UNIT 4

BIPOLAR JUNCTION TRANSISTORS

NPN -PNP -Operations-Early effect-Current equations – Input and Output

characteristics of CE, CB, CC – Hybrid -π model – h-parameter model, Ebers Moll
Model- Gummel Poon-model, Multi Emitter Transistor.

INTRODUCTION:

After having a good knowledge on the working of the diode, which is a single PN junction,
let us try to connect two PN junctions which make a new component called Transistor.
A Transistor is a three terminal semiconductor device that regulates current or voltage flow
and acts as a switch or gate for signals.

Why Do We Need Transistors?

Suppose that you have a FM receiver which grabs the signal you want. The received signal
will obviously be weak due to the disturbances it would face during its journey. Now if this
signal is read as it is, you cannot get a fair output. Hence we need to amplify the
signal. Amplification means increasing the signal strength.

This is just an instance. Amplification is needed wherever the signal strength has to be
increased. This is done by a transistor. A transistor also acts as a switch to choose between
available options. It also regulates the incoming current and voltage of the signals.

CONSTRUCTIONAL DETAILS OF A TRANSISTOR

The Transistor is a three terminal solid state device which is formed by connecting two
diodes back to back. Hence it has got two PN junctions. Three terminals are drawn out of
the three semiconductor materials present in it. This type of connection offers two types of
transistors. They are PNP and NPN which means an N-type material between two P types
and the other is a P-type material between two N-types respectively.

The construction of transistors is as shown in the following figure which explains the idea
discussed above.
The three terminals drawn from the transistor indicate Emitter, Base and Collector terminals.
They have their functionality as discussed below.

Emitter
 The left hand side of the above shown structure can be understood as Emitter.

 This has a moderate size and is heavily doped as its main function is to supply a
number of majority carriers, i.e. either electrons or holes.

 As this emits electrons, it is called as an Emitter.

 This is simply indicated with the letter E.

Base
 The middle material in the above figure is the Base.

 This is thin and lightly doped.

 Its main function is to pass the majority carriers from the emitter to the collector.

 This is indicated by the letter B.

Collector
 The right side material in the above figure can be understood as a Collector.

 Its name implies its function of collecting the carriers.

 This is a bit larger in size than emitter and base. It is moderately doped.

 This is indicated by the letter C.

The symbols of PNP and NPN transistors are as shown below.

The arrow-head in the above figures indicated the emitter of a transistor. As the collector
of a transistor has to dissipate much greater power, it is made large. Due to the specific
functions of emitter and collector, they are not interchangeable. Hence the terminals are
always to be kept in mind while using a transistor.

TRANSISTOR BIASING
As we know that a transistor is a combination of two diodes, we have two junctions here. As
one junction is between the emitter and base, that is called as Emitter-Base junction and
likewise, the other is Collector-Base junction.

Biasing is controlling the operation of the circuit by providing power supply. The function
of both the PN junctions is controlled by providing bias to the circuit through some dc
supply. The figure below shows how a transistor is biased.

By having a look at the above figure, it is understood that

 The N-type material is provided negative supply and P-type material is given positive supply to
make the circuit Forward bias.

 The N-type material is provided positive supply and P-type material is given negative supply to
make the circuit Reverse bias.

By applying the power, the emitter base junction is always forward biasedas the emitter
resistance is very small. The collector base junction is reverse biased and its resistance is a
bit higher. A small forward bias is sufficient at the emitter junction whereas a high reverse
bias has to be applied at the collector junction.

The direction of current indicated in the circuits above, also called as the Conventional
Current, is the movement of hole current which is opposite to the electron current.

OPERATION PNP TRANSISTOR

The operation of a PNP transistor can be explained by having a look at the following figure,
in which emitter-base junction is forward biased and collector-base junction is reverse
biased.
The voltage VEE provides a positive potential at the emitter which repels the holes in the P-
type material and these holes cross the emitter-base junction, to reach the base region. There
a very low percent of holes recombine with free electrons of N-region. This provides very
low current which constitutes the base current IB. The remaining holes cross the collector-
base junction, to constitute collector current IC, which is the hole current.

As a hole reaches the collector terminal, an electron from the battery negative terminal fills
the space in the collector. This flow slowly increases and the electron minority current flows
through the emitter, where each electron entering the positive terminal of VEE, is replaced by
a hole by moving towards the emitter junction. This constitutes emitter current IE.

