BEE Semiconductor Devices and Circuits 3 — Questions and Answers
Question 1: In a full-wave bridge rectifier with a transformer secondary voltage of 12 Vrms and ideal diodes, what is the average DC output voltage?
- 10.8 V (Correct answer)
- 7.64 V
- 16.97 V
- 5.4 V
Correct answer: 10.8 V
Vpeak = 12√2 ≈ 16.97 V; Vavg = (2/π)Vpeak ≈ 0.637 × 16.97 ≈ 10.8 V.
Question 2: The transconductance gm of a MOSFET in saturation is proportional to which of the following?
- VGS² − VT²
- VDS − VGS
- (VGS − VT) (Correct answer)
- 1/(VGS − VT)
Correct answer: (VGS − VT)
In saturation gm = ∂ID/∂VGS = kn(VGS − VT), so gm is proportional to (VGS − VT).
Question 3: Which parameter of a BJT is defined as the ratio of collector current to base current?
- α (alpha)
- β (beta) (Correct answer)
- gm (transconductance)
- Early voltage VA
Correct answer: β (beta)
β (hFE) = IC/IB is the common-emitter current gain of a BJT.
Question 4: A CMOS inverter draws maximum power during which condition?
- Static logic-high output
- Static logic-low output
- During input signal transitions (Correct answer)
- When both NMOS and PMOS are cut off
Correct answer: During input signal transitions
CMOS draws significant current only during transitions when both transistors are momentarily on, dissipating dynamic power.
Question 5: In a p-n junction, the diffusion capacitance is dominant under which bias condition?
- Strong reverse bias
- Zero bias
- Moderate to strong forward bias (Correct answer)
- Avalanche breakdown
Correct answer: Moderate to strong forward bias
Diffusion (storage) capacitance arises from minority carrier charge stored during forward bias and dominates in forward-biased junctions.
Question 6: An emitter-follower (common-collector) amplifier is primarily used for:
- High voltage gain with low current gain
- Impedance buffering with unity voltage gain (Correct answer)
- Phase inversion with high voltage gain
- High-frequency oscillation generation
Correct answer: Impedance buffering with unity voltage gain
The emitter-follower has voltage gain ≈1, very high input impedance, and low output impedance, making it ideal as a buffer.
Question 7: In a JFET, pinch-off voltage VP is the gate-source voltage at which:
- The channel resistance becomes zero
- The drain current reaches its maximum value IDSS
- The channel is completely depleted and ID approaches zero (Correct answer)
- Avalanche breakdown of the gate junction occurs
Correct answer: The channel is completely depleted and ID approaches zero
At VGS = VP (pinch-off), the depletion regions fully close the channel and the drain current drops to essentially zero.
BEE Semiconductor Devices and Circuits 3
In a full-wave bridge rectifier with a transformer secondary voltage of 12 Vrms and ideal diodes, what is the average DC output voltage?