Download AKSHAYA COLLEGE OF ENGINEERING AND TECHNOLOGY

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.
calculate 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 between drift and diffusion currents.
Drift current
Diffusion current
It is developed due to potential It is developed due to charge
gradient
concentration gradient
This phenomenon is found both in This is found only in semiconductors
metals and semiconductors
7. Differentiate between avalanche and zener break down.
Zener break down
Breakdown occurs due to heavily
doped junction and applied strong
electric field
Doping level is high
Breakdown occurs at lower
voltage compared to avalanche
breakdown
Avalanche breakdown
Breakdown occurs due to avalanche
multiplication between thermally generated
ions
Doping level is low
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, some 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 pside, 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 equation.
I=Io[eV/ηVT – 1]
I
Io
V
η
VT
=
=
=
=
=
diode current
reverse saturation current in amperes
applied voltage
1 for germanium diode, 2 for silicon diode
voltage equivalent of temperature
12. What is meant by depletion region?
In PN junction, the diffusion of holes and electrons start 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 two 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 four applications of light emitting diode.
 All kinds of visual displays
 In the optical devices such as optocouplers
 As on-off indicator in various types of electronic circuits.
 Remote control. burglar alarm
UNIT II - BJT AND ITS APPLICATIONS
1. Will a transistor result if two diodes are connected back to back?
A transistor has two p-n junctions. One junction is between the emitter and the
base and is called emitter base junction and the other junction is between the base
and the collector and is called collector base junction. Thus transistor is like two
pn junction diodes connected back to back.
2. State early effect. What are 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 reduces the
electrical base width. This effect is called “Early Effect” or “Base width
modulation”. The Early effect has two 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 four
parameters are not same. They have different units. In other words, they are
mixture of different 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 required 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 between Ic, β, IB and IcBo in a BJT.
i. In the active region:
i. IC = β IB + (1+ β)ICO
ii. In the cut-off region:
i. IC = (1+ β)ICO
ICBO is the collector current when the emitter current is zero. ICBO is
greater than ICO
10. Define α and β of a transistor.
α: It is defined as the ratio of the collector current resulting from
carrier injection to the total emitter current
α=
IC
IE
β: it is defined as the ratio of collector current to base current.
β=
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 obtained 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 :
Vo :
input voltage
output voltage
The above parameters are required 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
IB
=
-hfe
1+hoeRL’
15. What are power transistors?
Power transistors are one 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 Ω to 100 Ω. This makes the configuration an ideal for coupling between
various transistor stages
17. For a npn transistor IE = 12ma and β = 140. Determine the value of IB and Ic.
Ans:
IE = (1+ β) IB
IB = IE / (1+ β)
= 12mA / (1+140)
= 85.1 mA
Ic = β IB = 140 * 85.1 mA = 11.914 mA
18. Determine β and IE for a transistor if IB = 50μA and Ic = 3.6 mA.
Ans:
I β = 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 one type 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 between 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 changes 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 defined 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 three regions that are present 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 some 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.
Sl.no
1
2
3
4
5


BJT
Current controlled device
Bipolar device
Less input resistance
Less thermal stability
More thermal noise
FET
Voltage controlled device
Unipolar device
Input resistance greater than BJT
High thermal stability
Less thermal noise
Drain characteristics
Transfer characteristics
11. Differentiate between BJT and JFET
12. Give the drain current equation 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 between JFET and MOSFET
JFET
Gate is not insulated from channel
There are two types – N-channel and
P-channel
Cannot be operated in depletion and
enhancement modes
There is a continuous channel
MOSFET
Gate is insulated from channel by a thin layer
of SiO2
Four types - P-channel enhancement, Pchannel depletion, N-channel enhancement,
N-channel depletion
Can be operated in depletion and
enhancement modes
There is a continuous channel only in
depletion type, but not in enhancement type
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 between two 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 =
ρ
=
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 + βA) ). The reciprocal of the sensitivity is called the Desensitivity
11. What is the Barkhausen criterion for feedback oscillators?
-Aβ = 1
Aβ
=1
The above to conditions are required 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 2п radians.
 The magnitude of the product of the open loop gain of the amplifier (A)
and the feedback factor β is unity i.e. Aβ = 1
13. Mention the expression for frequency of oscillations for colpitts oscillator and
Hartley oscillator.
Hartley oscillator:
1
f=
2п
C (L1 + L2)
Colpitts oscillator:
1
f=
2п
Ceq =
L Ceq
C1 C2
C1 + C2
14. What is the advantage of clap oscillator over a colpitts oscillator.
 The frequency is stable and accurate.
 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 required
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 one 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 “η”
Hence
η =
rB1
rB2
=
rB1
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 below 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 types 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 two stage RC coupled amplifier with output of
one stage supplied back to the input of another stage. An AMV generates square
wave of known frequency. So it is called a “Square wave generator.