Hence we can understand that −

 The conduction in a PNP transistor takes place through holes.

 The collector current is slightly less than the emitter current.

 The increase or decrease in the emitter current affects the collector current.

OPERATION NPN TRANSISTOR

The operation of an NPN transistor can be explained by having a look at the following
figure, in which emitter-base junction is forward biased and collector-base junction is
reverse biased.
The voltage VEE provides a negative potential at the emitter which repels the electrons in the
N-type material and these electrons cross the emitter-base junction, to reach the base region.
There a very low percent of electrons recombine with free holes of P-region. This provides
very low current which constitutes the base current IB. The remaining holes cross the
collector-base junction, to constitute the collector current IC.

As an electron reaches out of the collector terminal, and enters the positive terminal of the
battery, an electron from the negative terminal of the battery VEE enters the emitter region.
This flow slowly increases and the electron current flows through the transistor.

Hence we can understand that −

 The conduction in a NPN transistor takes place through electrons.

 The collector current is higher than the emitter current.
 The increase or decrease in the emitter current affects the collector current.
ADVANTAGES
There are many advantages of a transistor such as −

 High voltage gain.

 Lower supply voltage is sufficient.
 Most suitable for low power applications.
 Smaller and lighter in weight.
 Mechanically stronger than vacuum tubes.
 No external heating required like vacuum tubes.
 Very suitable to integrate with resistors and diodes to produce ICs.
EARLY EFFECT
The Early effect is the variation in the width of the base in a BJT due to a variation in the
applied base-to-collector voltage, named after its discoverer James M. Early. A
greater reverse bias across the collector–base junction, for example, increases the collector–
base depletion width, decreasing the width of the charge neutral portion of the base.

Figure 1. Top: pnp base width for low collector–base reverse bias; Bottom: narrower
pnp base width for large collector–base reverse bias. Light colors are depleted regions.
In Figure 1 the neutral base width is dark blue, and the depleted base regions are light blue.
The neutral emitter and collector regions are dark red and the depleted regions pink. Under
increased collector–base reverse bias, the lower panel of Figure 1 shows a widening of the
depletion region in the base and the associated narrowing of the neutral base region.

The collector depletion region also increases under reverse bias, more than does that of the
base, because the collector is less heavily doped. The principle governing these two widths is
charge neutrality. The emitter–base junction is unchanged because the emitter–base voltage is
the same.

Figure 2. The Early voltage as seen in the output-characteristic plot of a BJT

Base-narrowing has two consequences that affect the current:

There is a lesser chance for recombination within the "smaller" base region.
The charge gradient is increased across the base, and consequently, the current of minority
carriers injected across the emitter junction increases.
Both these factors increase the collector or "output" current of the transistor with an increase
in the collector voltage. This increased current is shown in Figure 2. Tangents to the
characteristics at large voltages extrapolate backward to intercept the voltage axis at a voltage
called the Early voltage, often denoted by the symbol VA.
LARGE-SIGNAL MODEL
In the forward active region the Early effect modifies the collector current (IC) and the
forward common-emittercurrent gain (βF), as typically described by the following equations:
[1][2]

Where
VCE is the collector–emitter voltage
VT is the thermal voltage kT / q; see thermal voltage: role in semiconductor physics
VA is the Early voltage (typically 15 V to 150 V; smaller for smaller devices)
βF0 is forward common-emitter current gain at zero bias.
Some models base the collector current correction factor on the collector–base
voltage VCB (as described inbase-width modulation) instead of the collector–emitter
voltage VCE.[3] Using VCB may be more physically plausible, in agreement with the physical
origin of the effect, which is a widening of the collector–base depletion layer that depends
on VCB. Computer models such as those used in PSPICE use the collector–base voltageVCB.[4]
SMALL-SIGNAL MODEL
The Early effect can be accounted for in small-signal circuit models (such as the hybrid-pi
model) as a resistor defined as (see [5]

in parallel with the collector–emitter junction of the transistor. This resistor can thus account
for the finite output resistance of a simple current mirror or an actively loaded common-
emitter amplifier.
In keeping with the model used in SPICE and as discussed above using VCB the resistance
becomes:

,
which almost agrees with the textbook result. In either formulation, rO varies with DC reverse
bias VCB, as is observed in practice.[citation needed]
In the MOSFET the output resistance is given in Shichman–Hodges model [6] (accurate for
very old technology) as:

,
where VDS = drain-to-source voltage, ID = drain current and λ = channel-length
modulation parameter, usually taken as inversely proportional to channel length L. Because
of the resemblance to the bipolar result, the terminology "Early effect" often is applied to the
MOSFET as well.

