CET Electronic Components and Devices 3 — Questions and Answers
Question 1: Which parameter of a MOSFET is analogous to the transconductance of a BJT?
- Drain resistance (rd)
- Transconductance (gm) (Correct answer)
- Gate capacitance (Cgs)
- Threshold voltage (Vth)
Correct answer: Transconductance (gm)
Transconductance (gm) relates the change in drain current to the change in gate-source voltage in a MOSFET.
Question 2: A photodiode is operated in which mode for fastest response time?
- Forward bias
- Zero bias
- Reverse bias (photoconductive mode) (Correct answer)
- Avalanche mode
Correct answer: Reverse bias (photoconductive mode)
Reverse bias (photoconductive mode) widens the depletion region, reducing junction capacitance and speeding up response.
Question 3: What causes thermal runaway in a bipolar junction transistor?
- Excessive base current
- Increased collector current raising junction temperature, further increasing current (Correct answer)
- Low supply voltage
- Reverse breakdown of the emitter junction
Correct answer: Increased collector current raising junction temperature, further increasing current
Rising temperature increases leakage current, which raises collector current, generating more heat in a destructive positive feedback loop.
Question 4: Which type of capacitor is NOT polarized and is commonly used in AC filtering applications?
- Electrolytic
- Tantalum
- Film (polyester) (Correct answer)
- Wet electrolytic
Correct answer: Film (polyester)
Film capacitors are non-polarized and suitable for AC circuits, whereas electrolytic and tantalum types are polarized.
Question 5: In an enhancement-mode N-channel MOSFET, current flows when the gate-source voltage (VGS) is:
- Below threshold voltage
- Equal to zero
- Above threshold voltage (Correct answer)
- Negative
Correct answer: Above threshold voltage
An enhancement-mode N-channel MOSFET requires VGS to exceed the threshold voltage to create the conducting channel.
Question 6: What is the purpose of a bypass capacitor placed in parallel with the emitter resistor of a BJT amplifier?
- Increase DC stability
- Prevent oscillation
- Bypass AC signal to increase AC gain (Correct answer)
- Reduce power supply noise
Correct answer: Bypass AC signal to increase AC gain
A bypass capacitor short-circuits the emitter resistor at AC frequencies, eliminating the gain-reducing negative feedback.
Question 7: Which device is best described as a voltage-controlled variable resistor operating in its ohmic region?
- BJT in active region
- MOSFET in ohmic (linear) region (Correct answer)
- Zener diode
- UJT
Correct answer: MOSFET in ohmic (linear) region
A MOSFET biased in its ohmic (linear) region acts as a voltage-controlled resistor, with RDS controlled by VGS.
CET Electronic Components and Devices 3
Which parameter of a MOSFET is analogous to the transconductance of a BJT?