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9. Define transconductance of JFET. (Nov/Dec 2014)

Transconductance (gm) is defined as the ratio of small change in drain current ( Id) to
the corresponding change to gate source ( Vgs) at constant drain to source voltage
(Vds).

10. What is SCR? (Apr/May 2015)

A silicon-controlled rectifier (SCR) is a three terminal, three-junction semiconductor
device that acts as a true electronic switch. It is a unidirectional device. It control the
amount of power fed to the load.
11.Define Early effect(Nov/Dec 2016)
A variation of the base collector voltage results in a variation of the quasineutral
width in the base. The gradient of the minoritycarrier density in the base
therefore changes, yielding an increased collector current as the collectorbase
current is increased. This effect is referred to as the Early effect
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w .Ea Part - B

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1. Explain the input and output characteristics of a CE transistor configuration. List
out the comparisons between CE, CB and CC configurations.(Nov/Dec 2013). Input
Characteristic: The curve between IB and VBE for different values of VCE are shown in
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figure. Since the base emitter junction of a transistor is a diode, therefore the
characteristic is similar to diode one. With higher values of V CE collector gathers slightly

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more electrons and therefore base current reduces. Normally this effect is neglected.
(Early effect). When collector is shorted with emitter then the input characteristic is the

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characteristic of a forward biased diode when VBE is zero and IB is also zero.

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e t

Output Characteristic: The output characteristic is the curve between VCE and IC for
various values of IB. For fixed value of IB and is shown in figure. For fixed value of IB,
IC is not varying much dependent on VCE but slopes are greater than CE characteristic.
The output characteristics can again be divided into three parts.

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ww
w
(1) Active Region:

.Ea
In this region collector junction is reverse biased and emitter junction is forward
biased. It is the area to the right of V CE = 0.5 V and above IB= 0. In this region

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transistor current responds most sensitively to IB. If transistor is to be used as an
amplifier, it must operate in this region.
IEIC IB
Since, I C I COdc I E ngi
I C I COdc (I C I B )
or nee
(1dc )I Cdc I B
0r
I co
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I dc I
1
I g.n
e
C B CO
1 1

t
dc dc

dc
dc
1 dc
IC (1dc )I COdc I B
I I
C C 0
dcI I
BCO

If adc is truly constant then IC would be independent of VCE. But because of early
effect, αdc increases by 0.1% (0.001) e.g. from 0.995 to 0.996 as VCE increases from a
few volts to 10V. Then βdc increases from 0.995 / (1-0.995) = 200 to 0.996 / (1-0.996)
= 250 or about 25%. This shows that small change in a reflects large change in b.
Therefore the curves are subjected to large variations for the same type of transistors.
(2) Cut Off:
Cut off in a transistor is given by IB = 0, IC= ICO. A transistor is not at cut off if the base
current is simply reduced to zero (open circuited) under this condition,
IC = IE= ICO / ( 1-αdc) = ICEO
The actual collector current with base open is designated as I CEO. Since even in the
neighborhood of cut off, a dc may be as large as 0.9 for Ge, then IC=10

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ICO(approximately), at zero base current. Accordingly in order to cut off transistor it is

not enough to reduce IB to zero, but it is necessary to reverse bias the emitter junction
slightly. It is found that reverse voltage of 0.1 V is sufficient for cut off a transistor. In
Si, the α dc is very nearly equal to zero, therefore, I C = ICO. Hence even with IB= 0, IC=
IE= ICO so that transistor is very close to cut off. In summary, cut off means I E = 0, IC =
ICO, IB = -IC = -ICO , and VBE is a reverse voltage whose magnitude is of the order of
0.1 V for Ge and 0 V for Si.
(3).Saturation Region:
In this region both the diodes are forward biased by at least cut in voltage. Since the
voltage VBE and VBC across a forward is approximately 0.7 V therefore, V CE = VCB+
VBE = - VBC + VBE is also few tenths of volts. Hence saturation region is very close to
zero voltage axis, where all the current rapidly reduces to zero. In this region the
transistor collector current is approximately given by V CC / R C and independent of
base current. Normal transistor action is last and it acts like a small ohmic resistance.

