## LED and LCD
**1. Which semiconductor material emits infrared light in LEDs?**
a) GaAsP
b) AlGaP
c) GaAs
d) SiC
**Answer: c) GaAs** [1]
**Explanation:** GaAs produces infrared light at 850-950 nm wavelengths due to its bandgap properties, while others emit visible colors like red, green, or blue. [1]
**2. What voltage range do LCDs typically operate at?**
a) 1-3V
b) 3-12V
c) 12-24V
d) 24-50V
**Answer: b) 3-12V** [2]
**Explanation:** LCDs function efficiently between 3V and 12V, consuming low average current (1.2 µA to 6 µA) and producing minimal heat. [2]
**3. How do multi-color LEDs produce different colors?**
a) Single PN junction
b) Two PN junctions in anti-parallel
c) External filters
d) Temperature variation
**Answer: b) Two PN junctions in anti-parallel** [1]
**Explanation:** Applying positive or negative potential activates one of the two junctions, emitting light from the forward-biased diode. [1]
**4. What blocks light in LCD operation?**
a) Emitting crystals
b) Liquid crystals altering alignment
c) Backlight only
d) Magnetic fields
**Answer: b) Liquid crystals altering alignment** [2]
**Explanation:** Energized liquid crystals control light passage through pixels for color display, unlike light-emitting technologies. [2]
**5. Which LED color is absent in standard LED TV panels?**
a) Red
b) Green
c) Blue
d) Cyan
**Answer: d) Cyan** [3]
**Explanation:** LED TVs use red, green, blue sub-pixels; cyan is not a primary color in these panels. [3]
**6. What causes thermal runaway in LEDs?**
a) Constant current
b) Forward voltage change with temperature
c) Low voltage AC
d) High frequency
**Answer: b) Forward voltage change with temperature** [3]
**Explanation:** Temperature shifts forward voltage, risking burnout unless drivers maintain constant current. [3]
**7. What is the response time for LCD operation?**
a) 10 ms
b) 100 ms
c) 1 s
d) 10 s
**Answer: b) 100 ms** [2]
**Explanation:** Fast 100 ms opening time enables quick pixel changes without geometric distortion. [2]
**8. Which LCD type offers wide gamut with quantum dots?**
a) TN
b) IPS
c) Quantum Dot LCD
d) AFFS
**Answer: c) Quantum Dot LCD** [4]
**Explanation:** Quantum dot filters enhance color range for high-dynamic-range displays. [4]
**9. What do LED drivers primarily do?**
a) Increase voltage
b) Rectify AC to DC at low voltage
c) Generate heat
d) Block light
**Answer: b) Rectify AC to DC at low voltage** [1]
**Explanation:** Drivers convert high-voltage AC to precise low-voltage DC for LED stability. [1]
**10. Why are LCDs lighter than CRTs?**
a) Heavy crystals
b) Thinner design, low radiation
c) Magnetic dependency
d) High power use
**Answer: b) Thinner design, low radiation** [2]
**Explanation:** LCDs avoid bulky electron guns, working with CMOS ICs unaffected by fields. [2]
## Solar Cell
**1. What is the maximum power point (MPP) in a solar cell?**
a) Zero current
b) V-I point maximizing power
c) Open circuit
d) Short circuit
**Answer: b) V-I point maximizing power** [5]
**Explanation:** MPP on the V-I curve gives peak V times I output for optimal load efficiency. [5]
**2. How does light intensity affect solar cell photocurrent?**
a) Inversely proportional
b) Directly proportional
c) No effect
d) Logarithmic only
**Answer: b) Directly proportional** [5]
**Explanation:** More photons from higher intensity excite more electron-hole pairs, boosting current. [5]
**3. What is the photovoltaic effect basis?**
a) Heat generation
b) Light producing electricity in semiconductors
c) Magnetic induction
d) Chemical reaction
**Answer: b) Light producing electricity in semiconductors** [5]
**Explanation:** Photons absorbed create electron-hole pairs separated by junction field for current. [5]
**4. Typical open-circuit voltage for silicon solar cell?**
a) 0.1 V
