Physics 40 Solved Past Papers 2000 to 2024 Archives

Electronics Past Papers

Solved past paper MCQs for Electronics from official UHS, NUMS, SZABMU, DUHS, and KMU examinations. Includes verified distractor autopsies and step-by-step cognitive explanations.

Boards Included: DUHS ETEA MDCAT NUMS PMC PMDC SZABMU UHS
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Official printable A4 booklet containing all 40 verified past paper MCQs with unbroken options, 5-column rapid answer key, and Propolis cognitive error autopsies.

Interactive Mode:
#1 of 40 SZABMU 2024
Permalink
Diode is/an _ device, which can be used for rectification process.
A
Perfect conducting
B
Semiconductor
C
Perfect insulating
D
Insulating
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

Diodes are manufactured from materials whose electrical properties lie between those of good conductors and insulators.

Solution:

  • A P-N junction diode is forged by doping Silicon or Germanium to create P-type and N-type regions.


  • Silicon and Germanium are fundamentally semiconductor materials.


  • Because their conductivity can be precisely controlled by doping and biasing, they make perfect rectifying devices.


Why other options are incorrect:

  • Perfect conducting: A short circuit, cannot rectify.


  • Insulating / Perfect insulating: An open circuit, current could never flow.
#2 of 40 NUMS 2024
Permalink
For rectification transformer used is:
A
With low input voltage
B
Step up
C
Step down
D
With low input resistance
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

Most electronic devices operate on low-voltage Direct Current (DC), but power grids supply high-voltage Alternating Current (AC).

Solution:

  • Mains AC is typically \( 220\text{V} \) or \( 110\text{V} \). Electronic circuits (like phones and laptops) require much lower voltages (e.g., \( 5\text{V} \), \( 12\text{V} \)).


  • Before rectification occurs, the high mains voltage must be lowered to a safe, usable level to prevent destroying the diodes.


  • A Step down transformer is utilized at the very beginning of the power supply to achieve this voltage reduction.


Why other options are incorrect:

  • Step up: Would dangerously increase the voltage, destroying the circuit.


  • Low input voltage / Low input resistance: Do not describe the fundamental function of the transformer in this context.
#3 of 40 DUHS 2023
Permalink
A semi-conductor diode can be used as:
A
A transmitter
B
An amplifier
C
A full wave rectifier
D
A half wave rectifier
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

A single diode blocks one polarity of current and permits the other.

Solution:

  • Because a single diode can only conduct during one half of an AC cycle (either positive or negative depending on orientation), it effectively chops off half the wave.


  • Therefore, a single semiconductor diode acts purely as a half wave rectifier.


  • A full wave rectifier strictly requires a minimum of two diodes (with a center tap) or four diodes (bridge).


Why other options are incorrect:

  • Full wave rectifier: Requires multiple diodes.


  • Amplifier: Requires a transistor (active device with gain).


  • Transmitter: Requires complex oscillator and antenna circuits.
#4 of 40 NUMS 2023
Permalink
A device that converts AC into DC is called:
A
Transistor
B
Inductor
C
Capacitor
D
Diode
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

Rectification depends on unilateral conductivity.

Solution:

  • The core element in power electronics used to convert Alternating Current (AC) into Direct Current (DC) is the diode.


  • It performs this by providing near-zero resistance to forward current and near-infinite resistance to reverse current.


Why other options are incorrect:

  • Transistor: Used for amplification.


  • Capacitor/Inductor: Used for energy storage and filtering, but not the primary rectifying agents.
#5 of 40 NUMS 2023
Permalink
The conversion of alternating current into direct current is called:
A
Magnification
B
Amplification
C
Rectification
D
Resolution
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

The foundational terminology for AC manipulation is strictly defined.

Solution:

  • When an AC wave (which oscillates above and below zero) is passed through diodes, the output is restricted to one polarity.


  • This process of "straightening out" the alternating wave into a direct, single-polarity wave is scientifically termed Rectification.


Why other options are incorrect:

  • Amplification: Boosting signal strength.


  • Magnification/Resolution: Terms for optical enhancement, totally unrelated to electronics.
#6 of 40 NUMS 2023
Permalink
In full wave rectification ____ diodes are used.
A
2
B
1
C
3
D
4
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

There are two primary ways to achieve full-wave rectification: Center-tapped (\( 2 \) diodes) and Bridge (\( 4 \) diodes). Without specifying "center-tapped", the industry standard topology is the bridge rectifier.

Solution:

  • A Full Wave Bridge Rectifier does not require an expensive center-tapped transformer.


  • It utilizes an arrangement of exactly \( 4 \) diodes configured in a bridge loop.


  • During the positive half cycle, two diodes conduct. During the negative half cycle, the other two conduct.


  • for this specific NUMS question, \( 4 \) is the expected answer.


Why other options are incorrect:

  • \( 1 \): Makes a half-wave rectifier.


  • \( 2 \): Valid only if a "center-tapped transformer" is explicitly mentioned.


