Bipolar Junction Transistor (BJT) Viva Questions and Answers

Viva Questions and Answers on Bipolar Junction Transistor Experiment

1. Briefly discuss about a transistor?

We can simply say that, a “junction transistor is simply a sandwich of one type of semiconductor material between two layers of the other type”. A transistor has three terminals namely emitter, base and collector. A transistor is a current sensing device. Transistor was developed in the year 1948 at BELL laboratories. A transistor can be seen as two p-n junction placed back to back. The emitter is heavily doped, while the base is lightly doped. The collector is moderately doped. A transistor can be considered as the heart of electronic products. It is almost used in all electronic devices. There are mainly two types of transistors namely, pnp and npn transistors.

2. Discuss about a Bipolar Junction Transistor (BJT), explain about its terminals, types, and mention which type is better? Justify your answer?

In a Bipolar Junction Transistor, the current flow is mainly due to two polarities, holes and electrons. A BJT has three terminals, namely emitter, base and collector. BJT can be of two types, PNP or NPN transistor. Out of these two, NPN transistor is the best one and is most commonly used one. For NPN transistor, the mobility of electrons is more than that of holes. Due to this reason, the electron current is more than the hole current in NPN transistor.

3. Mention about the different configurations of a transistor?

The different configurations of a transistor are common emitter configuration, common base and common collector configurations.

a) Common Emitter (CE)

b) Common Base (CB)

c) Common collector(CC)

4. Explain about the Common Base configuration?

In a common base configuration, the base terminal is common between the input and output circuits. The applied input signal is fed to the emitter-base terminal and the output is collected from the collector-base terminal.

5. Discuss about the characteristics of CB configuration?

We know that, the input resistance of a CB configuration is low and the output resistance is high. Due to this, the current gain of a CB configuration is less than 1. The input and output signals in the common-base circuit are always in phase. The configuration of a CB is mentioned in question 4.

6. Mention about a transistor and explain why it is called so?

The explanation of transistor is mentioned in question number 1. The transistor got its name because a transistor transfers the input signal from low resistance to high resistance. There are mainly two types of transistors-pnp and npn.

7. Explain the parameters α, β, γ in transistor. Give the relation between them?

These parameters represent current gains in CB, CE and CC configurations respectively.

8. Give the comparison of common emitter, common base and common collector configurations?

9. Define cut-off, active and saturation regions in a transistor?

The operation of a transistor can be defined in three regions namely active region, saturation region and cut-off region.

Cut off region:  In a cut-off region, both the emitter junction (Je) and the collector junction (J_c) are in reverse biased condition

Active region: In an active region, the emitter junction is forward biased condition and the collector junction is reverse biased.

Saturation region: In a saturation region, both the emitter and collector junctions are in forward biased condition.

10. How you understand about input and output characteristics?

We can say that the Input Characteristics describes the changes in input current with the variation in the values of input voltage keeping the output voltage constant. The output characteristics can be defined as the plot of output current versus output voltage with constant input current.

11. How you explain about the physical structure of a BJT?

A typical BJT can be seen as one N type material is sandwiched between two P type materials or One P type material is sandwiched between two N type materials, The physical structure of a BJT has its widths (Collector>Emitter>Base) and the ratio of doping concentration (Emitter>Collector>Base).

12. Discuss in detail about Early Effect?

The Early effect can be defined as; it is the variation in base width of BJT due to variation in the applied base-collector voltage.

13. How you understand about the emitter current increase with increase in reverse bias at the collector junction?

It is due to the effect of breakdown at collector junction.

14. What you understand about the collector reverse saturation current?

The reverse saturation current is the current due to the minority carriers flow through collector junction when it is connected in reverse bias.

15. Give the collector current expression for BJT in Common Base configuration?

At the Cut-off region, Ic= 0

Active region Ic= βIb

Saturation Ic < βIb

16. What is the value of V_CE Sat for the transistor BC107?

For BC107, Vce sat=0.2V

17. How a transistor performs as an electronic switch?

It is a well known fact that transistor is commonly used as switch in electronic circuits. We know that, when transistor is in cut off region, it works as off switch (off state condition) and when transistor is in saturation region, it works as on switch (on state condition).

18. Explain whether a BJT is a current controlled device or a voltage-controlled device?

BJT is a current controlled current device

19. Give the representations of NPN and PNP transistors.

20. Give the characteristics of a common emitter BJT configuration.

21. How a common-emitter configuration gets its name?

In a common emitter configuration, the emitter terminal is common to both input and output. That is for both the input and output, the base region is needed.

22. Define “load-line”. And give its significance? Compare a.c. load line and d.c. load line.

The load line specifies the operating point (Q point) of a transistor. Dc load line can be defined as the line based on the relation between output voltage and currents when input is zero, Ac load line is the line which process through the Q point with slope of (-1/Rac, Rac= Rc//Rl).

23. What you understand about Q- point?

The Q-point of a transistor specifies the correct biasing for the required amplification.

24. Explain about biasing and its need for transistors?

Biasing can be defined as the process of giving proper supply voltage and resistances for obtaining the desired operating point. For the proper working of transistor, biasing is needed. Without proper biasing, a transistor cannot work properly.

25. Explain about the different methods of transistor biasing. Which type is best and why?

The different types of transistor biasing are; Fixed bias, Emitter feedback bias, Collector to base bias, Collector- emitter feedback bias and self bias (voltage divider bias). Out of all these biasing techniques, voltage divider bias (self bias) is best and is commonly used, because self bias is more stable for a greater range of input signal

26. Define thermal runaway?

Thermal runaway is a situation where increase in temperature alters the conditions in a way that causes an additional increase in temperature.

27. What is the normal value of thermistor temperature co efficient?

The value of a thermistor temperature may be positive (+) or negative (–) based on the type of material used.

28. What is a two port network? Explain its types, and which one is best for BJT?

A two port network has two ports, input and output ports. Z, Y, h, g are the ABCD parameters of a two port network. The best one is hybrid parameters because this parameter gives all parameters about amplifier such as input, output impedance, voltage and current gains.

29. Give the h-parameter values for a transistor?

30. Give the comparison and applications of a CE, CB and CC amplifiers?

Property	CB	CE	CC
I/P resistance	Low	Moderate	High
O/P resistance	High	Moderate	Low
Current Gain	1	High	High
Voltage Gain	≈150	≈ 500	>1
Phase Shift	0 or 360°	180°	0 or 360°
Uses	High freq ckts	Audio freq ckts	For Impedance Matching

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.