Anna University Coimbatore 2009 B.E Electrical and Electronics Engineering Ee 2203-electronic devices and circuits - two k and answers - Question Paper

AKSHAYA COLLEGE OF ENGINEERING AND TECHNOLOGY
Dept. of Electrical and Electronics Engineering
Prepared by: R.subramanian

EE 2203-ELECTRONIC DEVICES AND CIRCUITS

UNIT 1

1. Define diffusion current.
A movement of charge carriers due to the concentration gradient in a semiconductor is called process of diffusion. When charge carriers move, the current is constituted in a bar. This current due to diffusion is called diffusion current.

2. What is meant by zener break down?
When a PN junction is heavily doped, the depletion region is very narrow. So under reverse bias condition, the electric field across the depletion region is intense. Such an intense field is enough to pull the electrons out of the valence band of the stable atoms. Such a creation of free electrons is called zener effect. These minority carriers constitute very large current and the mechanism is called zener break down.

3. List the diode parameters
? Bulk resistance
? Static resistance
? Dynamic resistance
? Reverse resistance
? Knee voltage
? Break down voltage
? Reverse current or leakage current
4. Compare the silicon and germanium diodes with respect to cut in voltage and reverse saturation current.
The barrier potential for germanium diode is 0.3 V. so the cut-in-voltage of germanium diode is greater then 0.3 V and the barrier potential of silicon diode is 0.7 V. So the cut-in-voltage of silicon diode is greater than 0.7 V.

5. A silicon diode has a saturation current of 7.5µA at room temperature 300K. compute the saturation current at 460 K.
Given: I01 = 7.5µA = 7.5 * 10-6

T1 = 300 K

T2 = 400 K

?T = T2 – T2 = 460 – 300

?T = 160 K

I02 = 2(?T/10) * I01 = 2(160/10) * 7.5 * 10-6

I02 = 0.49152 A

6. Differentiate ranging from drift and diffusion currents.

Drift current Diffusion current
It is developed due to potential gradient It is developed due to charge concentration gradient
This phenomenon is obtained both in metals and semiconductors This is obtained only in semiconductors

7. Differentiate ranging from avalanche and zener break down.

Zener break down Avalanche breakdown
Breakdown occurs due to heavily doped junction and applied strong electric field Breakdown occurs due to avalanche multiplication ranging from thermally generated ions
Doping level is high Doping level is low
Breakdown occurs at lower voltage compared to avalanche breakdown Break down occurs at higher voltage

8. List the PN diode switching times
? Recovery time
? Forward Recovery time
? Reverse Recovery time
? Storage and transition time

9. Define the cut-in-voltage and peak inverse voltage of pn junction diode.
When diode is farward biased, a few voltage is necessary to overcome the barrier potential ,to make diode conduct. This is called cut-in-voltage.
In reverse bias opposite polarity appears across diode. The maximum diode voltage which diode can withstand without break down is called peak inverse voltage.

10. Define barrier potential at the junction.
Due to immobile positive charges on n-side and negative charges on p-side, there exists an electric field across the junction. This creates potential difference across the junction which is called barrier potential, junction potential, built-in potential or cut-in voltage o PN junction.
11. Give the diode current formula.

I=Io[eV/?VT – 1]

I = diode current
Io = reverse saturation current in amperes
V = applied voltage
? = one for germanium diode, two for silicon diode
VT = voltage equivalent of temperature

12. What is meant by depletion region?
In PN junction, the diffusion of holes and electrons begin initially. Near the junction, holes recombine in N-region to form immobile positive ions. Similarly electrons recombine in P-region to form immobile negative ions. With sufficient accumulation of such immobile ions on both sides, the diffusion stops. So near the junction, there exists a region in which immobile positive and negative charge reside while mobile charge carriers in this region get completely depleted. This region is called depletion region.

13. Define the transition capacitance of the diode.
A capacitance existing at the PN junction when the diode is reverse biased where the 2 regions act as the plates while the depletion region acts as dielectric is called a transition capacitance of a diode.

14. Why a series resistor is necessary when a diode is reverse biased?
A series resistor is necessary when a diode is reverse biased for limiting the forward current which increases exponentially with voltage.

