# Electronic Devices and Circuits Questions and Answers – The JFET Volt-Ampere Characteristics

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

1. For low value of VDS, the JFET behaves like a __________
a) Voltage Variable Resistor
b) Constant Voltage Device
c) Amplifier
d) Switch

Explanation: When VDS voltage is very less, there will not be much current flow, since as the VDS 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, Vp=-4V.What is the value of rds at Vgs = 0V?
a) 2KΩ
b) 5.2KΩ
c) 10KΩ
d) 9.8KΩ

Explanation: rds=L/(2aqND µnW) = Nd = 2V/qa2=1.33×1021 atoms/m3
µn=0.15m2/v-sec
On substituting the values, we get rds=9.8KΩ.

3. A JFET has ID=10mA, IDSS=1A, Vp=-1v, what is the value of Vgs?
a) -0.9V
b) 1V
c) -1V
d) 0.5V

Explanation:

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

Explanation: For a P- channel FET, since the pinch off lies on right side of the origin, the current Id will increase from VP and rise to its left until it reaches IDSS. Usually IDSS 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 Vgs=-2V and Vds=5V was found to be 2mA, what will be the value of ID at Vgs=0V and Vds=5V?
a) 0A
b) 2mA
c) Lesser than 0A
d) Greater than 2mA

Explanation: We know that for a n channel FET, the current will flow only if Vgs is greater than Vp, since in this case current is flowing, which indicates Vgs is greater than pinch off. But as Vgs increases the flow of current also increases, hence it will be greater than 2mA.
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6. For a p-channel FET, the current ID at Vgs = 2V and Vds = 5V and Vgs = 3V was found to be 2mA, what will be the value of ID at Vgs = 0V and Vds = 5V?
a) 0A
b) 2mA
c) Lesser than 0A
d) Greater than 2mA

Explanation: We know that for a p channel FET, the current will flow only if Vgs is lesser than Vgs, but here pinch off voltage is 3V, when Vgs=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 ID = IDSS/4, at this point the value of trans conductance is __________________ (ID=10mA, Vp=-5V).
a) 1mA/V
b) 2mA/V
c) 3mA/V
d) 4mA/V

Explanation:

8. What is the value of resultant gm if two non identical FETs are connected in parallel?
a) (µ1 rd1 + µ2 rd2)/ (rd1+rd2)
b) (µ1 rd2 + µ2 rd1)/ (rd1+rd2)
c) 0
d) µ1 rd1+ µ2 rd2

Explanation: we know that µ=gm rd
But gm’=gm1+gm2
And rd’=rd1 ×rd2/rd1+ rd2
From the above two equations, gm’=(µ1 rd2 + µ2 rd1)/ (rd1+rd2).

9. What will be the value of rd, if two identical FETs are connected in parallel?
a) Doubles
b) Reduces to half
c) 0
d) Infinite

Explanation: rd’=rd1 ×/rd1+ rd2
If rd1=rd1=rd => then rd’=rd2/2rd =rd/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

Explanation: gm’=IDS/Vgs at VDS=0V, if two FET are identical then,
gm1=gm2=gm
But gm’=gm1+gm2
Therefore gm’=gm+gm=2gm.

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