  • \( 3 \): Not a functional rectifier configuration.
#7 of 40 UHS 2022
Permalink
When the temperature of semiconductor suddenly drops to zero kelvin, then a semiconductor acts as:
A
Super conductor
B
Semi-conductor
C
Conductor
D
Insulator
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

The electrical conductivity of intrinsic semiconductors is highly temperature-dependent. Thermal energy is required to break covalent bonds and excite electrons from the valence band to the conduction band.

Solution:

  • At Absolute Zero (\( 0\text{ K} \)), there is zero thermal energy available in the lattice.


  • Consequently, no covalent bonds are broken, and absolutely no electrons can cross the bandgap into the conduction band.


  • With zero free electrons and zero holes, the material has infinite resistivity. Therefore, it acts perfectly as an insulator.


Why other options are incorrect:

  • Conductor / Super conductor: Conductivity drops to zero at \( 0\text{ K} \) for semiconductors, unlike metals which become better conductors or superconductors at ultra-low temperatures.


  • Semi-conductor: At \( 0\text{ K} \), it loses its defining semi-conductive property and behaves purely as an insulator.
#8 of 40 UHS 2022
Permalink
A process in which only one half of alternating current is converted into direct current, such process is called:
A
Half wave rectification
B
Magnification
C
Amplification
D
Full wave rectification
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

Rectification is the process of converting bidirectional alternating current (AC) into unidirectional direct current (DC).

Solution:

  • If a circuit utilizes both the positive and negative halves of the AC cycle, it is Full Wave Rectification.


  • If a circuit uses a single diode to block one half of the AC cycle (letting only the positive or negative half pass), it is termed Half wave rectification.


Why other options are incorrect:

  • Full wave rectification: Converts both halves.


  • Amplification & Magnification: Refers to increasing signal strength/size, completely unrelated to AC/DC conversion.
#9 of 40 SZABMU 2022
Permalink
Efficiency of full wave rectifier circuit is almost _ than half wave rectifier circuit:
A
Four times
B
Double
C
Sixteen times
D
Same as
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

Rectifier efficiency (\( \eta \)) represents the ratio of DC output power to the applied AC input power.

Solution:

  • A Half-Wave Rectifier wastes an entire half-cycle of the input wave. Its maximum theoretical efficiency is roughly \( 40.6\% \).


  • A Full-Wave Rectifier utilizes both the positive and negative half-cycles of the input wave. Its maximum theoretical efficiency is roughly \( 81.2\% \).


  • Comparing the two: \( 81.2\% \) is exactly double of \( 40.6\% \).


Why other options are incorrect:

  • Four times / Sixteen times: These drastically overestimate the efficiency ratio.


  • Same as: Incorrect, because utilizing the blocked half-cycle inherently increases power efficiency.
#10 of 40 ETEA 2022
Permalink
Output current of a half wave rectifier is:
A
AC current
B
Unidirectional current
C
Zero always
D
Straight line parallel to vertical axis
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

The defining function of any rectifier is to transform bidirectional current into current that flows in only one direction.

Solution:

  • A half-wave rectifier uses a diode to block the negative half of the AC cycle.


  • The resulting output consists of pulses of positive current separated by gaps of zero current.


  • Even though the magnitude of the current fluctuates (pulsates), it never drops below zero to flow backward. Thus, it is purely a unidirectional current.


Why other options are incorrect:

  • AC current: AC flows in both directions; the rectifier has eliminated this.


  • Straight line parallel to vertical axis: This describes an infinite slope, which represents zero physical reality in this context (an ideal steady DC would be parallel to the horizontal axis).


  • Zero always: Incorrect, as it conducts during the positive half-cycle.
#11 of 40 ETEA 2022
Permalink
The resistance of full wave rectifier is:
A
Less than the resistance of half wave rectifier
B
Equal to the resistance of half wave rectifier
C
More than the resistance of half wave rectifier
D
Negligible in comparison to the resistance of half wave rectifier
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

The total internal forward resistance of a rectifier circuit depends on the number of diodes in the conduction path during any given half-cycle.

Solution:

  • In a Half-Wave Rectifier, current flows through only \( 1 \) diode during the active half-cycle. Therefore, the forward resistance is \( R_d \).


  • In a Full-Wave Bridge Rectifier (the most common un-tapped configuration), current must pass through \( 2 \) diodes in series during every half-cycle. Therefore, the forward resistance is \( 2 R_d \).


  • Since \( 2 R_d > R_d \), the internal resistance of the FWR circuit is more than the resistance of a half wave rectifier.


Why other options are incorrect:

  • A single diode will physically offer less resistance than two diodes in series, making "less than" or "equal to" incorrect when comparing bridge to half-wave architectures.
#12 of 40 ETEA 2022
Permalink
When AC is converted to DC, the process is called:
A
Resolution
B
Magnification
C
Amplification
D
Rectification
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

Electronic systems often require power supplies that convert alternating current from the mains into stable direct current.

Solution:

  • The formal engineering term for converting alternating current (AC) to direct current (DC) is rectification.


