Level 1 — RecognitionDiodes & Applications

Diodes & Applications

⏱ 20 minutes30 marksprintable — key stays hidden on paper

Chapter: 2.3 Diodes & Applications Level: 1 — Recognition Time limit: 20 minutes Total marks: 30

Section A — Multiple Choice (1 mark each) [10 marks]

Select the single best answer.

Q1. In a half-wave rectifier fed by a sinusoid, how much of each input cycle appears at the output?

A) 100% B) 75% C) 50% D) 25%

Q2. A full-wave bridge rectifier uses how many diodes?

A) 1 B) 2 C) 4 D) 6

Q3. A Zener diode used for voltage regulation is normally operated in which region?

A) Forward conduction B) Reverse breakdown C) Cut-off D) Saturation

Q4. The colour of light emitted by an LED is determined mainly by:

A) forward current B) the semiconductor's band-gap energy C) case size D) reverse voltage

Q5. Compared with a silicon PN junction diode, a Schottky diode has:

A) a higher forward voltage drop B) a lower forward voltage drop
C) no reverse leakage D) slower switching

Q6. A varactor (varicap) diode is valued because its ______ varies with reverse voltage.

A) resistance B) inductance C) junction capacitance D) forward voltage

Q7. A photodiode is normally operated under:

A) forward bias B) reverse bias C) zero current D) breakdown

Q8. A diode clamping circuit is used to:

A) limit peak amplitude B) shift the DC level of a waveform
C) rectify AC D) regulate voltage

Q9. In a positive diode clipper, the diode:

A) removes part of the waveform above a level B) adds DC offset
C) stores charge D) emits light

Q10. The datasheet parameter $I_R$ typically refers to:

A) forward current B) reverse (leakage) current
C) rated ripple D) internal resistance

Section B — Matching (1 mark each) [8 marks]

Match each device/term (Q11–Q18) to the best description (i–viii). Each used once.

#	Device / Term
Q11	Rectifier diode
Q12	Zener diode
Q13	LED
Q14	Solar cell
Q15	Schottky diode
Q16	Varactor diode
Q17	$V_F$
Q18	PIV / $V_{RRM}$

Descriptions:

(i) Converts light energy into electrical energy
(ii) Voltage-controlled capacitor for tuning
(iii) Maintains near-constant voltage in reverse breakdown
(iv) Converts AC to pulsating DC
(v) Emits light when forward biased
(vi) Metal–semiconductor junction with fast switching
(vii) Forward voltage drop of a conducting diode
(viii) Maximum reverse voltage a diode can withstand

Section C — True/False WITH Justification (2 marks each) [12 marks]

State True or False (1 mark) and give a one-line justification (1 mark).

Q19. "A full-wave rectifier produces a higher average DC output than a half-wave rectifier for the same AC input."

Q20. "An LED conducts and lights up when it is reverse biased."

Q21. "In diode logic, connecting diodes with cathodes to a common output pulled high forms an AND-type arrangement (output low if any input is low)."

Q22. "A Schottky diode has a stored minority-charge recovery time comparable to a normal silicon diode."

Q23. "Ideally the Zener current must be kept above the minimum knee current for regulation to hold."

Q24. "Exceeding the maximum forward current rating on a datasheet is safe as long as reverse voltage is low."

Answer keyMark scheme & solutions

Section A (1 mark each)

Q1: C — A half-wave rectifier passes only one polarity (half) of each cycle → 50%. Q2: C — A bridge rectifier requires 4 diodes arranged in a bridge. Q3: B — Regulation exploits the near-constant reverse breakdown voltage. Q4: B — Photon energy $E=hf \approx E_g$ ; band-gap sets wavelength/colour. Q5: B — Metal–semiconductor junction gives ~0.2–0.4 V drop, lower than ~0.7 V Si. Q6: C — Reverse bias widens depletion region, changing junction capacitance. Q7: B — Reverse bias makes photocurrent proportional to light (photoconductive mode). Q8: B — A clamp adds/shifts a DC level without changing peak-to-peak. Q9: A — A clipper removes (limits) part of the waveform beyond a threshold. Q10: B — $I_R$ = reverse leakage current in datasheets.

Section B (1 mark each)

Q11 → (iv); Q12 → (iii); Q13 → (v); Q14 → (i); Q15 → (vi); Q16 → (ii); Q17 → (vii); Q18 → (viii).

Section C (True=1, Justification=1)

Q19: TRUE. Full-wave uses both halves, so average $V_{dc}=\frac{2V_m}{\pi}$ vs half-wave $\frac{V_m}{\pi}$ — twice the DC.

Q20: FALSE. LEDs emit only when forward biased (carriers recombine across junction); reverse bias gives no light and risks breakdown.

Q21: TRUE. With cathodes to output pulled up through a resistor, any input pulling its diode's anode low pulls the output low → output high only when all inputs high (AND behaviour). (Accept True with correct reasoning about common-cathode/pull-up giving AND.)

Q22: FALSE. A Schottky is a majority-carrier device with negligible minority-charge storage, so its recovery time is much shorter than a normal Si diode.

Q23: TRUE. Below the knee current the device leaves the sharp breakdown region and voltage no longer stays constant, so $I_Z>I_{Z(min)}$ is required.

Q24: FALSE. Maximum forward current is an independent thermal/current limit; exceeding it causes overheating and failure regardless of reverse voltage.

Mark distribution

Section A: 10 × 1 = 10
Section B: 8 × 1 = 8
Section C: 6 × 2 = 12
Total = 30

[
  {"claim":"Full-wave average = 2Vm/pi is twice half-wave Vm/pi","code":"Vm=symbols('Vm',positive=True); half=Vm/pi; full=2*Vm/pi; result = simplify(full/half - 2) == 0"},
  {"claim":"Half-wave passes 50 percent of cycle","code":"result = (180/360)*100 == 50"},
  {"claim":"Bridge rectifier uses 4 diodes","code":"result = 4 == 4"},
  {"claim":"Schottky forward drop ~0.3V is less than Si ~0.7V","code":"schottky=Rational(3,10); si=Rational(7,10); result = schottky < si"}
]