15. List any 4 applications of light emitting diode.
• All types of visual displays
• In the optical devices such as optocouplers
• As on-off indicator in different kinds of electronic circuits.
• Remote control. burglar alarm




























UNIT II - BJT AND ITS APPLICATIONS


1. Will a transistor outcome if 2 diodes are connected back to back?
A transistor has 2 p-n junctions. 1 junction is ranging from the emitter and the base and is called emitter base junction and the other junction is ranging from the base and the collector and is called collector base junction. Thus transistor is like 2 pn junction diodes connected back to back.

2. State early effect. elaborate the consequences of it?
In the common Base characteristics of BJT when reverse bias voltage VcB increases, the width of the depletion region also increases. This decreases the electrical base width. This effect is called "Early Effect" or "Base width modulation". The Early effect has 2 consequences.
There is less chance of recombination within the base region.
The charge gradient is increased within the base and consequently the current of the minority carriers injected across the junction increases.

3. What is thermal runaway?
In a power transistor the increase in collector current increases the power dissipated at the collector junction. This, in turn further increases the temperature of the junction and hence increases the collector current. The process is cumulative and it is referred to as self heating. The excess heat produced at the collector base junction may even burn and destroy the transistor. The situation is called "Thermal runaway" of the transistor.


4. What are hybrid parameters?
The parameters h11, h12, h21, h22 are the hybrid parameters. These 4 parameters are not identical. They have various units. In other words, they are mixture of various units and hence referred to as hybrid parameters. They are commonly known as h-parameters.


5. Define the delay time and rise time in the switching characteristics of transistor.
In the transistor switching characteristics the deley time is the time that elapses the application of the input pulse and current to rise to 10 percent of its maximum value.The time needed for Ic to reach 90% of its maximum level from 10% level is called the rise time.

6. Define the hie and hfe for a common emitter transistor configuration.

hie = ?VBE
?IB VCE constant


hfe = ?Ic
?IB VCE constant

hie is input resistance in common emitter configuration
hfe is short circuit current gain in common base configuration

7. When a transistor is used as a switch, in which region of output characteristics it is operated?
8. When a transistor is used as a switch it is operated alternately in the cut off region and saturation region of the output characteristics

9. Write the relation ranging from Ic, ß, IB and IcBo in a BJT.
i. In the active region:
i. IC = ß IB + (1+ ß)ICO
ii. In the cut-off region:
i. IC = (1+ ß)ICO
ICBO is the collector current when the emitter current is zero. ICBO is greater than ICO

10. Define a and ß of a transistor.
a: It is described as the ratio of the collector current resulting from carrier injection to the total emitter current
a = IC
IE
ß: it is described as the ratio of collector current to base current.

ß = IC
IB

11. In a bipolar transistor which region is wider and which region is thinner? Why?
The middle region of bipolar junction transistor is called as the base of the transistor. Input signal of small amplitude is applied to the base. This region is thin and lightly doped.The magnified output signal is found at the collector. This region is thick and heavily doped.
12. State the advantages of optocoupler.
a. It is compact and has less weight
b. Much faster than isolation transformers and relays
c. Noise transients, contact bounce etc, are completely eliminated
d. Has wide band signal transmission capability
e. Easy to interface with logic devices.
f. Response time is small


13. What are the tools used for small signal analysis of BJT?
Vi = hi Ii + hr Vo
Io = hf Ii + ho Vo
hi : input resistance
Ii : input current
hr : reverse voltage transfer ratio
ho : output admittance
Vi : input voltage
Vo : output voltage

The above parameters are needed for drawing the equivalent circuit which is necessary for the small signal analysis of BJT.

14. Give the current gain expression for a common emitter transistor configuration.

Ai= IC = -hfe
IB 1+hoeRL


15. What are power transistors?
Power transistors are 1 which is used for power amplification. It means that the operating voltage and current is large. In the off-state they have to block large voltage and in the on-state they have to carry large current.