  • This is achieved using "rectifier" circuits built around diodes.


Why other options are incorrect:

  • Amplification: Boosting the power of a signal.


  • Magnification: Visual enlargement.


  • Resolution: Clarity or detail in imaging/data.
#13 of 40 DUHS 2022
Permalink
The potential difference applied across a diode to reduce depletion layer is:
A
Junction potential
B
Reverse biasing
C
Forward biasing
D
Biasing voltage
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

The depletion layer in a P-N junction acts as a barrier to charge carriers. Applying external voltage (biasing) manipulates the width of this layer.

Solution:

  • When the positive terminal of a battery is connected to the P-type material and the negative terminal to the N-type material, holes and electrons are pushed toward the junction.


  • This external potential opposes the built-in junction potential.


  • As a result, the width of the depletion layer decreases, allowing current to flow. This specific configuration is known as Forward biasing.


Why other options are incorrect:

  • Reverse biasing: Increases the width of the depletion layer.


  • Junction potential: The internal built-in barrier itself, not an externally applied voltage.


  • Biasing voltage: Too generic; could refer to either forward or reverse bias.
#14 of 40 NUMS 2022
Permalink
A device that converts AC into DC is called:
A
Diode
B
Capacitor
C
Transistor
D
Inductor
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

Converting AC to DC fundamentally requires a one-way electrical valve.

Solution:

  • A P-N junction diode is explicitly designed to conduct current easily in one direction (forward bias) and block it in the other (reverse bias).


  • When an AC signal is applied, the diode strips away one half of the waveform, executing the core task of rectification (AC to DC).


Why other options are incorrect:

  • Capacitor: Stores charge, smooths DC, but cannot rectify.


  • Inductor: Opposes changes in current, cannot rectify.


  • Transistor: Used to amplify or switch signals.
#15 of 40 NUMS 2022
Permalink
The conversion of alternating current into direct current is called:
A
Amplification
B
Rectification
C
Magnification
D
Resolution
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

Standard terminology defines the various manipulations of electrical signals.

Solution:

  • The deliberate process of blocking or inverting the reverse-flowing portion of an alternating current (AC) so that it flows exclusively in one direction (DC) is termed Rectification.


Why other options are incorrect:

  • Amplification: Increasing the magnitude of a signal.


  • Magnification / Resolution: Terms primarily used in optics and imaging, completely unrelated to AC/DC conversion.
#16 of 40 PMDC 2021
Permalink
Which has less ripple factor?
A
Half wave rectifier
B
None
C
Full wave rectifier
D
Both
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

The ripple factor (\( \gamma \)) measures the smoothness of the DC output. A lower ripple factor means a smoother DC output with fewer AC fluctuations.

Solution:

  • For a Half-Wave Rectifier (HWR), the standard ripple factor is approximately \( 1.21 \) (or \( 121\% \)), meaning the AC ripple is actually larger than the DC component.


  • For a Full-Wave Rectifier (FWR), the standard ripple factor is approximately \( 0.48 \) (or \( 48\% \)).


  • Since \( 0.48 < 1.21 \), the Full wave rectifier has significantly less ripple factor.


Why other options are incorrect:

  • Half-wave rectifiers have huge gaps between pulses, leading to massive ripples.


  • They cannot be "Both" or "None" as their mathematical constants for ripple factor distinctly differ.
#17 of 40 PMDC 2021
Permalink
Ripple factor of full wave rectifier?
A
1.21
B
0.84
C
0.482
D
0.21
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

Ripple factor mathematically describes the ratio of the RMS value of the AC component to the DC component in the output.

Formula:

$$ R.F = \sqrt{(F.F)^2 - 1} $$

Solution:

  • The Form Factor (\( F.F \)) for a full-wave rectifier is \( \frac{I_{rms}}{I_{DC}} = \frac{\pi}{2\sqrt{2}} \approx 1.11 \).


  • Substitute this into the ripple factor formula: \( R.F = \sqrt{(1.11)^2 - 1} \).


  • \( R.F = \sqrt{1.2321 - 1} = \sqrt{0.2321} \approx 0.482 \).


Why other options are incorrect:

  • \( 1.21 \): This is the ripple factor for a half-wave rectifier.


  • \( 0.84 \) & \( 0.21 \): These are incorrect distractors not tied to standard rectifier outputs.
#18 of 40 PMDC 2021
Permalink
If \( f = 50\text{Hz} \) then in half wave no of pulses are
A
100
B
150
C
50
D
60
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

In a half-wave rectifier, the diode conducts only during the positive half-cycle of the AC input and blocks the negative half-cycle.

Formula:

$$ f_{output} = f_{input} \quad \text{(For HWR)} $$

Solution:

  • For every \( 1 \) full cycle of AC input, a half-wave rectifier produces exactly \( 1 \) output pulse.


  • If the input frequency is \( 50 \text{ Hz} \) (meaning \( 50 \) full cycles per second), the output will correspondingly have \( 50 \) pulses per second.


