Analog Circuits Questions and Answers – Biasing of JFET and MOSFET

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Biasing of JFET and MOSFET”.

1. Which of the following statements are true?

P: JFET is biased to operate it in active region
Q: MOSFET is biased to operate it in saturation region

a) Both P and Q are correct
b) P is correct and Q is incorrect
c) P is incorrect and Q is correct
d) Both P and Q are incorrect
View Answer

Answer: c
Explanation: While transistors are biased to work in the active region, to act as amplifiers, FET devices are instead biased in the saturation region to work as an amplifier, whether it be a JFET or a MOSFET. In saturation, current IDS changes with respect to VGS, and small changes in VGS cause proportionate changes in IDS, and the device can act as an amplifier.

2. In the given situation for n-channel JFET, we get drain-to-source current is 5mA. What is the current when VGS = – 6V?
Find the current when VGS is – 6V for n-channel JFET if current is 5mA
a) 5 mA
b) 0.5A
c) 0.125 A
d) 0.5A
View Answer

Answer: c
Explanation: IDS = IDSS(1-VGS/VP)2
When VGS = 0, IDSS = IDS = 5mA
When VGS = -6V, IDS = 5mA(1 + 4)2
IDS = 5 x 25 = 125 mA.
advertisement
advertisement

3. Consider the following circuit. Given that VDD = 15V, VP = 2V, and IDS = 3mA, to bias the circuit properly, select the proper statement.
RD less than 6kΩ if VDD is 15V, VP is 2V, & IDS is 3mA
a) RD < 6kΩ
b) RD > 6kΩ
c) RD > 4kΩ
d) RD < 4kΩ
View Answer

Answer: a
Explanation: In given circuit, VGS = -5V
VDS = VDD – IDSRD
To bias properly VDS > |VP| – |VGS|
VDS > -3
15 – 3mA*RD > -3
-3mA*RD > -18
RD < 6kΩ.

4. Consider the circuit shown. VDS=3 V. If IDS=2mA, find VDD to bias circuit.
Find VDD to bias circuit if VDS is 3V & IDS is 2mA
a) -30V
b) 30V
c) 33V
d) Any value of voltage less than 12 V
View Answer

Answer: c
Explanation: VDS = VDD – IDS(10k + 5k)
3 = VDD – 2(15)
3 = VDD – 30
VDD = 33 V.

5. To bias a e-MOSFET ___________
a) we can use either gate bias or a voltage divider bias circuit
b) we can use either gate bias or a self bias circuit
c) we can use either self bias or a voltage divider bias circuit
d) we can use any type of bias circuit
View Answer

Answer: a
Explanation: To bias an e-MOSFET, we cannot use a self bias circuit because the gate to source voltage for such a circuit is zero. Thus, no channel is formed and without the channel, the MOSFET doesn’t work properly. If self bias circuit is used, then D-MOSFET can be operated in depletion mode.

6. Consider the following circuit. Process transconductance parameter = 0.50 mA/V2, W/L=1, Threshold voltage = 3V, VDD = 20V. Find the operating point of circuit.
Find operating point if transconductance parameter is 0.50 mA/V2, W/L is 1, voltage is 3V
a) 20V, 25mA
b) 13V, 22mA
c) 12.72V, 23.61mA
d) 20V, 23.61mA
View Answer

Answer: c
Explanation: IDS = [k’W/L(VGS – VT)2]/2
VGS = 20 x 35/55 = 12.72 V
IDS = 0.25 (9.72)2
IDS = 23.61 mA.
advertisement

7. Given VDD = 25V, VP = -3V. When VGS = -3V, IDS = 10mA. Find the operating point of the circuit.
Find operating point of the circuit if VDD is 25V, VP is -3V, VGS is -3V & IDS is 10mA
a) -3.83V, 0.766mA
b) -2.345V, 0.469mA
c) 3.83V, 0.469mA
d) 2.3V, 0.7mA
View Answer

Answer: b
Explanation: When VGS = VP then IDSS = IDS = 10mA
Also, in above circuit, VGS = -IDSRS = – IDSx5k
Thus, IDS = IDSS(1-VGS/VP)2
Solving we get, IDS = 0.766mA, 0.469mA
Thus we get VGS = -3.83V, -2.345V
However, VGS should lie between 0 and VP.

8. Consider the following circuit. IDSS = 2mA, VDD = 30V. Find R, given that VP = – 2V.
Find R if VP is –2V, IDSS is 2mA & VDD is 30V
a) 10kΩ
b) 4kΩ
c) 2kΩ
d) 5kΩ
View Answer

Answer: b
Explanation: IDSS = 2mA
IDS = (VDD – 15)/50k = 0.3mA
VGS = VP[1 – \(\sqrt{\frac{I_{DS}}{D_{SS}}}\)]
VGS = -2 x (1 – \(\sqrt{.15}\)) = – 1.22V
Thus, VGS + IDS x (R) = 0
R = 1.22/0.3mA = 4kΩ.
advertisement

9. Which of the following statements are true?

A: In a self bias circuit, the current IDS is not stable.
B: Source capacitance, CS, parallel to RS, reduces stability.

a) Both statements are correct and B is the correct reasoning
b) Both statements are correct but B is not the correct reason for it
c) Statement A is correct while statement B is wrong
d) Both statements are incorrect
View Answer

Answer: d
Explanation: In a self bias circuit, the current IDS is stable due to the presence of source resistance RS in the circuit. The source resistance helps provide negative feedback to keep current stable. Capacitor CS is a bypass capacitor to prevent decrease in voltage gain.

10. For a MOSFET, the pinch-off voltage is -3V. Gate to source voltage is 20V. W/L ratio is 5. Process transconductance parameter is 40μA/V2. Find drain to source current in saturation.
a) 0.10 mA
b) 0.05mA
c) – 0.05mA
d) – 50A
View Answer

Answer: c
Explanation: ISD = k’W(VSG – |VT|)2/2L
ISD = 20*5*(-20-3)2 = 52900μA = 0.05mA.

Sanfoundry Global Education & Learning Series – Analog Circuits.

To practice all areas of Analog Circuits, here is complete set of 1000+ Multiple Choice Questions and Answers.

If you find a mistake in question / option / answer, kindly take a screenshot and email to [email protected]

advertisement
advertisement
Subscribe to our Newsletters (Subject-wise). Participate in the Sanfoundry Certification contest to get free Certificate of Merit. Join our social networks below and stay updated with latest contests, videos, internships and jobs!

Youtube | Telegram | LinkedIn | Instagram | Facebook | Twitter | Pinterest
Manish Bhojasia - Founder & CTO at Sanfoundry
Manish Bhojasia, a technology veteran with 20+ years @ Cisco & Wipro, is Founder and CTO at Sanfoundry. He lives in Bangalore, and focuses on development of Linux Kernel, SAN Technologies, Advanced C, Data Structures & Alogrithms. Stay connected with him at LinkedIn.

Subscribe to his free Masterclasses at Youtube & discussions at Telegram SanfoundryClasses.