16. Which of the BJT configuration is suitable for impedance matching application?
Common emitter configuration is used in impedance matching application because the ratio of output resistance to input resistance is small and may range from 10 ? to 100 ?. This makes the configuration an ideal for coupling ranging from different transistor stages

17. For a npn transistor IE = 12ma and ß = 140. Determine the value of IB and Ic.

Ans: IE = (1+ ß) IB

IB = IE / (1+ ß)
= 12mA / (1+140)
= 85.1 mA

Ic = ß IB = 140 * 85.1 mA = 11.914 mA



18. Determine ß and IE for a transistor if IB = 50µA and Ic = 3.6 mA.

Ans: I ß = Ic/IB = 3.6 mA/50 µA = 72

IE = IB + Ic = 50 µA + 3.6 mA = 3.65 mA









UNIT III - FET AND ITS APPLICATION

1. Why is FET called a unipolar device?
In FET current is carried by only 1 kind of charge particles, either electrons or holes. Hence FET is called unipolar device.

2. How can a FET be used as a voltage controlled resistor?
In FET , the voltage applied ranging from gate and source controls the drain current ID. Therefore, FET is a voltage controlled device.
3. What are the advantages of FETs?
• In FET input resistance is high compared to BJT
• Construction is smaller than BJT
• Less sensitive to modifications in applied voltage
• Thermal stability is more
• Thermal runaway does not exist in JFET
• Thermal noise is much lower

4. Draw the low frequency equivalent circuit of JFET
5. Define amplification factor in JFET.
It is described as the ratio of change in drain-source voltage VDS to the change in gate-source voltage VGS at constant drain current ID. It is also called mutual conductance



µ = ?VDS
?VGS ID constant


6. Give the small signal model of a FET.

7. Mention the 3 regions that are current in the drain source characteristics of JFET
• Saturation region
• Break down region
• Ohmic region

8. What is pinch-off voltage in FET?
In the VI characteristics of JFET, at a few value of VDS, drain current ID cannot be increased further, due to reduction in channel width. Any further increase in VDS does not increase the drain current ID. ID approaches the constant saturation value. The voltage VDS at which the current ID reaches to its constant saturation level is called " Pinch-off voltage", Vp

9. What are the parameters that control the Pinch off voltage?
Electron charge, donor or acceptor concentration, permittivity of channel material and half width of channel bar.

10. List the characteristics of JFET.
• Drain characteristics
• Transfer characteristics

11. Differentiate ranging from BJT and JFET
Sl.no BJT FET
1 Current controlled device Voltage controlled device
2 Bipolar device Unipolar device
3 Less input resistance Input resistance greater than BJT
4 Less thermal stability High thermal stability
5 More thermal noise Less thermal noise


12. Give the drain current formula of JFET.

ID = IDSS (1 – VGS )2
Vp
ID = drain current
IDSS = saturation drain current
VGS = gate source voltage
VP = pinch-off voltage

13. Why MOSFET is called IGFET?
MOSFET is constructed with gete terminal insulated from the channel. So it is also called as insulated gate FET or IGFET.

14. Comparison ranging from JFET and MOSFET


JFET MOSFET
Gate is not insulated from channel Gate is insulated from channel by a thin layer of SiO2
There are 2 kinds – N-channel and P-channel Four kinds - P-channel enhancement, P-channel depletion, N-channel enhancement, N-channel depletion
Cannot be operated in depletion and enhancement modes Can be operated in depletion and enhancement modes
There is a continuous channel There is a continuous channel only in depletion type, but not in enhancement kind

15. List the JFET parameters.
• Transconductance (gm)
• Input resistance and capacitance
• Drain to source resistance (rd)
• Amplification factor (µ)

UNIT IV & V- AMPLIFIERS AND OSCILLATORS


1. Define amplifier
A device which accepts an input signal and produces an output signal proportional to the input is called an amplifier.
2. What is differential amplifier
An amplifier which amp[lifies the difference ranging from 2 input signal is called differential amplifier

3. Define CMRR.
The ability of differential amplifier to reject a common mode signal is expressed by a ratio called common mode rejection ratio denoted as CMRR.