  • Therefore, the number of pulses is \( 50 \).


Why other options are incorrect:

  • \( 100 \): This would be the number of pulses for a Full-Wave Rectifier, where \( f_{output} = 2 \times f_{input} \).


  • \( 60 \) & \( 150 \): Incorrect values not mathematically linked to a \( 50\text{Hz} \) input.
#19 of 40 PMC 2021
Permalink

What is the typical range of the forbidden energy band gap in electrical conductors?
A
100 - 1000 eV
B
0.5 - 10 eV
C
10000 - 10000000 eV
D
0.01 - 0.1 eV
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

Band theory of solids divides materials into conductors, semiconductors, and insulators based on the gap between the valence and conduction bands.

Solution:

In excellent conductors (metals), the valence and conduction bands overlap, meaning the band gap is effectively zero. In certain weak conductors or semi-metals, a very narrow energy gap exists, which typically ranges from 0.01 to 0.1 eV.

Why other options are incorrect:

  • 0.5 to 10 eV describes the band gap of typical semiconductors and insulators.


  • Other extremely high values are physically non-existent for solid-state molecular structures.
#20 of 40 PMC 2021
Permalink

If a half-wave rectifier is connected to a 50 Hz AC mains, the number of DC pulses generated per second in the output voltage is:
A
25
B
50
C
100
D
75
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

The ripple frequency of a half-wave rectifier is exactly equal to the frequency of the input AC supply.

Solution:

A half-wave rectifier permits only one polarity of current (one peak) to pass through per input cycle, while blocking the other. Since the input frequency is \( 50\text{ Hz} \) (50 full cycles per second), there will be exactly 50 pulses of DC output per second.

Why other options are incorrect:

  • 100 is the output pulse frequency of a full-wave rectifier.


  • 25 and 75 do not correspond to the physics of half-wave rectification.
#21 of 40 PMC 2021
Permalink

A transformer is commonly used prior to the rectification stage in a DC power supply to:
A
Step up the voltage
B
Step down the voltage
C
Equalize the voltage
D
None of these
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

Standard household mains voltage is typically very high (e.g., 220V AC), while most digital and solid-state electronic devices operate on low DC voltages (e.g., 5V to 12V).

Solution:

The primary function of the transformer in a consumer electronic power supply is to step down the high AC mains voltage to a safe, low AC level before passing it through the rectifying diodes and filters.

Why other options are incorrect:

  • Step up is incorrect because electronic devices do not require hundreds or thousands of volts to operate.
#22 of 40 PMC 2021
Permalink

A basic half-wave rectifier passes only the:
A
Upper half-cycle
B
Lower half-cycle
C
Both half-cycles
D
None of these
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

A half-wave rectifier operates by selectively allowing only half of the input AC waveform to pass to the load.

Solution:

Under standard conventional setups, the diode is arranged to be forward-biased during the positive phase. It therefore permits only the positive (upper) half-cycle of the wave to pass while completely cutting off the negative (lower) half-cycle.

Why other options are incorrect:

  • Passing both half-cycles describes a full-wave rectifier.
#23 of 40 PMDC 2020
Permalink
In full wave rectification, the diodes are used:
A
1
B
3
C
4
D
2
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

Full-wave rectification requires processing both halves of the AC input waveform, which necessitates more than one diode.

Solution:

  • A Half-Wave Rectifier uses exactly \( 1 \) diode.


  • A Full-Wave Center-Tapped Rectifier uses \( 2 \) diodes.


  • A Full-Wave Bridge Rectifier uses \( 4 \) diodes.


  • for this specific exam question, the minimum configuration required for full wave rectification—using a center tap—utilizes \( 2 \) diodes.


Why other options are incorrect:

  • \( 1 \) diode only allows half-wave rectification.


  • \( 3 \) diodes is not a standard full-wave configuration.


  • While \( 4 \) diodes are used in a bridge configuration, \( 2 \) is the correct answer historically mapped for the general "full wave rectification" baseline involving center-tapped transformers.
#24 of 40 NUMS 2019 Reconduct
Permalink

In a half wave rectifier, the diode conducts during:
A
One half of the input cycle
B
A portion of the positive half of the input cycle
C
A portion of the negative half of the input cycle
D
Both halves of the input cycles
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

A half-wave rectifier uses a single diode that only permits current flow in one direction.

Solution:

An ideal diode conducts only when it is forward-biased. In an AC sinusoidal wave cycle:

  • During the positive half-cycle, the diode is forward-biased and conducts current.


  • During the negative half-cycle, the diode is reverse-biased and blocks current.


Therefore, the diode conducts for exactly one half of the input cycle (e.g., \( 180^\circ \) out of a full \( 360^\circ \) wave cycle).

Why other options are incorrect:

  • Both halves describes the conduction pattern of a full-wave rectifier instead.