b) 0.5-0.6 V
c) 2 V
d) 5 V
**Answer: b) 0.5-0.6 V** [5]
**Explanation:** Single-junction silicon cells produce about 0.5-0.6 V max under light. [5]
**5. Common materials for solar cells include?**
a) Copper only
b) Silicon, CdTe, CIGS
c) Aluminum
d) Plastic
**Answer: b) Silicon, CdTe, CIGS** [5]
**Explanation:** Semiconductors like silicon enable photovoltaic conversion efficiently. [5]
**6. Effect of light intensity on open-circuit voltage?**
a) Decreases
b) Increases logarithmically
c) Constant
d) Halves
**Answer: b) Increases logarithmically** [5]
**Explanation:** Higher intensity raises charge separation, logarithmic Voc rise. [5]
**7. What separates charges in solar cells?**
a) External battery
b) Built-in junction electric field
c) Resistor
d) Capacitor
**Answer: b) Built-in junction electric field** [5]
**Explanation:** Field at p-n junction directs electrons and holes to terminals. [5]
**8. Silicon solar cell efficiency factor?**
a) Independent of light
b) Proportional to photon energy absorption
c) Fixed at 100%
d) Inversely to temperature
**Answer: b) Proportional to photon energy absorption** [5]
**Explanation:** Absorbed photons above bandgap generate usable carriers. [5]
**9. Dark current in photodiode without light?**
a) High
b) Negligible
c) Maximum
d) Zero exactly
**Answer: b) Negligible** [6]
**Explanation:** Reverse current minimal absent incident light. [6]
**10. Recombination in forward-biased diode produces?**
a) Nothing
b) Heat, light, radiation
c) Only current
d) Voltage drop
**Answer: b) Heat, light, radiation** [6]
**Explanation:** Electron-hole recombination releases energy in various forms. [6]
## Diode as Circuit Element
**1. Diode forward bias drop for silicon?**
a) 0.3 V
b) 0.7 V
c) 1.4 V
d) 0 V
**Answer: b) 0.7 V** [6]
**Explanation:** Silicon diodes drop ~0.7 V; germanium ~0.3 V in forward bias. [6]
**2. Purpose of zener diode Q-point?**
a) Forward conduction
b) Voltage regulation at breakdown
c) Switching only
d) Light emission
**Answer: b) Voltage regulation at breakdown** [6]
**Explanation:** Zener maintains constant voltage across it in reverse breakdown. [6]
**3. In anti-parallel diodes, voltage drop cancels source?**
a) Current flows
b) Zero drop across resistance, zero current
c) Full conduction
d) Breakdown
**Answer: b) Zero drop across resistance, zero current** [6]
**Explanation:** Combined drops equal source voltage, no resistor current. [6]
**4. Photodiode current without light?**
a) Photocurrent
b) Dark current, negligible
c) Zener current
d) Forward current
**Answer: b) Dark current, negligible** [6]
**Explanation:** Minimal reverse saturation current absent illumination. [6]
**5. Diode primary function?**
a) Amplify
b) One-way current control
c) Store charge
d) Oscillate
**Answer: b) One-way current control** [6]
**Explanation:** Allows forward, blocks reverse current flow. [6]
**6. Example diode circuit current calculation?**
a) Ignores drops
b) Accounts for 0.7 V Si, 0.3 V Ge
c) Assumes zero drop
d) Uses breakdown
**Answer: b) Accounts for 0.7 V Si, 0.3 V Ge** [6]
**Explanation:** Drops subtracted from supply for accurate current. [6]
**7. Capacitor-diode circuit identification?**
a) All resistors
b) Capacitors and diodes mix
c) Only diodes
d) Inductors
**Answer: b) Capacitors and diodes mix** [6]
**Explanation:** Common in filters, rectifiers with specific roles. [6]
**8. Forward recombination effect?**
a) Blocks light
b) Produces heat/light/radiation
c) Increases resistance
d) No energy
**Answer: b) Produces heat/light/radiation** [6]
**Explanation:** Carrier annihilation releases photon/heat/phonon energy. [6]
**9. Diode in rectifier role?**
a) AC to AC
b) AC to DC conversion
c) DC to AC
d) Filtering only