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Comparison of CE, CB and CC configurations:
Parameters CB CE CC

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Current gain (Ai)

.Ea
Voltage gain (Vi)
Low
High
High
High
High
Low

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Input resistance (Ri)
Output resistance (Ro)
Low
High
Medium
Medium
High
Low

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2. With a neat sketch explain the construction and characteristics of
enhancement MOSFET.( Apr/May 2015, Apr/May 2017)
MOSFET structure and channel formation
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rin
g.n
e t
Figure shows the construction of an N-channel E-MOSFET. The main difference
between the construction of DE-MOSFET and that of E-MOSFET, as we see from the
figures given below the E-MOSFET substrate extends all the way to the silicon
dioxide (SiO2) and no channels are doped between the source and the drain. Channels
are electrically induced in these MOSFETs, when a positive gate-source voltage VGS is
applied to it.

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Operation of EMOSFET:

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w .Ea
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As its name indicates, this MOSFET operates only in the enhancement mode and has
no depletion mode. It operates with large positive gate voltage only. It does not

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conduct when the gate-source voltage V GS 0. This is the reason that it is called
normally-off MOSFET. In these MOSFET’s drain current I D flows only when VGS
exceeds VGST [gate-to-source threshold voltage].
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When drain is applied with positive voltage with respect to source and no potential is

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applied to the gate two N-regions and one P-substrate from two P-N junctions connected
back to back with a resistance of the P-substrate. So a very small drain current that is,

e
reverses leakage current flows. If the P-type substrate is now connected to the source
-
t
terminal, there is zero voltage across the source substrate junction, and the drain-substrate
junction remains reverse biased. When the gate is made positive with respect to the source
and the substrate, negative (i.e. minority) charge carriers within the substrate are attracted
to the positive gate and accumulate close to the-surface of the substrate. As the gate
voltage is increased, more and more electrons accumulate under the gate. Since these
electrons cannot flow across the insulated layer of silicon dioxide to the gate, so they
accumulate at the surface of the substrate just below the gate. These accumulated minority
charge carriers N -type channel stretching from drain to source. When this occurs, a
channel is induced by forming what is termed an inversion layer (N-type). Now a drain
current starts flowing. The strength of the drain current depends upon the channel
resistance which, in turn, depends upon the number of charge carriers attracted to the
positive gate. Thus drain current is controlled by the gate potential. Since the conductivity
of the channel is enhanced by the positive bias on the gate so this device is also called the
enhancement MOSFET or E- MOSFET.The minimum value of gate-to-source voltage VGS
that is required to form the inversion layer
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(N-type) is termed the gate-to-source threshold voltage VGST. For VGS below VGST, the
drain current ID = 0. But for VGS exceeding VGST an N-type inversion layer connects
the source to drain and the drain current ID is large. Depending upon the device being
used, VGST may vary from less than 1 V to more than 5 V.JFETs and DE-MOSFETs are
classified as the depletion-mode devices because their conductivity depends on the
action of depletion layers. E-MOSFET is classified as an enhancement-mode device
because its conductivity depends on the action of the inversion layer. Depletion-mode
devices are normally ON when the gate-source voltage VGS = 0, whereas the
enhancement-mode devices are normally OFF when VGS = 0.
Characteristics of EMOSFET:

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Drain characteristics of an N-channel E-MOSFET are shown in figure. The lowest

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curve is the VGST curve. When VGS is lesser than VGST, ID is approximately zero. When
VGS is greater than VGST, the device turns- on and the drain current I D is controlled by

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the gate voltage. The characteristic curves have almost vertical and almost horizontal
parts. The almost vertical components of the curves correspond to the ohmic region,
and the horizontal components correspond to the constant current region. Thus E-
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MOSFET can be operated in either of these regions i.e. it can be used as a variable-
voltage resistor (WR) or as a constant current source.
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e t
Figure shows a typical transconductance curve. The current IDSS at VGS <=0 is very
small, being of the order of a few nano-amperes. When the V GS is made positive, the drain
current ID increases slowly at first, and then much more rapidly with an increase in V GS.
The manufacturer sometimes indicates the gate-source threshold voltage V GST at which the
drain current ID attains some defined small value, say 10 u A. A current I D (0N,
corresponding approximately to the maximum value given on the drain characteristics and
the values of VGS required to give this current VGs QN are also usually given on the
manufacturers data sheet. The equation for the transfer characteristic does not obey
equation. However it does follow a similar “square law type” of relationship. The
equation for the transfer characteristic of E-MOSFETs is given as:
2
ID=K(VGS-VGST)

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3. Explain the construction and characteristics of UJT with a neat sketch.