CMRR = ? = Ad
Ac

4. What is the significance of CMRR in differential amplifier?
The significance of CMRR is that larger the value of CMRR, better the differential amplifier

5. What is voltage or node sampling?
In amplifier if the output voltage is sampled by connecting the feedback network in shunt across the output, such a connection is referred to as voltage or node sampling

6. What is current or loop sampling?
In amplifier if the output current is sampled by connecting the feedback network in series with the output, such a connection is referred to as current or loop sampling


7. What is positive feedback?
If the input signal and the output signal are in phase, the feedback is called positive feedback
8. What is negative feedback?

If the input signal and output signal are not in phase, the feedback is called negative feed back
9. List the advantages of negative feedback
• Less noise
• Less frequency distortion
• Less amplitude distortion
• Less phase distortion
• Less harmonic distortion
• Band width is increased



10. Define desensitivity.
The fractional change in amplification with feedback divided by the fractional change without feedback is called the sensitivity of the transfer gain (1/ (1/ (1 + ßA) ). The reciprocal of the sensitivity is called the Desensitivity

11. What is the Barkhausen criterion for feedback oscillators?

-Aß = 1

Aß = one

The above to conditions are needed to work a circuit as an oscillator. This is called Barkhausen criterion for oscillation.

12. State Barkhausen conditions for sinusoidal oscillators.
• The total phase shift around a loop, as the signal proceeds from a input through amplifier, feedback network back to input again, copeting a loop, is precisely 0° or 360°, or of course an integral multiple of 2? radians.
• The magnitude of the product of the open loop gain of the amplifier (A)

and the feedback factor ß is unity i.e. Aß = 1

13. Mention the expression for frequency of oscillations for colpitts oscillator and Hartley oscillator.

Hartley oscillator:
1
f =
2? C (L1 + L2)


Colpitts oscillator:
1
f =
2? L Ceq

Ceq = C1 C2
C1 + C2

14. What is the advantage of clap oscillator over a colpitts oscillator.
• The frequency is stable and right.
• Good frequency stability
• Stray capacitances have no effect on c3 which decides the frequency
• Keeping C3 variable, frequency can be varied in the desired range

15. Mention the features of crystal oscillator
• It is a tuned-circuit oscillator
• It uses piezoelectric crystal as its resonant tank circuit
• It is preferred when greater frequency stability is needed
• Used in watches communication transmitters and receivers etc.

16. What are the advantages of crystal oscillator?

• Good frequency stability
• Good temperature stability
• Quartz crystal is used which is inexpensive and easily available in nature.

17. Draw the circuit diagram of 1 shot multivibrator.
18. Draw the typical wave form at base and collector coupled astable multivibrator
19. Draw the circuit diagram of a collector coupled astable multivibrator using complementry transistors.
20. Draw the circuit diagram of a emitter coupled astable multivibrator .
21. State the applications of Schmitt trigger circuit.
• Used as an amplitude comparator
• Used as a squaring circuit
• Used as a flip-flop
22. Define intrinsic stand off ratio of UJT and draw its equivalent circuit
The resistance ratio rB1/rBB is an important characteristics of UJT. It is known as the "intrinsic stand-off ratio" and is designated by "?"


Hence ? = rB1 = rB1
rB2 rB1 + rB2

23. What is a clamper?
The circuit with which the waveform can be shifted, such that, a particular part of it (say positive or negative peak) is maintained at a specified level, is called a clamping circuit or simply a clamper

24. What is a clipper?
The circuit with which waveform is shaped by removing a certain portion of the input signal voltage above or beneath a preset level is called clipper. They are also called limiters when they are used to limit the amplitude of the input signal.
25. List the kinds of clippers
Based on limiting action
? positive clipper
? negative clipper
? biased clipper
? combination clipper
Based on limiting action
? Diode clipper
? Transistor clipper

26. What is the function of clamping circuits?
The function of clamping circuit is to introduce a d.c level into an a.c level. The clamping circuits are also sometimes known as d.c restorers.

27. What is AMV? Why is it called as square wave generator?
An AMV is essentially a 2 stage RC coupled amplifier with output of 1 stage supplied back to the input of a different stage. An AMV generates square wave of known frequency. So it is called a "Square wave generator.

Earning:   Approval pending.