**Answer: b) AC to DC conversion** [6]
**Explanation:** Passes one polarity half-cycles. [6]
**10. Cut-in voltage effect at 1 kHz?**
a) Irrelevant
b) Determines conduction timing
c) Halves frequency
d) Doubles voltage
**Answer: b) Determines conduction timing** [6]
**Explanation:** 0.7 V threshold sets forward bias onset. [6]
## Rectifiers: Half Wave, Full Wave, and Bridge
**1. Half-wave rectifier efficiency max?**
a) 81.2%
b) 40.6%
c) 100%
d) 50%
**Answer: b) 40.6%** [7]
**Explanation:** Uses only half AC cycle, lower DC output relative to RMS. [7]
**2. Full-wave rectifier efficiency max?**
a) 40.6%
b) 81.2%
c) 90%
d) 46%
**Answer: b) 81.2%** [7]
**Explanation:** Utilizes both AC halves for higher DC power conversion. [7]
**3. Bridge rectifier TUF vs center-tapped?**
a) Worse
b) Better
c) Equal
d) Half
**Answer: b) Better** [7]
**Explanation:** Bridge uses full transformer winding efficiently without tap. [7]
**4. Center-tapped PIV vs bridge?**
a) Lower
b) Higher
c) Same
d) Double bridge
**Answer: b) Higher** [7]
**Explanation:** Center-tapped diodes face twice peak voltage. [7]
**5. Bridge rectifier diodes needed?**
a) 1
b) 2
c) 4
d) 3
**Answer: c) 4** [8]
**Explanation:** Four diodes conduct alternately for full rectification. [8]
**6. Full-wave average voltage for 66 V peak?**
a) 21 V
b) 42 V
c) 66 V
d) 33 V
**Answer: b) 42 V** [9]
**Explanation:** Average = (2 Vm / π) ≈ 0.637 Vm for full-wave. [9]
**7. Half-wave uses?**
a) Both cycles
b) Positive half only
c) Negative half
d) Full with filter
**Answer: b) Positive half only** [7]
**Explanation:** Single diode passes one polarity. [7]
**8. Ripple factor lowest in?**
a) Half-wave
b) Full-wave
c) Bridge
d) None
**Answer: b) Full-wave** [7]
**Explanation:** Both halves smooth output better. [7]
**9. Bridge rectification efficiency?**
a) 40.6%
b) 81.2%
c) 46%
d) 90%
**Answer: b) 81.2%** [8]
**Explanation:** Measures DC content presence, same as full-wave. [8]
**10. Capacitive filter in rectifier?**
a) High-pass
b) Low-pass, smooths ripple
c) Blocks DC
d) Amplifies
**Answer: b) Low-pass, smooths ripple** [7]
**Explanation:** Charges to peak, discharges slowly through load. [7]
more
## LED and LCD (Additional Set)
**1. What determines the direction of electric field in an LCD?**
a) Magnetic field
b) Electric field
c) Electromagnetic field
d) Gallois field
**Answer: b) Electric field** [5]
**Explanation:** Electric field induced by voltage changes liquid crystal molecule orientation for light modulation. [5]
**2. How many LEDs in a 7-segment display?**
a) 8
b) 7
c) 10
d) 9
**Answer: b) 7** [5]
**Explanation:** Seven LEDs form segments to display decimal numerals efficiently. [5]
**3. What is the backplane in LCD?**
a) DC voltage between segment and common
b) AC voltage between segment and common
c) Power consumption amount
d) Intensity adjuster
**Answer: b) AC voltage between segment and common** [5]
**Explanation:** Backplane applies AC to drive segments via EX-OR gates. [5]
**4. LCD displays images by?**
a) Emitting light
b) Blocking white light
c) Reflecting only
d) Generating heat
**Answer: b) Blocking white light** [3]
**Explanation:** Voltage alters liquid crystal properties to control light passage through polarizers. [3]
**5. LED TVs offer richer color than LCD panels due to?**
a) No backlight
b) Modified white light color
c) Thicker panels
d) Higher power
**Answer: b) Modified white light color** [6]
**Explanation:** Backlight adjustment compensates for LCD limitations in color reproduction. [6]
**6. Which effect produces electro-optical changes in LCD?**
a) Thermal
b) Piezoelectric
c) Electro-optical
d) Photoelectric
**Answer: c) Electro-optical** [5]
**Explanation:** Voltage orients molecules along or perpendicular to field based on chemical structure. [5]
**7. LCD advantage over CRT?**
a) Geometric distortion
