current, the curve becomes steeper, so a small voltage change causes larger current change, reducing dynamic resistance.
In a forward-biased silicon diode, most applied voltage mainly drops across
A Depletion region
B Metal contacts
C Neutral regions
D Lead wires
Under forward bias, depletion region shrinks and its voltage drop becomes small. Most additional applied voltage appears across the resistive neutral p and n regions and contacts, producing series resistance effects at higher currents.
Temperature rise in a diode typically causes knee voltage to
A Increase
B Decrease
C Stay same
D Become zero
As temperature increases, carrier concentration rises and the barrier potential reduces. Hence a silicon diode needs slightly less forward voltage to conduct a given current, so its cut-in (knee) voltage decreases.
For a reverse-biased diode below breakdown, current is nearly
A Proportional to V
B Zero always
C Very oscillatory
D Constant
Below breakdown, reverse current is mainly reverse saturation current due to minority carriers. It changes very little with reverse voltage, but increases strongly with temperature because more carriers are generated.
Zener regulator maintains output voltage mainly by changing
A Zener current
B Load resistance
C Transformer ratio
D Input frequency
In breakdown, Zener voltage stays nearly constant. When supply or load changes, current through the Zener adjusts automatically (within limits), keeping output voltage almost fixed across the load.
If load current increases in a Zener regulator, Zener current usually
A Increases
B Decreases
C Becomes negative
D Doubles always
Total current through the series resistor splits into load current and Zener current. When the load draws more, less current remains for the Zener, so Zener current falls while voltage stays nearly constant.
A Zener diode stops regulating properly when its current falls below
A Peak current
B Reverse current
C Avalanche current
D Knee current
Zener regulation is stable only in the breakdown region above the knee current. If Zener current drops below this minimum, the diode leaves breakdown and output voltage starts varying with load or supply.
Main purpose of selecting correct series resistor in Zener circuit is to
A Increase ripple
B Raise threshold
C Limit Zener power
D Remove DC
The series resistor controls current through the Zener and load. It ensures Zener current stays within safe range under worst-case input and load, preventing power dissipation from exceeding rating.
Tunnel diode negative resistance region lies between
A Knee and breakdown
B Peak and valley
C Saturation and cutoff
D Open and short
Tunnel diode current rises to peak due to tunneling, then decreases with increasing voltage until valley current. This falling-current section between peak and valley is the negative resistance region.
Heavy doping in tunnel diode mainly produces
A Thick depletion
B High barrier
C No junction
D Thin depletion
Very heavy doping reduces depletion width drastically. A thin barrier allows carriers to tunnel through even at low voltages, creating the special V–I characteristic with peak, valley, and negative resistance.
Tunnel diode is biased in negative resistance region mainly for
A Oscillation generation
B Stable DC supply
C Rectifier filtering
D Heat control
Negative resistance can cancel circuit losses and sustain oscillations. When biased properly, tunnel diode with an LC network can produce high-frequency oscillations useful in RF and microwave circuits.
LED efficiency is higher when semiconductor has
A Indirect band gap
B Zero band gap
C Direct band gap
D Metallic band
In direct band-gap materials, electron–hole recombination directly releases energy as photons. This produces strong light output. Indirect band-gap materials lose more energy as heat, reducing efficiency.
LED wavelength mainly depends on
A Junction area
B Band gap value
C Wire resistance
D Supply ripple
Photon energy equals band gap energy. A larger band gap gives higher-energy photons and shorter wavelength. Therefore material choice controls LED color from infrared to visible range.
LCD segment becomes visible mainly due to change in
A Charge storage
B Diode conduction
C Thermal emission
D Polarization rotation
Liquid crystals rotate polarized light depending on electric field. With crossed polarizers, this controls light transmission through a segment. The segment appears dark or bright depending on alignment.
Backlighting in LCD is required mainly because LCD is
A Light modulator
B Light emitter
C Current source
D Voltage source
LCD does not generate light. It only controls how much light passes through. In low ambient light, a backlight provides illumination so the display remains clearly visible.
Solar cell I–V curve in sunlight shows current at zero voltage equals
A Open voltage
B Short current
C Fill factor
D Peak power
At zero terminal voltage, the circuit is effectively shorted. The solar cell delivers maximum current called short-circuit current (Isc), mainly dependent on light intensity and cell area.
Series resistance in a solar cell mainly reduces
A Voc only
B Band gap
C Fill factor
D Light absorption
Series resistance causes voltage drop at higher current, reducing the “squareness” of the I–V curve. This lowers maximum power output and decreases fill factor, reducing overall efficiency.
