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Thursday, 7 May 2020

Field Effect Transistor (FET) Viva Questions and Answers

Viva Questions and Answers on Field Effect Transistor Experiment

1. Explain the differences between a BJT and a JFET and compare them?

The main difference of BJT (Bipolar Junction Transistor) and FET (Field Effect Transistor) is BJT is a current controlled device, while FET is a voltage controlled device (Control by the change in voltage at the gate terminal).  The current in BJT is produced due to the movements of both electrons and holes (current flow is due to both majority and minority carriers). In FET, the current is produced due to either electrons or holes (current flow due to majority charge carriers only). The main advantage of BJT and FET is that in electrical and electronics circuits, both devices can be used as switches and amplifiers. As compared to BJT, FETs are faster switching devices. In BJT, the Current-Voltage (I-V) characteristics are linear, but in FET, it is non-linear.  For radiation, FET is good, while BJT is sensitive. The noise effect is more in BJT as compared to FET.


2. Briefly explain about unipolar device. Is JFET a unipolar device, Justify your answer?

A Unipolar device can be defined as a device, in which the current is produced due to only one polarity. In other words, we can say that in unipolar devices, the current flow is due to either electrons or holes (majority carriers only).  We can clearly say that JFET is a unipolar device, because in JFET, the current flow is produced due to only one carrier. In N-channel JFET, the charge flow is due to electrons and in P- channel JFET, the current flow is due to holes.

3. Give the symbols of JFET and MOSFET?


4. List the major applications of a JFET?

The main applications of a JFET are:

JFET can be used as a constant current source and as a buffer amplifier.
Can be used as RF amplifier and cascade amplifier
Can be used as chopper and  in electronic switches.
Can be used as high impedance wide band amplifier
Used as a voltage variable resistor, or voltage development resistor.
Can be used as variable resistor in circuits such as: Op amps, mixer circuits and memory cell etc.

5. Briefly explain the characteristic parameters of a FET and give the relations between them?


Trans conductance gm=dId/dVgs
Drain resistance Rd=dVds/dId
Amplification factor µ=-dVds/dVgs
µ=RdXgm

6. Mention the differences between n-channel and p-channel JFETs?

We all know that JFET is a unipolar device. The charge flow in JFET is only due to the majority carriers 9either electrons or holes). The N-channel JFET is made by N-type material. So in N channel JFET, the current flows from Drain to source due to electrons (the majority carriers) only. The p-channel JFET is made by P-type material. So in P channel JFET, the current flows from Drain to source due to holes  (the majority carriers) only.


7.  List about commercially available JFET devices.

The commercially available JFETs are;

2N3954A- This is a low noise, low drift monolithic dual, N channel JFET
BF245A, J111, J112:  These are N channel JFET
2SK2394 – This is a N-channel JFET, having 15V and 6 to 32 mA current
BF256B – This is an N-channel amplifier
BSR57,BSR58 – These are N-channel low frequency, low noise amplifiers.
J109 – This is a n-channel switch
J175, J176 – P-channel switch
J113 – This is a JFET chopper transistor.
MMBF5486 – This is an n-channel RF-amplifier.

8. List the types of JFET configurations. Give the applications of each configuration?

The types of configurations of JFET and their applications are:

Common Drain configuration: This is used in Buffer circuits
Common Source configuration: This is applicable in Amplifier circuits
Common Gate: configuration: This is applicable in high frequency amplifier circuits.

9. Discuss about pinch off voltage?

Pinch off voltage can be termed as “the drain to source voltage following which drain current turn into almost constant and JFET goes into saturation region”.  This is defined only if Vgs is 0V.


10.  Sketch the regions in transfer characteristics, output characteristics of JFET?


11. Give the drain current and Trans conductance equation in JFET?

The drain current can be obtained as:

Id = Idss(1-(Vgs/Vp))2

The trans-conductance can be obtained by

gm = gmo(1-( Vgs/Vp))

12. Explain about MOSFET and the possible types in a MOSFET?

Metal Oxide Semiconductor Field Effect Transistor (MOSFET), is also known as Insulated Gate Field Effect Transistor (IGFET).  The types of MOSFETs are enhancement mode and depletion mode MOSFETs. Both the enhancement and depletion mode has P-channel and n- channel enhancement MOSFET and depletion MOSFTs.


13. Which parameter of a transistor varies with temperature?

The list of parameters that varies with temperature is:

Reverse saturation current, current gains and trans-conductance

14. Discuss about ‘load line’?

Load line is a graph obtained by plotting the output side voltage, current relation based on input voltage or current. The Operating point (Q point) is the inter section point of AC and DC load lines.


15. Discuss “operating point” of a transistor?

The Operating point species output voltage and currents at required operating region.

16. Explain the need of capacitor used to shunt the emitter resistor of self-biased transistor?

Current series feedback (-ve) is required to get more stability for DC input. For Ac input, the current series feedback is not required.

17. What is the advantage of self-bias circuit over other possible biasing circuits?

The main advantage of a self-bias circuit is it is more stable with respect to temperature, β.

18. Define Stability factor and mention its significance?

Stability factor gives the changes within the output current due to changes in temperature, transistor (β), leakage current, base emitter voltage

19.  Mention the advantage of a FET biasing circuit over BJT biasing?

FET is a voltage controlled current device so there is no need of extra circuitry, while BJT is a current controlled device.

20. Explain about MOSFET?  How is it different from a JFET? 

Metal oxide semiconductor Field Effect Transistor (MOSFET) can be operated in both depletion and enhancement modes. The Junction Field effect Transistor (JFET) can be operated in depletion mode only.

21. What are the advantages of  FET over BJT?

The main advantages of FETs are FET has high input impedance, lower noise, low to medium gain as compared to BJT.

22. Is it possible to interchange the source and drain terminals in a FET circuit?  Is it possible to do the same with the emitter and collector terminals of a BJT circuit?

Yes, we can interchange the drain and source terminals in a FET.  It is not possible to change  the emitter and collector in a BJT, because emitter and collectors dimensions and doping concentration is different in BJT.

23. Discuss the need of coupling capacitors.

The main use of a coupling capacitor is to filter the Dc term from the Input signal and also from Collector output in amplifiers.

24. Explain about thermal stabilization?

Thermal stabilization is a process of maintaining a constant operating point as temperature varies.

25. Discuss the process of reversal of phase in a BJT CE Amplifier.

We know that in a BJT CE amplifier, as the Base voltage increases, the base current also increases. Due to this, collector current increases and the voltage drop across Rc increases. Hence the output voltage decreases.

26. Explain the result of amplifier biased at cutoff or at saturation region?

In cutoff region, the collector current Ic is 0. In saturation region, the collector emitter voltage Vce is almost Zero.

27. Discuss the significance of bandwidth of an amplifier?

“The Bandwidth species the input signal frequency range that can be applied to amplifier to get maximum gain”.

28. Discuss about the Gain-Bandwidth Product and mention its significance?

“The gain-bandwidth product can be defined as the product of gain of a device and its bandwidth”. For any circuit (system), the gain bandwidth product is always constant. In any case, if gain increases, the bandwidth decreases and vice versa.

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