This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “The JFET Volt-Ampere Characteristics”.

1. For low value of V_{DS}, the JFET behaves like a __________

a) Voltage Variable Resistor

b) Constant Voltage Device

c) Amplifier

d) Switch

View Answer

Explanation: When V

_{DS}voltage is very less, there will not be much current flow, since as the V

_{DS}changes, the current value changes very little. Hence we can say that FET works as a voltage controlled Resistor.

2. If a JFET with length L=10µm, a=2µm, W=8µm, V_{p}=-4V.What is the value of r_{ds} at V_{gs} = 0V?

a) 2KΩ

b) 5.2KΩ

c) 10KΩ

d) 9.8KΩ

View Answer

Explanation: r

_{ds}=L/(2aqN

_{D}µ

_{nW}) = Nd = 2V/qa

^{2}=1.33×10

^{21}atoms/m

^{3}

µn=0.15m

^{2}/v-sec

On substituting the values, we get r

_{ds}=9.8KΩ.

3. A JFET has I_{D}=10mA, I_{DSS}=1A, V_{p}=-1v, what is the value of V_{gs}?

a) -0.9V

b) 1V

c) -1V

d) 0.5V

View Answer

4. Where does the transfer curve lie for a p- channel FET?

a) First quadrant

b) Second quadrant

c) Third quadrant

d) Fourth quadrant

View Answer

Explanation: For a P- channel FET, since the pinch off lies on right side of the origin, the current Id will increase from V

_{P}and rise to its left until it reaches I

_{DSS}. Usually I

_{DSS}lies on positive Y axis, therefore we can say transfer curve for the p-channel FET lies on first quadrant.

5. For an n-channel FET, the current v at V_{gs}=-2V and V_{ds}=5V was found to be 2mA, what will be the value of I_{D} at V_{gs}=0V and V_{ds}=5V?

a) 0A

b) 2mA

c) Lesser than 0A

d) Greater than 2mA

View Answer

Explanation: We know that for a n channel FET, the current will flow only if V

_{gs}is greater than V

_{p}, since in this case current is flowing, which indicates V

_{gs}is greater than pinch off. But as V

_{gs}increases the flow of current also increases, hence it will be greater than 2mA.

6. For a p-channel FET, the current I_{D} at V_{gs} = 2V and V_{ds} = 5V and V_{gs} = 3V was found to be 2mA, what will be the value of I_{D} at V_{gs} = 0V and V_{ds} = 5V?

a) 0A

b) 2mA

c) Lesser than 0A

d) Greater than 2mA

View Answer

Explanation: We know that for a p channel FET, the current will flow only if V

_{gs}is lesser than V

_{gs}, but here pinch off voltage is 3V, when V

_{gs}=5V which is greater than pinch off voltage the channel will be destroyed resulting in no conventional current flow.

7. A FET is biased at I_{D} = I_{DSS}/4, at this point the value of trans conductance is __________________ (I_{D}=10mA, V_{p}=-5V).

a) 1mA/V

b) 2mA/V

c) 3mA/V

d) 4mA/V

View Answer

8. What is the value of resultant g_{m} if two non identical FETs are connected in parallel?

a) (µ_{1} r_{d1} + µ_{2} r_{d2})/ (r_{d1}+r_{d2})

b) (µ_{1} r_{d2} + µ_{2} r_{d1})/ (r_{d1}+r_{d2})

c) 0

d) µ_{1} r_{d1}+ µ_{2} r_{d2}

View Answer

Explanation: we know that µ=g

_{m}r

_{d}

But g

_{m’}=g

_{m1}+g

_{m2}

And r

_{d’}=r

_{d1}×r

_{d2}/r

_{d1}+ r

_{d2}

From the above two equations, g

_{m’}=(µ

_{1}r

_{d2}+ µ

_{2}r

_{d1})/ (r

_{d1}+r

_{d2}).

9. What will be the value of r_{d}, if two identical FETs are connected in parallel?

a) Doubles

b) Reduces to half

c) 0

d) Infinite

View Answer

Explanation: r

_{d’}=r

_{d1}×/r

_{d1}+ r

_{d2}

If r

_{d1}=r

_{d1}=r

_{d }=> then r

_{d’}=r

_{d}

^{2}/2r

_{d}=r

_{d}/2.

10. What will be the value of trans conductance if two Identical FETs are connected in parallel?

a) Doubles

b) Reduces to half

c) 0

d) Infinite

View Answer

Explanation: g

_{m’}=I

_{DS}/V

_{gs}at V

_{DS}=0V, if two FET are identical then,

g

_{m1}=g

_{m2}=g

_{m}

But g

_{m’}=g

_{m1}+g

_{m2}

Therefore g

_{m’}=g

_{m}+g

_{m}=2g

_{m}.

**Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.**

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