  • The diode conducts for the entire positive/negative half-cycle, not just a small portion of it (assuming an ideal diode).
#25 of 40 MDCAT 2018
Permalink
In case of half rectification, resistance of diode during negative half of A.C is:
A
Very low
B
A few ohms
C
Very high
D
Negligible
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

A diode acts as a one-way valve for electrical current based on its biasing.

Solution:

  • During the positive half cycle of an AC input, the diode is forward-biased, offering very low resistance (ideal: zero).


  • During the negative half cycle, the polarity reverses, and the diode becomes reverse-biased.


  • In a reverse-biased state, the depletion region widens, resulting in a very high resistance (ideal: infinite) which blocks current flow, effectively creating the half-wave rectification.


Why other options are incorrect:

  • Very low, negligible, or a few ohms describe the forward-biased state, not the negative half (reverse-biased) state.
#26 of 40 MDCAT 2018
Permalink
The direction of current through the load resistance of a full-wave rectification circuit:
A
Inverts for positive cycle
B
Remains constant
C
Changes for every cycle
D
Inverts for negative cycle
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

The primary goal of any rectifier (half-wave or full-wave) is to convert bidirectional AC into unidirectional DC.

Solution:

  • In a full-wave rectifier, different diode pairs conduct during opposite halves of the AC cycle.


  • However, the physical topology of the circuit routes the current from both halves so that it always enters the load resistance from the exact same terminal.


  • Because the current always flows through the load in the same direction, the direction remains constant (unidirectional).


Why other options are incorrect:

  • If the current inverted or changed direction for any cycle, the output would still be alternating current (AC), rendering the rectifier useless.
#27 of 40 MDCAT 2017
Permalink
Output voltage of rectifier is not smooth. It can be made smooth by using a circuit known as:
A
Wheat stone circuit
B
Filter circuit
C
Bridge circuit
D
Ripple circuit
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

The output of a standard rectifier is a pulsating direct current, meaning it flows in one direction but its magnitude fluctuates.

Solution:

  • To convert this pulsating DC into a steady, constant DC voltage (similar to a battery), smoothing components are required.


  • Capacitors or inductors are introduced to bypass AC ripples or choke them, respectively.


  • A circuit designed specifically for this smoothing operation is called a filter circuit.


Why other options are incorrect:

  • Wheat stone circuit: Used for measuring unknown electrical resistance.


  • Bridge circuit: A configuration of diodes used for rectification, not post-rectification smoothing.


  • Ripple circuit: Not a standard electronic term; "ripple" is the very flaw we are trying to remove.
#28 of 40 MDCAT 2017
Permalink
In the following figure what happens for the positive half cycle of the input?

D1 D2 D3 D4
A
\( D_1 \) and \( D_3 \) conducts
B
\( D_1 \) and \( D_2 \) conducts
C
\( D_3 \) and \( D_4 \) conducts
D
\( D_2 \) and \( D_4 \) conducts
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

In a standard full-wave bridge rectifier, diodes conduct in diagonally opposite pairs depending on the instantaneous polarity of the alternating input voltage.

Solution:

  • During the positive half-cycle of the AC input voltage, the terminal polarity forward-biases the first pair of diagonally opposite diodes, labeled \( D_1 \) and \( D_3 \) in the diagram.


  • Simultaneously, the opposite pair (\( D_2 \) and \( D_4 \)) is reverse-biased and blocks current.


  • Therefore, \( D_1 \) and \( D_3 \) conduct during the positive half-cycle.


Why other options are incorrect:

  • Option D: \( D_2 \) and \( D_4 \) conduct during the alternating negative half-cycle.
  • Options B and C: Adjacent pairs (such as \( D_1 \) & \( D_2 \), or \( D_3 \) & \( D_4 \)) cannot conduct simultaneously in a bridge rectifier as they would cause an input short circuit.
#29 of 40 ETEA 2012
Permalink
Conversion of alternating current into direct current is called:
A
Amplification
B
Oscillation
C
Rectification
D
Regeneration
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

The fundamental process of translating an alternating waveform (which changes direction periodically) into a unidirectional waveform is a core electronics principle.

Solution:

  • An alternating current (AC) flows in both directions, whereas a direct current (DC) flows only in one direction.


  • The specific process of converting AC to DC using electronic components (like diodes) is formally defined as rectification.


Why other options are incorrect:

  • Amplification: Increasing the amplitude of a signal.


  • Oscillation: Converting DC into AC.


  • Regeneration: Feeding a portion of the output back into the input to boost signal strength.
#30 of 40 MDCAT 2000
Permalink
Consider the circuit diagram below representing a forward-biased silicon diode:

p (+)n (−)\(I_{\text{forward}}\)

In this forward-biased \(\text{p-n}\) junction, the electric current is predominantly transported across the junction by:
A
Minority carriers moving under a strong built-in field
B
Majority carriers diffusing across the junction barrier
C
Electrons tunneling directly through the forbidden bandgap
D
Thermionic emission of electrons from the metal contacts
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

In forward bias, external energy lowers the potential barrier, allowing majority carriers (holes from the p-side and electrons from the n-side) to diffuse across the junction, recombining as they enter the opposite regions and establishing a large forward current.

