# Analog Circuits Questions and Answers – Characteristics of JFET

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

1. JFET is a ______ carrier device.
a) Unipolar
b) Bipolar
c) Minority
d) Majority

Explanation: The current flow in the device is due to majority carriers. In an n-type JFET, it is due to the electrons and in a p-type JFET- it is due to the holes.

2. The n-channel JFET, the pinch off voltage is ______________
a) not greater than 0
b) greater than or equal to 0
c) less than or equal to 0
d) not less than 0

Explanation: The pinch off voltage for an N-channel JFET is negative. The depletion region would extend into the N-channel if the reverse bias in the gate to source voltage increases which means that the gate to source voltage has to be negative since the gate is N-type.

3. The built-in barrier potential in a N-channel JFET is ___________
a) less than the internal pinch-off voltage
b) equal to the internal pinch-off voltage
c) greater than the internal pinch-off voltage
d) not related to the internal pinch-off voltage

Explanation: Pinch-off would require more voltage than the voltage required to establish the p-n barrier voltage. This is evident from the dependence of such voltage on the doping concentration.

4. If channel thickness increases, the internal pinch-off voltage ___________
a) Decreases
b) Increases
c) Remains the same
d) Increases logarithmically

Explanation: The internal pinch off voltage is directly proportional to the channel thickness. If the channel thickness increases, the pinch off voltage increases.

5. If the doping concentration of the gate increases, the internal pinch-off voltage ___________
a) Increases logarithmically
b) Increases linearly
c) Increases exponentially
d) Decreases linearly

Explanation: The internal pinch-off voltage is linearly proportional to the doping concentration. Hence, it would increase with the increase in the doping concentration. The built-in-barrier potential is logarithmically proportional to the doping concentration of the gate.
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6. The cut-off frequency of a JFET is that time when the magnitude of the input current is ___________
a) Greater than the output current
b) Less than the output current
c) Equal to the output current
d) Twice the output current

Explanation: The cut-off frequency is an important feature of the JFET due to the present of capacitive effects. It has been seen that the output current becomes a function of frequency in high-frequency applications and hence we have to choose a cut-off frequency so that the output current is equal to the input current.

7. The cut-off frequency of a JFET is ___________
a) linearly related to the transconductance of the JFET
b) inversely proportional to the transconductance of the JFET
c) exponentially related to the transconductance of the JFET
d) logarithmically related to the transconductance of the JFET

Explanation: The cut-off frequency is seen to be linearly related to the transconductance of the JFET. This is typically due to the reactance of the capacitors.

8. How is the transconductance at saturation related to the pinch off voltage of the JFET?
a) Inversely proportional
b) Directly proportional
c) Inverse-squarely related
d) Directly and proportional to square of the pinch-off voltage

Explanation: The transconductance is seen to be inversely related to the pinch of voltage. The transconductance is seen to be inversely related to the channel length while the pinch off voltage is directly proportional to the channel length.

9. When an N-channel JFET reaches pinch-off, the increase in the drain to source voltage results in shifting of the pinch-off position towards the ___________
a) Gate
b) Drain
c) Source
d) Does not shift

Explanation: Pinch off is said to be reached if the drain to source voltage is equal to the difference between the gate to source and the threshold voltage. So, this pinch off happens at a certain distance from the source and the gradual decrease in the channel length will happen faster if the voltage along the channel length increases faster. It can be readily observed that equality is reached at a distance less than the previous case and hence the pinch-off is shifted towards the source.

10. An N-channel JFET is ___________
a) Always ON
b) Always OFF
c) Enhancement mode JFET
d) Has a p-type substrate

Explanation: An N-channel is always ON depletion mode JFET since the channel for current flow from source to drain is always present. This is in contrast to a P-channel JFET which needs to be provided with a channel for the flow of current.

11. A P-channel JFET is___________
a) Always ON
b) Always OFF
c) Depletion mode JFET
d) Has an n-type substrate

Explanation: The P-channel JFET doesn’t have a built-in channel for the flow of current. This is because the conduction in a P-channel JFET can begin after a certain voltage is applied at the gate which would lead to widening the channel between the source and the drain.

12. How is the metallurgical channel thickness between the gate and the substrate related to the doping concentration of the channel?
a) Inversely proportional to the square root of the doping concentration
b) Logarithmically related to the square root of the doping concentration
c) Directly proportional to the square root of the doping concentration
d) Exponentially related to the square root of the doping concentration

Explanation: The channel thickness is inversely related to the square root of the doping concentration of the channel. This is because the electric field developed is proportional to the channel doping concentration while the relation between the potential, electric field and doping concentration is visible from the Poisson’s equation.

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