(Apr/May 2015,Nov/Dec 2016))
UJT Construction:

The symbol for the unijunction transistor looks very similar to that of the junction field

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effect transistor or JFET, except that it has a bent arrow representing the Emitter (E) input.
From the equivalent circuit above, that the N-type channel basically consists of two

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resistors RB2 and RB1 in series with an equivalent (ideal) diode, D representing the p-n
junction connected to their center point. This Emitter p-n junction is fixed in position
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along the ohmic channel during manufacture and can therefore not be changed.
Resistance RB1 is given between the Emitter, E and terminal B 1, while resistance RB2 is

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given between the Emitter, E and terminal B 2.As the physical position of the p-n junction
is closer to terminal B 2 than B1 the resistive value of R B2will be less than RB1.The total

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resistance of the silicon bar (its Ohmic resistance) will be dependent upon the
semiconductors actual doping level as well as the physical dimensions of the N-type

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silicon channel but can be represented by R BB. If measured with an ohmmeter, this static
resistance would typically measure somewhere between about 4kΩ and 10kΩ’s for most
common UJT’s such as the 2N1671, 2N2646 or the 2N2647.These two series resistances
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produce a voltage divider network between the two base terminals of the unijunction

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transistor and since this channel stretches from B 2 to B1, when a voltage is applied across
the device, the potential at any point along the channel will be in proportion to its position

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between terminals B2 and B1. The level of the voltage gradient therefore depends upon the
amount of supply voltage.When used in a circuit, terminal B 1 is connected to ground and
the Emitter serves as the input to the device. Suppose a voltage V BB is applied across the
UJT between B2 and B1 so that B2 is biased positive relative to B 1. With zero Emitter input
t
applied, the voltage developed across R B1 (the lower resistance) of the resistive voltage
divider can be calculated as:
Unijunction Transistor RB1 Voltage

v RB1 V
RB1 RB1 R B2 BB

\
For a unijunction transistor, the resistive ratio of R B1 to RBB shown above is called the
intrinsic stand-off ratio and is given the Greek symbol: η (eta). Typical standard
values of η range from 0.5 to 0.8 for most common UJT’s.If a small positive input
voltage which is less than the voltage developed across resistance, R B1 ( ηVBB ) is now
applied to the Emitter input terminal, the diode p-n junction is reverse biased, thus
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offering a very high impedance and the device does not conduct. The UJT is switched
“OFF” and zero current flows. However, when the Emitter input voltage is increased
and becomes greater than VRB1 (or ηVBB + 0.7V, where 0.7V equals the p-n junction
diode volt drop) the p-n junction becomes forward biased and the unijunction
transistor begins to conduct. The result is that Emitter current, ηI E now flows from the
Emitter into the Base region. The effect of the additional Emitter current flowing into
the Base reduces the resistive portion of the channel between the Emitter junction and
the B1 terminal. This reduction in the value of RB1resistance to a very low value means
that the Emitter junction becomes even more forward biased resulting in a larger
current flow. The effect of this results in a negative resistance at the Emitter terminal.
Likewise, if the input voltage applied between the Emitter and B 1terminal decreases to
a value below breakdown, the resistive value ofR B1 increases to a high value. Then the
Unijunction Transistor can be thought of as a voltage breakdown device.So we can
see that the resistance presented by R B1 is variable and is dependent on the value of

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Emitter current, IE. Then forward biasing the Emitter junction with respect to B 1 causes
more current to flow which reduces the resistance between the Emitter, E and B 1.In
other words, the flow of current into the UJT’s Emitter causes the resistive value of
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RB1 to decrease and the voltage drop across it, VRB1must also decrease, allowing more

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current to flow producing a negative resistance condition.
UJT Characteristics:

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g.n
The static emitter characteristic (a curve showing the relation between emitter voltage
VE and emitter current IE) of a UJT at a given inter base voltage VBB is shown in

e
figure. From figure it is noted that for emitter potentials to the left of peak point,
emitter current IE never exceeds IEo . The current IEo corresponds very closely to the
reverse leakage current ICo of the conventional BJT. This region, as shown in the t
figure, is called the cut-off region. Once conduction is established at V E = VP the
emitter potential VE starts decreasing with the increase in emitter current I E. This
Corresponds exactly with the decrease in resistance R B for increasing current IE. This
device, therefore, has a negative resistance region which is stable enough to be used
with a great deal of reliability in the areas of applications listed earlier. Eventually, the
valley point reaches, and any further increase in emitter current I E places the device in
the saturation region, as shown in the figure. Three other important parameters for the
UJT are IP, VV and IV and are defined below:
Peak-Point Emitter Current. Ip. It is the emitter current at the peak point. It
represents the rnimrnum current that is required to trigger the device (UJT). It is
inversely proportional to the interbase voltage VBB.
Valley Point Voltage VV The valley point voltage is the emitter voltage at the valley
point. The valley voltage increases with the increase in interbase voltage VBB.
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Valley Point Current IV The valley point current is the emitter current at the valley
point. It increases with the increase in inter-base voltage VBB.
4. With a neat sketch explain the construction and working characteristics of a
SCR.(NOV/Dec 2014, NOV/Dec 2016,May 2017))
Principle of operation of SCR:

The SCR is a four-layer, three-junction and a three-terminal device. The end P-region
is the anode, the end N-region is the cathode and the inner P-region is the gate. The

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anode to cathode is connected in series with the load circuit. Essentially the device is a
switch. Ideally it remains off (voltage blocking state), or appears to have an infinite

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impedance until both the anode and gate terminals have suitable positive voltages with
respect to the cathode terminal. The thyristor then switches on and current flows and
.Ea
continues to conduct without further gate signals. Ideally the thyristor has zero
impedance in conduction state. For switching off or reverting to the blocking state,

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there must be no gate signal and the anode current must be reduced to zero. Current
can flow only in one direction.

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In absence of external bias voltages, the majority carrier in each layer diffuses until
there is a built-in voltage that retards further diffusion. Some majority carriers have

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enough energy to cross the barrier caused by the retarding electric field at each
junction. These carriers then become minority carriers and can recombine with
majority carriers. Minority carriers in each layer can be accelerated across each
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junction by the fixed field, but because of absence of external circuit in this case the
sum of majority and minority carrier currents must be zero. A voltage bias, as shown in
g.n
figure, and an external circuit to carry current allow internal currents which include the
following terms: The current Ix is due to



Majority carriers (holes) crossing junction J1
Minority carriers crossing junction J1 e
Holes injected at junction J2 diffusing through the N-region and crossing junction
t
J1 and
 Minority carriers from junction J2 diffusing through the N-region and crossing
junction J1.
Similarly I2 is due to six terms and I3 is due to four terms.
Characteristics of SCR:

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There are three modes of operation for an SCR depending upon the biasing given to it:

ww 1. Forward blocking mode(off state)

2. Forward conduction mode(on state)

w 3. Reverse blocking mode(off state)

.Ea
Forward blocking mode:
In this mode of operation anode is given a positive potential while cathode is given
negative voltage keeping gate at zero potential i.e. disconnected. In this case junction
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J1 andJ3 are forward biased while J2 is reversed biased due to which only a small
leakage current flows from anode to cathode till applied voltage reaches it break over

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value at which J2undergoes avalanche breakdown and at this break over voltage it
starts conducting but below break over voltage it offers very high resistance to the flow

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of current through to it and said to be in off state.
Forward conduction mode:

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SCR can be brought from blocking mode to conduction mode in two ways - either by
increasing the voltage across anode to cathode beyond break over voltage or by

g.n
application of positive pulse at gate. Once it starts conducting no more gate voltage is
required to maintain it in on state. Now there are two ways to turn it off i.e. Reduce the

e
current flowing through it below a minimum value called holding current. Apply a
negative pulse at gate which will bring it in off state instantaneously.
Reverse blocking mode: t
SCR are available with reverse blocking capability. Reverse blocking capability adds
to the forward voltage drop because of the need to have a long, low doped P1 region.
(If one cannot determine which region is P1, a labeled diagram of layers and junctions
can help). Usually, the reverse blocking voltage rating and forward blocking voltage
rating are the same. The typical application for reverse blocking SCR is in current
source inverters.SCR incapable of blocking reverse voltage are known as
asymmetrical SCR, abbreviated ASCR. They typically have a reverse breakdown
rating in the 10's of volts. ASCR are used where either a reverse conducting diode is
applied in parallel (for example, in voltage source inverters) or where reverse voltage
would never occur (for example, in switching power supplies or DC traction
choppers).Asymmetrical SCR can be fabricated with a reverse conducting diode in the
same package. These are known as RCT, for reverse conducting thyristor.

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5. Explain the construction and operation of NPN transistor with neat sketch. Also
comment on the characteristics of NPN transistor.
(Nov/Dec 2014)
The NPN Transistor
In the previous tutorial we saw that the standard Bipolar Transistor or BJT, comes in two
basic forms. An NPN (Negative-Positive-Negative) type and a PNP (Positive-Negative-
Positive) type, with the most commonly used transistor type being the NPN Transistor. We
also learnt that the junctions of the bipolar transistor can be biased in one of three different
ways – Common Base, Common Emitter and Common Collector.
In this tutorial about bipolar transistors we will look more closely at the “Common
Emitter” configuration using the Bipolar NPN Transistor with an example of the
construction of a NPN transistor along with the transistors current flow characteristics
is given below.
A Bipolar NPN Transistor Configuration

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w .Ea
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(Note: Arrow defines the emitter and conventional current flow, “out” for a Bipolar
NPN Transistor.)
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The construction and terminal voltages for a Bipolar NPN Transistor are shown above.
The voltage between the Base and Emitter ( V BE ), is positive at the Base and negative
e
at the Emitter because for an NPN transistor, the Base terminal is always positive with
respect to the Emitter. Also the Collector supply voltage is positive with respect to the
Emitter ( VCE ). So for a bipolar NPN transistor to conduct the Collector is always
t
more positive with respect to both the Base and the Emitter.

NPN Transistor Connection

Then the voltage sources are connected to an NPN transistor as shown. The Collector is
connected to the supply voltage VCC via the load resistor, RL which also acts to limit the

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maximum current flowing through the device. The Base supply voltage V B is connected to
the Base resistor RB, which again is used to limit the maximum Base current.
So in a NPN Transistor it is the movement of negative current carriers (electrons)
through the Base region that constitutes transistor action, since these mobile electrons
provide the link between the Collector and Emitter circuits. This link between the input
and output circuits is the main feature of transistor action because the transistors
amplifying properties come from the consequent control which the Base exerts upon
the Collector to Emitter current.
Then we can see that the transistor is a current operated device (Beta model) and that a
large current ( Ic ) flows freely through the device between the collector and the emitter
terminals when the transistor is switched “fully-ON”. However, this only happens when a
small biasing current ( Ib ) is flowing into the base terminal of the transistor at the same
time thus allowing the Base to act as a sort of current control input.
The transistor current in a bipolar NPN transistor is the ratio of these two currents

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( Ic/Ib ), called the DC Current Gain of the device and is given the symbol of hfe or
nowadays Beta, ( β ). The value of β can be large up to 200 for standard transistors,
and it is this large ratio between Ic and Ib that makes the bipolar NPN transistor a
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useful amplifying device when used in its active region as Ibprovides the input and Ic

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provides the output. Note that Beta has no units as it is a ratio.
Also, the current gain of the transistor from the Collector terminal to the Emitter terminal,

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Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons diffusing
across the junction). As the emitter current Ie is the sum of a very small base current plus a

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very large collector current, the value of alpha α, is very close to unity, and for a typical
low-power signal transistor this value ranges from about 0.950 to 0.999

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UNIT – III AMPLIFIERS
Part– A
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1. Draw the hybrid model of CE amplifier. (Apr/May 2015,Nov/Dec 2016)
g.n
+
+
e t
==

2. What is meant by hybrid parameters? ( Nov/Dec 2014)

The parameters which has a combination of units are called hybrid parameters. The
hybrid parameters are input impedance, output impedance, current gain and voltage
gain.

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