b) Affected by magnetic fields
c) High power and heat
d) No radiation, thin design
**Answer: d) No radiation, thin design** [3]
**Explanation:** Compact structure with low emission and field immunity. [3]
**8. LED vs LCD TV thickness?**
a) LED thicker
b) LCD thinner
c) Equal
d) LED requires more power only
**Answer: a) LED thicker** [3]
**Explanation:** LED panels achieve thinner profiles than traditional LCDs. [3]
**9. LCD pixel control mechanism?**
a) Heat
b) Twisted crystals blocking light
c) Direct emission
d) Magnetic twist
**Answer: b) Twisted crystals blocking light** [4]
**Explanation:** Electricity untwists crystals to allow or block backlight selectively. [4]
**10. Refresh rate range for LCD operation?**
a) 1-10 Hz
b) 30-60 Hz
c) 100-200 Hz
d) DC only
**Answer: b) 30-60 Hz** [1]
**Explanation:** Standard rates prevent flicker in display updates. [1]
## Solar Cell (Additional Set)
**1. Which factor most limits silicon solar cell efficiency?**
a) Perfect absorption
b) Bandgap mismatch with spectrum
c) Infinite carriers
d) Zero recombination
**Answer: b) Bandgap mismatch with spectrum** [11]
**Explanation:** Silicon bandgap misses high/low energy photons, capping theoretical efficiency. [11]
**2. Fill factor measures?**
a) Maximum power ratio to Voc Isc
b) Open circuit only
c) Current density
d) Area coverage
**Answer: a) Maximum power ratio to Voc Isc** [12]
**Explanation:** Indicates curve squareness and quality near ideal rectangle. [12]
**3. Temperature effect on solar cell voltage?**
a) Increases linearly
b) Decreases
c) No change
d) Oscillates
**Answer: b) Decreases** [11]
**Explanation:** Higher temperature reduces bandgap, lowering open-circuit voltage. [11]
**4. Monocrystalline vs polycrystalline efficiency?**
a) Equal
b) Mono higher
c) Poly higher
d) Both 100%
**Answer: b) Mono higher** [12]
**Explanation:** Fewer defects in single crystal improve carrier collection. [12]
**5. Series resistance impact on IV curve?**
a) No effect
b) Reduces slope near Voc
c) Increases fill factor
d) Shifts left
**Answer: b) Reduces slope near Voc** [12]
**Explanation:** Limits current at high voltage, lowering power output. [12]
**6. Anti-reflection coating purpose?**
a) Increase reflection
b) Minimize light loss
c) Heat dissipation
d) Color change
**Answer: b) Minimize light loss** [11]
**Explanation:** Reduces Fresnel reflection at air-semiconductor interface. [11]
**7. Shunt resistance low value causes?**
a) Ideal curve
b) Soft knee at Isc
c) Higher Voc
d) No power loss
**Answer: b) Soft knee at Isc** [12]
**Explanation:** Leaks current around junction, reducing fill factor. [12]
**8. Spectral response peak for silicon?**
a) UV
b) IR
c) 800-1000 nm
d) Visible only
**Answer: c) 800-1000 nm** [11]
**Explanation:** Matches bandgap for optimal photon absorption. [11]
**9. Bypass diode prevents?**
a) Overvoltage
b) Hot spots in shaded cells
c) Efficiency gain
d) Current block
**Answer: b) Hot spots in shaded cells** [12]
**Explanation:** Provides path for reverse current in modules. [12]
**10. Quantum efficiency definition?**
a) Total power
b) Carriers per incident photon
c) Voltage gain
d) Area factor
**Answer: b) Carriers per incident photon** [11]
**Explanation:** Measures internal conversion effectiveness. [11]
## Diode as Circuit Element (Additional Set)
**1. Reverse recovery time affects?**
a) Forward conduction
b) Switching speed
c) Breakdown voltage
d) Light emission
**Answer: b) Switching speed** [13]
**Explanation:** Time to sweep out stored charge during turn-off. [13]
**2. Schottky diode advantage?**
a) High forward drop
b) Fast switching, low drop
c) High breakdown
d) Light emitting