Formula:

$$I = I_s \left( e^{\frac{qV}{\eta k_B T}} - 1 \right)$$

Solution:

  • Forward bias lowers the built-in potential barrier from \(V_0\) to \((V_0 - V)\).


  • Majority holes in the p-region and majority electrons in the n-region obtain sufficient thermal energy to overcome the reduced barrier and diffuse across the junction.


  • Thus, forward current is primarily a majority carrier diffusion current.


Why other options are incorrect:

  • Option A: Minority carriers moving under the junction field cause reverse saturation leakage current.
  • Option C: Direct bandgap tunneling is significant only in heavily doped Zener/tunnel diodes, not standard forward-biased diodes.
  • Option D: Thermionic emission is the operating principle of vacuum tubes, not solid-state \(\text{p-n}\) junctions.
#31 of 40 MDCAT 2000
Permalink
Consider the bridge rectifier circuit shown below:

AC InAC In+ DC− DC

During each individual half-cycle of the AC input voltage, how many diodes conduct current simultaneously in series with the load resistor?
A
1
B
2
C
3
D
4
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

In a bridge rectifier, the four diodes operate in diagonally opposite pairs. During any given half-cycle, one diagonal pair of diodes is forward-biased and conducts current in series through the load, while the other pair is reverse-biased and blocks current.

Formula:

Qualitative concept / No formula required.

Solution:

  • Positive half-cycle: Diodes \(D_1\) and \(D_3\) conduct (total = 2 diodes).


  • Negative half-cycle: Diodes \(D_2\) and \(D_4\) conduct (total = 2 diodes).


  • Therefore, exactly 2 diodes conduct simultaneously during each half-cycle.


Why other options are incorrect:

  • Option A: Only 1 diode conducts at a time in a half-wave rectifier or center-tapped full-wave rectifier.
  • Option C: A bridge rectifier never activates three diodes concurrently during normal operation.
  • Option D: All 4 diodes conducting at once would cause a direct short-circuit of the AC source.
#32 of 40 MDCAT 2000
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RD1D2+6V0V

In the circuit shown above, diodes \( D_1 \) and \( D_2 \) have an ideal forward resistance of \( 50\ \Omega \) and infinite reverse resistance. If a \( +6.0\text{ V} \) DC potential is applied at the left terminal and \( R = 100\ \Omega \), what is the total current flowing through \( R \)?
A
0.040 A
B
0.020 A
C
0.000 A
D
0.060 A
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Correct Key: Option A Diagnostic Explanation
Concept:

When a positive potential is applied to the common input, analyze the biasing state of each parallel diode branch.

Formula:

$$I_{\text{total}} = \frac{V}{R_{\text{branch}} + R_L}$$

Solution:

  • \( D_1 \) has its anode connected to \( +6\text{ V} \), so it is forward-biased with \( R_{D1} = 50\ \Omega \).


  • \( D_2 \) has its cathode connected to \( +6\text{ V} \), so it is reverse-biased and acts as an open circuit (\( R_{D2} = \infty \)).


  • Current flows exclusively through the branch with \( D_1 \).


  • Total resistance: \( R_{\text{total}} = R_{D1} + R = 50\ \Omega + 100\ \Omega = 150\ \Omega \).


  • Circuit current: \( I = \frac{6.0\text{ V}}{150\ \Omega} = 0.040\text{ A} = 40\text{ mA} \).


Why other options are incorrect:

  • Option B: 0.020 A incorrectly treats both diodes as conducting in series (\( 6/300 \)).
  • Option C: 0.000 A assumes both diodes are reverse-biased.
  • Option D: 0.060 A neglects the diode forward resistance (\( 6/100 \)).
#33 of 40 MDCAT 2000
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+V+I-V-IVk

The I-V characteristic curve shown above represents the operational behavior of which electronic device?
A
Ohmic carbon resistor
B
Semiconductor PN junction diode
C
Ideal constant current source
D
Linear air-core inductor
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

The standard I-V curve of a PN junction exhibits an exponential increase in forward current above the knee voltage \( V_k \) (\( \approx 0.7\text{ V} \) for Si) and a small reverse saturation leakage current that breaks down at \( V_Z \).

Formula:

$$I = I_0 \left( e^{\frac{q V}{\eta k T}} - 1 \right)$$

Solution:

  • In the forward quadrant, current remains negligible until \( V \ge V_k \), after which it rises exponentially.


  • In the reverse quadrant, only a tiny saturation leakage current flows until reverse breakdown is reached.


  • This non-linear asymmetric profile is the characteristic signature of a semiconductor diode.