Shunt resistance in a solar cell represents
A Ideal insulation
B Leakage paths
C Metal contacts
D Doping level
Low shunt resistance indicates unwanted leakage across the junction, bypassing the load. This reduces current available to the external circuit and lowers voltage, decreasing power and efficiency.
A diode used for demodulation is commonly called
A Flyback diode
B Zener diode
C Tunnel diode
D Detector diode
In AM demodulation, a detector diode rectifies the RF carrier and extracts the audio envelope. With an RC filter, the output follows the modulation signal, producing the recovered audio.
A peak detector circuit mainly uses diode and
A Inductor
B Transformer
C Capacitor
D Thermistor
The diode charges a capacitor to the peak of the input waveform. When input falls, diode becomes reverse biased and the capacitor holds the peak value, slowly discharging through load resistance.
In half-wave rectifier, average DC output is approximately
A 2Vm/π
B Vm/π
C Vm/2
D Vm
For an ideal half-wave rectifier with sinusoidal input peak Vm, only one half-cycle conducts. The average value over a full cycle becomes Vm/π, which is lower than full-wave rectification.
In full-wave rectifier, average DC output is approximately
A Vm/π
B Vm/2
C Vm
D 2Vm/π
Full-wave rectifier uses both halves of the sine wave. The average of the rectified waveform over one cycle is 2Vm/π, giving higher DC output and better efficiency than half-wave.
Bridge rectifier advantage over center-tap is
A Needs center tap
B Lower diode count
C Better transformer use
D No filtering needed
Bridge rectifier does not require a center-tapped transformer and uses the full secondary winding in both half-cycles. This improves transformer utilization and is common in power supply designs.
Capacitor filter works best when load current is
A Very low
B Very high
C Zero always
D Alternating only
With lighter load, the capacitor discharges slowly between peaks, so ripple reduces greatly. With heavy load, discharge is faster, ripple increases, and voltage drops more between cycles.
Peak inverse voltage in center-tap full-wave is about
A Vm
B Vm/2
C 4Vm
D 2Vm
In a center-tapped full-wave rectifier, when one diode conducts, the other is reverse biased and must withstand roughly the sum of peak voltages from both half windings, giving PIV ≈ 2Vm.
In CE amplifier, bypass capacitor is mainly used to
A Increase DC bias
B Increase AC gain
C Reduce input noise
D Raise cutoff
The emitter resistor stabilizes DC bias but reduces AC gain due to negative feedback. A bypass capacitor provides low reactance for AC, reducing emitter degeneration and increasing midband gain.
Coupling capacitor in amplifier mainly blocks
A AC signal
B Heat flow
C DC component
D Magnetic field
Coupling capacitors pass AC signals while blocking DC. This allows different amplifier stages to have independent biasing without DC from one stage disturbing the operating point of the next stage.
In CE amplifier, midband gain decreases mainly due to
A Loading effect
B Low temperature
C Reverse bias
D Junction breakdown
When the next stage or load has finite resistance, it forms a voltage divider with the amplifier output resistance. This reduces output voltage swing and lowers effective voltage gain in practical circuits.
Lower cutoff frequency in RC-coupled amplifier is mainly set by
A Collector resistor
B Heat sink
C Supply voltage
D Coupling capacitors
At low frequencies, capacitor reactance increases. Coupling and bypass capacitors then limit signal transfer and gain. This defines the lower cutoff frequency where gain drops below midband value.
Upper cutoff frequency mainly depends on
A Transformer core
B Junction capacitances
C DC bias only
D Wire color
At high frequencies, internal transistor capacitances and wiring capacitances provide feedback and shunting paths. These reduce gain as frequency rises, setting the upper cutoff and limiting bandwidth.
Miller effect mainly increases
A Output voltage
B DC current
C Power supply
D Input capacitance
In an inverting amplifier, capacitance between input and output appears multiplied at the input. This increases effective input capacitance, reducing high-frequency response and lowering upper cutoff frequency.
Input impedance of emitter follower is generally
A Very low
B Zero
C Very high
D Negative
In a common collector (emitter follower), small base current controls large emitter current, and feedback raises input resistance. This makes it useful as a buffer between high-resistance sources and loads.
Output impedance of emitter follower is generally
A Very low
B Very high
C Infinite
D Unstable
Emitter follower provides strong current drive and local feedback. This lowers output impedance, so the output voltage changes little with load variations, making it good for impedance matching.