**Answer: b) Fast switching, low drop** [13]
**Explanation:** Metal-semiconductor junction avoids minority carrier storage. [13]
**3. Avalanche vs Zener breakdown?**
a) Same mechanism
b) Avalanche impact ionization, Zener tunneling
c) Both tunneling
d) Both thermal
**Answer: b) Avalanche impact ionization, Zener tunneling** [13]
**Explanation:** Occurs at different voltage ranges and doping levels. [13]
**4. Varactor diode used for?**
a) Rectification
b) Variable capacitance tuning
c) LED
d) Clamping
**Answer: b) Variable capacitance tuning** [13]
**Explanation:** Reverse bias width modulates junction capacitance. [13]
**5. Diode clipping circuit does?**
a) Amplifies
b) Limits voltage peaks
c) Inverts
d) Filters
**Answer: b) Limits voltage peaks** [13]
**Explanation:** Clamps signal above/below diode thresholds. [13]
**6. Germanium diode forward voltage?**
a) 0.7 V
b) 0.3 V
c) 1.2 V
d) 0 V
**Answer: b) 0.3 V** [13]
**Explanation:** Lower bandgap than silicon results in smaller drop. [13]
**7. PIN diode application?**
a) Light emission
b) RF switching/attenuation
c) Voltage regulation
d) Rectification only
**Answer: b) RF switching/attenuation** [13]
**Explanation:** Wide intrinsic layer for low capacitance, high power. [13]
**8. Dynamic resistance of diode?**
a) Infinite forward
b) dV/dI small in forward
c) Zero reverse
d) Constant
**Answer: b) dV/dI small in forward** [13]
**Explanation:** Measures nonlinearity; low value indicates good conduction. [13]
**9. Tunnel diode characteristic?**
a) Normal rectification
b) Negative resistance region
c) High forward drop
d) Slow switching
**Answer: b) Negative resistance region** [13]
**Explanation:** Heavy doping enables quantum tunneling. [13]
**10. LED forward voltage depends on?**
a) Current only
b) Material bandgap/color
c) Size alone
d) Temperature fixed
**Answer: b) Material bandgap/color** [14]
**Explanation:** Energy gap sets photon wavelength and voltage. [14]
## Rectifiers (Additional Set)
**1. Ripple factor half-wave unfiltered?**
a) 0.48
b) 1.21
c) 0.04
d) Infinite
**Answer: b) 1.21** [15]
**Explanation:** High due to single half-cycle pulses. [15]
**2. Center-tap full-wave diodes?**
a) 4
b) 2
c) 1
d) 3
**Answer: b) 2** [15]
**Explanation:** Each half uses one diode from tapped secondary. [15]
**3. Bridge rectifier advantage?**
a) Needs center tap
b) No tap, uses full winding
c) Lower PIV
d) Single diode
**Answer: b) No tap, uses full winding** [15]
**Explanation:** Higher transformer utilization factor. [15]
**4. Form factor full-wave?**
a) 1.11
b) 1.57
c) 1.8
d) 1
**Answer: a) 1.11** [16]
**Explanation:** RMS to average voltage ratio lower than half-wave. [16]
**5. Peak inverse voltage bridge?**
a) 2 Vm
b) Vm
c) Vm/2
d) Infinite
**Answer: b) Vm** [15]
**Explanation:** Each diode sees peak input once. [15]
**6. Inductor filter best with?**
a) Half-wave
b) Full-wave
c) Reduces ripple more
d) Increases frequency
**Answer: b) Full-wave** [15]
**Explanation:** Higher pulse rate aids smoothing. [15]
**7. Full-wave output frequency for 50 Hz AC?**
a) 50 Hz
b) 100 Hz
c) 25 Hz
d) DC pure
**Answer: b) 100 Hz** [16]
**Explanation:** Both halves rectified double frequency. [16]
**8. Transformer utilization bridge vs center-tap?**
a) Half
b) Double
c) Equal
d) None
**Answer: b) Double** [15]
**Explanation:** Full secondary voltage used. [15]
**9. Half-wave with capacitor filter ripple?**
a) Lowest
b) Highest among types
c) Zero
d) Same as full
**Answer: b) Highest among types** [15]
**Explanation:** Longer discharge time between pulses. [15]
**10. Efficiency formula rectifier?**
a) Pdc / Pac
b) (8/π²) * 100% for full-wave
c) 50% fixed
d) Ripple dependent
**Answer: b) (8/π²) * 100% for full-wave** [16]
**Explanation:** Theoretical maximum 81.2% assuming ideal diodes. [16]