Why other options are incorrect:

  • Option A: An ohmic resistor produces a linear line through the origin (\( I = V/R \)).
  • Option C: A constant current source produces a horizontal line independent of voltage.
  • Option D: An inductor exhibits frequency-dependent AC reactance and a linear DC response, not an exponential I-V curve.
#34 of 40 MDCAT 2000
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RD1D2V_out

In the circuit shown above, identical ideal diodes \( D_1 \) and \( D_2 \) are connected in parallel with opposite polarities across an AC sinusoidal input voltage \( v(t) = V_m \sin(\omega t) \). What is the waveform of the voltage across the load resistor \( R \)?
A
Half-wave rectified waveform
B
Full-wave rectified unidirectional DC waveform
C
Constant smooth DC level
D
Unrectified AC sinusoidal waveform identical to the input
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

When two identical ideal diodes are connected in inverse-parallel (antiparallel), one diode conducts during the positive half-cycle while the other conducts during the negative half-cycle.

Formula:

$$\text{Positive Half-Cycle: } D_1 \text{ ON}, D_2 \text{ OFF} \implies v_R(t) = +V_m \sin(\omega t)$$

$$\text{Negative Half-Cycle: } D_2 \text{ ON}, D_1 \text{ OFF} \implies v_R(t) = -V_m |\sin(\omega t)|$$

Solution:

  • During the positive half-cycle, \( D_1 \) is forward-biased and conducts current through \( R \) from left to right.


  • During the negative half-cycle, \( D_2 \) is forward-biased and conducts current through \( R \) from right to left.


  • Because both half-cycles pass through \( R \) with opposite polarities, no rectification occurs and the output voltage matches the original AC input wave.


Why other options are incorrect:

  • Option A: Half-wave rectification blocks one of the half-cycles, which does not happen here because the inverse-parallel diode conducts it.
  • Option B: Full-wave rectification requires current to pass through the load in the same direction on both half-cycles.
  • Option C: A constant DC level requires rectification followed by large smoothing filters.
#35 of 40 MDCAT 2000
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+Vm+Vm0V (Gap)t

The voltage waveform shown above represents the unfiltered output of which type of circuit?
A
Full-wave bridge rectifier
B
Pure DC battery supply
C
Symmetrical AC inverter
D
Half-wave rectifier
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

A half-wave rectifier passes current during one half-cycle and blocks current during the other, producing an output of positive voltage pulses separated by zero-voltage gaps.

Formula:

$$v_{\text{out}}(t) = \begin{cases} V_m \sin(\omega t) & 0 \le \omega t < \pi \\ 0 & \pi \le \omega t < 2\pi \end{cases}$$

Solution:

  • The waveform shows conduction during the positive half-cycle followed by zero output during the negative half-cycle.


  • This periodic pattern with zero-voltage gaps is the signature of half-wave rectification.


Why other options are incorrect:

  • Option A: Full-wave rectifiers produce continuous pulses without zero-voltage gaps.
  • Option B: Pure DC is a flat, continuous horizontal line.
  • Option C: Symmetrical AC alternates between positive and negative polarities.
#36 of 40 MDCAT 2000
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+Vm+Vm+Vmt

The continuous pulsating unidirectional waveform shown above represents the unfiltered output of which circuit?
A
Full-wave rectifier (Bridge or Center-tapped)
B
Half-wave rectifier
C
High-pass RC filter circuit
D
Unbiased operational amplifier differentiator
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

A full-wave rectifier inverts the negative half-cycles of an AC input into positive pulses, producing continuous, unidirectional output pulses across the load.

Formula:

$$v_{\text{out}}(t) = |V_m \sin(\omega t)| \quad \text{for all } t$$

Solution:

  • The output waveform contains contiguous positive sinusoidal pulses with no dead-time gaps between half-cycles.


  • This continuous pulsating DC output is characteristic of full-wave rectifiers (both bridge and center-tapped topologies).


Why other options are incorrect:

  • Option B: Half-wave rectifiers have zero-voltage gaps between alternate pulses.
  • Option C: High-pass RC filters produce differentiated AC spikes centered around zero volts.
  • Option D: Differentiators output cosine waves from sine inputs, alternating between positive and negative polarities.
#37 of 40 MDCAT 2000
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A full-wave bridge rectifier operating from a \( 50\text{ Hz} \) line delivers a DC current of \( I_{\text{dc}} = 100\text{ mA} \) to a load. What filter capacitance \( C \) is required to maintain the peak-to-peak ripple voltage below \( 2.0\text{ V} \)?
A
1000 µF
B
500 µF
C
250 µF
D
2000 µF
View Answer & Propolis Autopsy
Correct Key: Option B Diagnostic Explanation
Concept:

In a full-wave rectifier, the ripple frequency is doubled (\( 2f = 100\text{ Hz} \)), so the peak-to-peak ripple voltage is given by \( V_r = \frac{I_{\text{dc}}}{2 f C} \).

Formula:

$$V_r = \frac{I_{\text{dc}}}{2 f C} \implies C = \frac{I_{\text{dc}}}{2 f \cdot V_r}$$

Solution:

  • Given: \( I_{\text{dc}} = 100\text{ mA} = 0.10\text{ A} \), \( f = 50\text{ Hz} \) (so \( 2f = 100\text{ Hz} \)), and \( V_r = 2.0\text{ V} \).