In JFET, drain current saturation starts near
A Gate forward bias
B Pinch-off condition
C Zener breakdown
D Thermal runaway
As Vds increases, depletion region near drain widens until the channel pinches off. Beyond this, drain current becomes nearly constant and the device operates in saturation region.
Self-bias in JFET commonly uses
A Gate resistor only
B Collector resistor
C Transformer tap
D Source resistor
In self-bias, a source resistor creates a voltage drop due to drain current, making Vgs negative for n-channel JFET. This stabilizes operating point without needing a separate gate supply.
In JFET, gate current is nearly zero because gate junction is
A Forward biased
B Shorted
C Reverse biased
D Broken
The gate forms a reverse-biased p–n junction with the channel. Reverse bias prevents significant current flow, giving high input impedance and allowing voltage control of channel width.
MOSFET has extremely high input impedance mainly due to
A No oxide layer
B Gate insulation
C High base current
D Metal channel
The gate is insulated from the channel by a thin oxide. This blocks DC gate current, so input resistance becomes very large. Only small charging current flows due to gate capacitance.
MOSFET ohmic region is also called
A Cutoff region
B Breakdown region
C Negative region
D Linear region
At small Vds, MOSFET channel acts like a voltage-controlled resistor and Id increases roughly linearly with Vds. This region is called linear or ohmic region and is used in analog switching.
MOSFET saturation occurs when Vds is
A Less than Vgs
B Greater than Vgs−Vt
C Equal to zero
D Negative always
In an enhancement MOSFET, saturation begins when Vds exceeds Vgs − Vt. The channel pinches near drain and current becomes mostly controlled by Vgs, useful for amplification.
CMOS logic mainly uses
A BJTs only
B Zener diodes
C Complementary MOSFETs
D Tunnel diodes
CMOS uses nMOS and pMOS devices together so that one is off when the other is on. This gives very low static power consumption and strong noise margins in digital circuits.
Gain–bandwidth improvement is a common result of
A Positive feedback
B Reverse biasing
C Zener action
D Negative feedback
Negative feedback reduces gain but increases bandwidth by flattening frequency response. It also improves stability, reduces distortion and noise, and makes amplifier performance less sensitive to component variations.
Feedback factor represents the fraction of
A Output fed back
B Input added to output
C Noise removed
D Power dissipated
Feedback factor is the portion of output signal returned to the input through the feedback network. In negative feedback, it is subtracted from the input, controlling overall gain and stability.
Barkhausen condition is linked to
A Rectification
B Oscillation
C Filtering
D Regulation
For sustained oscillations, loop gain magnitude must be unity and total phase shift must be 0° or 360°. This is Barkhausen criterion and it is used to analyze oscillator circuits.
Clipping distortion in an amplifier occurs when
A Signal too small
B Temperature low
C Signal too large
D Frequency low
If input signal drives the transistor beyond linear region into cutoff or saturation, output cannot follow the waveform fully. Peaks flatten, producing clipping distortion and adding unwanted harmonics.
Harmonic distortion means output contains
A Only DC
B Only noise
C Only input frequency
D Multiple harmonics
In non-linear amplification, output includes frequencies that are integer multiples of the input frequency. These harmonics change waveform shape, reduce fidelity, and are measured as harmonic distortion.
Intermodulation distortion appears when amplifier handles
A One DC source
B Two frequencies
C Only noise
D Only pulses
When two different input frequencies pass through a non-linear device, new frequencies form at sums and differences of the originals. This is intermodulation distortion and is harmful in communication systems.
A transistor used as a switch ideally operates in
A Active region only
B Breakdown region
C Cutoff and saturation
D Reverse active
In switching, transistor is either OFF (cutoff, no current) or ON (saturation, low voltage drop). Avoiding the active region reduces power loss and improves switching reliability.
Heat sink helps mainly by reducing
A Junction temperature
B Junction voltage
C Doping density
D Depletion width
Power devices generate heat during operation. A heat sink increases surface area for heat transfer to air, keeping junction temperature within safe limits and preventing thermal runaway or damage.
CRO is mainly used to display
A Resistance value
B Voltage waveform
C Current gain
D Doping profile
A cathode-ray oscilloscope displays voltage variations with time. It helps measure amplitude, frequency, phase difference, and waveform shape, making it useful for analyzing rectifier ripple and amplifier signals.
A signal generator is used to provide
A Pure DC only
B Heat pulses
C Test waveforms
D Magnetic flux
Signal generators produce controlled sine, square, and other waveforms over a range of frequencies and amplitudes. They are used to test amplifier gain, frequency response, and circuit behavior in labs.