Electronic Devices and circuits lab viva questions part4-FET

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FET

1.What are the comparisons and differences between a BJT and a JFET?

BJT current controlled current device, Fet voltage controlled current device, in BJT current produced by holes and electrons, but in FET current produced due to any one only,

2.What is meant by a unipolar device?  Why is a JFET known as a Unipolar Device?

In which device current produced due to one polarity (electron or hole), in jFET current produced due to one only(N channel JFET- electrons, p- channel JFET- holes).

3.Draw the symbols of JFET, MOSFET?

    

4.What are the typical applications of a JFET?

Buffer, RF amplifier, cascade amplifier, variable resistor in op amps, mixer circuit, memory cell

5.What are the characteristic parameters of a FET?  What are the relations between them?

Trans conductance gm=dId/dVgs

Drain resistance Rd=dVds/dId

Amplification factor µ=-dVds/dVgs

µ=RdXgm

6.What are n-channel and p-channel JFETs?  How are they different from each other?

In N channel jfet , the channel made by Ntype material so current exist from Drain to source due to Electrons (ajority carriers) only. In P channel jfet , the channel made by P type material so current exist from Drain to source due to holes  (ajority carriers) only

7.Give the names/numbers of a few commercially available JFET devices.

2N3954A- low noise low drift monolithic dual, N channel jfet

BF245A, J111,J112:  N channel jfet

8.What are the various possible configurations in which a JFET can be connected?  What are the typical applications of each?

Common Drain: Buffer

Common Sourse: Amplifier

Common Gate: Amplifier (high frequency)

9.Explain pinch off voltage and region?

Pinch off voltage is the drain to source voltage after which drain current becomes almost constant

10.Explain the regions in Transfer characteristics, output characteristics of JFET?

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

I_d= I_dss(1-(Vgs/Vp))²;    gm=gmo(1-( Vgs/Vp))

12.What is a MOSFET?  What are the possible types in a MOSFET?

Metal oxide semiconductor FET

Types are N channel, P channel – Enhancement and Depletion Type Mosfets.

13.Which transistor parameters vary with temperature?

Reverse saturation current, current gains, trans conductance

14.What is meant by ‘load line’?

Load line indicates the output side voltage, current relation based on input voltage or current. Inter section point of AC,DC load lines indicates Operating point(Q point)

15.What is meant by the “operating point” of a transistor?

Operating point specifies output voltage and currents at required operating region.

16.Why is a capacitor used to shunt the emitter resistor of self-biased transistor?

To get more stability we need to establish current series feedback (-ve) for DC input but for Ac input that is not required so..

17.Why is the self-bias circuit preferred over other possible biasing circuits?

More stable with respect to Tempareture, β,

18.Define Stability factor and explain its significance.

Stability factor represents changes with in the output current due to changes in Temperature, transistor (β), leackage current,  base emitter voltage

19.What is the advantage of a FET biasing circuit above BJT biasing?

FET is a voltage controlled current device so no need of extra circuitry

10.What is the need for the capacitor C_S?

To get more stability we need to establish current series feedback (-ve) for DC input but for Ac input that is not required so..

11.Why are the coupling capacitors required?

To filter the Dc term from the Input signal , Collector output in amplifiers.

12.What is meant by thermal stabilization?
Maintain a constant operating point when temperature varies
13.Explain why reversal of phase occurs in a BJT CE Amplifier.\
As Base voltage increases, base current increases, then collector current increases so voltage drop across Rc increases so out put voltage decreses.
14.What happens if an amplifier is biased at cutoff or at saturation?
In cutoff region Ic is 0, in saturation region Vce is almost Zero.
15.What is the significance of the bandwidth of an amplifier?
Bandwidth specifies the input signal frequency range that can be applied to amplifier to get maximum gain.
16.What is meant by Gain-Bandwidth Product?  What is its significance?
The name itself expressing it is the  product of gain of a device and its bandwidth. For any system (circuit) gain bandwidth product is constant, if gain increases bandwidth decreases vice versa.

17.What are the advantages of using a FET instead of a BJT?

FET has high input impedance, lower noise, low to medium gain,

19.Can we interchange the source and drain terminals in a FET circuit?  Can we do the same with the emitter and collector terminals of a BJT circuit?

We can interchange drain and source but we cannot change emitter and collector because emitter and collectors dimensions and doping concentration is different

20.What is a MOSFET?  How is it different from a JFET?  What are its typical applications?

Metal oxide semiconductor can be operated in both depletion and enhancement modes, but Junction field effect Transistor can be operated in depletion mode only.

21.Explain why reversal of phase does not occur in a BJT CC Amplifier?

As Base voltage increases, base current increases, then collector current increases so voltage drop across Re increases so output voltage increases
22. Why is the Common Collector Amplifier called as Emitter Follower?
In common collector amplifier out will be taken across emitter so its called as emitter follower

Special purpose Devices:

1.Explain the working of SCR.

2.Define holding current.

Holding current is the specified minimum current that must be flowing from
anode to cathode (with no gate current) and still be guaranteed that the SCR
will not turn off.

3.What are the specifications of the SCR ?
gate trigger voltage, gate trigger current, holding current, on-state voltage, peak gate power dissipation.

4.What is UJT, explain the input characteristics of it?
unijunction transistors are constructed from separate P-type and N-type semiconductor materials forming a single (hence its name Uni-Junction) PN-junction within the main conducting N-type channel of the device.

5.What is the symbol and principle of operation of Tunnel Diode?

According to the classical laws of physics a charged particle sin order to cross an energy barrier should possess energy at least equal to the energy barrier. Hence the particle will cross the energy barrier if its energy is greater than the barrier and cannot cross the barrier if its energy is less than the energy barrier. But quantum mechanically there exists non zero probability that the particle with energy less than the energy barrier will cross the barrier as if it tunnels across the barrier. This is called as Tunneling effect. 

6.Explain the V-I characteristics of Tunnel diode?

7.What is the symbol and principle of operation of Varactor Diode?

According to the classical laws of physics a charged particle sin order to cross an energy barrier should possess energy at least equal to the energy barrier. Hence the particle will cross the energy barrier if its energy is greater than the barrier and cannot cross the barrier if its energy is less than the energy barrier. But quantum mechanically there exists non zero probability that the particle with energy less than the energy barrier will cross the barrier as if it tunnels across the barrier. This is called as Tunneling effect. 

8.What is the symbol and principle of operation of SchottkyBarrier  Diode?

9.What is the symbol and principle of operation of photo Diode?

10.What is the symbol and principle of operation of LED?