  • Required capacitance: \( C = \frac{0.10\text{ A}}{2 \times 50 \times 2.0\text{ V}} = \frac{0.10}{200} = 0.00050\text{ F} = 500\ \mu\text{F} \).


Why other options are incorrect:

  • Option A: 1000 µF is the capacitance required for a half-wave rectifier.
  • Option C: 250 µF would allow a larger ripple voltage of 4 V.
  • Option D: 2000 µF would reduce ripple to 0.5 V, exceeding the required 2.0 V specification.
#38 of 40 MDCAT 2000
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VinVout+Vref

In the series diode clipping circuit shown above, what is the output voltage \( V_{\text{out}} \) when the input AC voltage \( V_{\text{in}} \) exceeds \( V_{\text{ref}} + V_D \)?
A
Zero volts permanently
B
Reversed negative input voltage
C
Constant infinite voltage
D
Vin - VD (the diode conducts, transmitting the positive input above threshold)
View Answer & Propolis Autopsy
Correct Key: Option D Diagnostic Explanation
Concept:

A series diode conducts whenever its anode potential exceeds its cathode potential by at least the forward cut-in drop: \( V_{\text{in}} > V_K + V_D \).

Formula:

$$V_{\text{out}} = V_{\text{in}} - V_D \quad (\text{when } V_{\text{in}} \ge V_K + V_D)$$

$$V_{\text{out}} = 0\text{ V} \quad (\text{when } V_{\text{in}} < V_K + V_D, \text{ diode OFF})$$

Solution:

  • When \( V_{\text{in}} \ge V_{\text{ref}} + V_D \), the diode becomes forward-biased.


  • Current flows through the load, producing an output voltage of \( V_{\text{out}} = V_{\text{in}} - V_D \).


Why other options are incorrect:

  • Option A: The output is zero only when the diode is reverse-biased (\( V_{\text{in}} < V_{\text{ref}} + V_D \)).
  • Option B: The diode blocks negative voltages rather than inverting them.
  • Option C: Passive circuits cannot produce infinite voltages.
#39 of 40 MDCAT 2000
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In a four-diode full-wave bridge rectifier circuit, how many diodes conduct simultaneously during each half-cycle of the input \( \text{AC} \) waveform?

Bridge RectifierLoad
A
All four diodes conduct continuously in parallel
B
Only one diode conducts at a time per half-cycle
C
Two diodes conduct in series per half-cycle
D
Three diodes conduct while one remains in reverse breakdown
View Answer & Propolis Autopsy
Correct Key: Option C Diagnostic Explanation
Concept:

A bridge rectifier uses four diodes arranged in a bridge loop. Diode pairs conduct on alternate half-cycles to maintain unidirectional current through the load.

Formula:

$$V_{\text{load, peak}} = V_m - 2 V_{\text{barrier}}$$

Solution:

  • During the positive half-cycle, two diagonally opposite diodes (e.g., \( D_1 \) and \( D_3 \)) are forward-biased and conduct in series with the load.


  • During the negative half-cycle, the other two diodes (\( D_2 \) and \( D_4 \)) are forward-biased and conduct in series with the load.


  • At any point during conduction, exactly two diodes are active simultaneously.


Why other options are incorrect:

  • Option A: If all four diodes conducted simultaneously, the secondary winding would be short-circuited.
  • Option B: Single-diode conduction occurs in half-wave and center-tapped full-wave rectifiers, not bridge rectifiers.
  • Option D: Rectifiers operate symmetrically with balanced pairs of diodes; three-diode conduction does not occur.
#40 of 40 MDCAT 2000
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In a standard current-voltage (\( \text{I-V} \)) characteristic plot of a \( \text{P-N} \) junction diode, the forward-bias conduction region is located in:

+V-V+I-IV_knee
A
The first quadrant (positive voltage and positive current)
B
The second quadrant (negative voltage and positive current)
C
The third quadrant (negative voltage and negative current)
D
The fourth quadrant (positive voltage and negative current)
View Answer & Propolis Autopsy
Correct Key: Option A Diagnostic Explanation
Concept:

Under forward bias, a positive voltage applied to the anode results in a positive forward current from anode to cathode.

Formula:

$$V_f > 0, \quad I_f > 0 \implies (V_f, I_f) \in \text{Quadrant I}$$

Solution:

  • The horizontal axis represents applied voltage (\( +V \) to the right, \( -V \) to the left).


  • The vertical axis represents diode current (\( +I \) upward, \( -I \) downward).


  • Because forward bias corresponds to positive voltage (\( +V \)) and positive current (\( +I \)), the forward conduction curve lies in the first quadrant.


Why other options are incorrect:

  • Option B: The second quadrant (\( -V, +I \)) would represent power generation, which does not occur in passive diodes.
  • Option C: The third quadrant (\( -V, -I \)) represents reverse-bias leakage and breakdown.
  • Option D: The fourth quadrant (\( +V, -I \)) is non-physical for passive semiconductor diodes.
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