TRANSISTOR CONFIGURATION

A Transistor has 3 terminals, the emitter, the base and the collector. Using these 3 terminals
the transistor can be connected in a circuit with one terminal common to both input and
output in a 3 different possible configurations.

The three types of configurations are Common Base, Common Emitter and Common
Collector configurations. In every configuration, the emitter junction is forward biased and
the collector junction is reverse biased.

COMMON EMITTER CONNECTION (OR CE CONFIGURATION)

Definition: The configuration in which the emitter is connected between the collector and
base is known as a common emitter configuration. The input circuit is connected between
emitter and base, and the output circuit is taken from the collector and emitter. Thus, the
emitter is common to both the input and the output circuit, and hence the name is the
common emitter configuration. The common emitter arrangement for NPN and PNP
transistor is shown in the figure below.
 NPN TRANSISTOR PNP TRANSISTOR
BASE CURRENT AMPLIFICATION FACTOR (Β)

The base current amplification factor is defined as the ratio of the output and input current in
a common emitter configuration. In common emitter amplification, the output current is the
collector current IC, and the input current is the base current IB.

In other words, the ratio of change in collector current with respect to base current is known
as the base amplification factor. It is represented by β (beta).

Relation Between Current Amplification Factor (α) & Base Amplification Factor (β)

The relation between Β and α can be derived as

We Known,

Now,
Substituting the value of ΔIE in equation (1), we get,

The above equation shows that the when the α reaches to unity, then the β reaches to infinity.
In other words, the current gain in a common emitter configuration is very high, and because
of this reason, the common emitter arrangement circuit is used in all the transistor
applications.

COLLECTOR CURRENT

In CE configuration, the input current IB and the output current IC are related by the equation
shown below.
If the base current is open (i.e., IB = 0). The collector current is current to the emitter, and this
current is abbreviated as ICEO that means collector- emitter current with the base open.

Substitute the value ΔIB in equations (1), we get,

CHARACTERISTICS OF COMMON EMITTER (CE) CONFIGURATION

The characteristic of the common emitter transistor circuit is shown in the figure below. The
base to emitter voltage varies by adjusting the potentiometer R 1. And the collector to emitter
voltage varied by adjusting the potentiometer R 2. For the various setting, the current and
voltage are taken from the milliammeters and voltmeter. On the basis of these readings, the
input and output curve plotted on the curve.
INPUT CHARACTERISTIC CURVE

The curve plotted between base current I B and the base-emitter voltage V EB is called Input
characteristics curve. For drawing the input characteristic the reading of base currents is taken
through the ammeter on emitter voltage VBEat constant collector-emitter current. The curve for
different value of collector-base current is shown in the figure below.

The curve for common base configuration is similar to a forward diode characteristic. The base
current IB increases with the increases in the emitter-base voltage VBE. Thus the input resistance of
the CE configuration is comparatively higher that of CB configuration.

The effect of CE does not cause large deviation on the curves, and hence the effect of a change in
VCE on the input characteristic is ignored.

Input Resistance: The ratio of change in base-emitter voltage VBE to the change in base current
∆IB at constant collector-emitter voltage VCE is known as input resistance, i.e.,
OUTPUT CHARACTERISTIC

In CE configuration the curve draws between collector current IC and collector-emitter voltage
VCE at a constant base current IB is called output characteristic. The characteristic curve for the
typical NPN transistor in CE configuration is shown in the figure below.

In the active region, the collector current increases slightly as collector-emitter V CE current
increases. The slope of the curve is quite more than the output characteristic of CB configuration.
The output resistance of the common base connection is more than that of CE connection.

The value of the collector current IC increases with the increase in VCE at constant voltage IB, the
value β of also increases.

When the VCE falls, the IC also decreases rapidly. The collector-base junction of the transistor
always in forward bias and work saturate. In the saturation region, the collector current becomes
independent and free from the input current IB

In the active region IC = βIB, a small current IC is not zero, and it is equal to reverse leakage current
ICEO.

Output Resistance: The ratio of the variation in collector-emitter voltage to the collector-emitter
current is known at collector currents at a constant base current IB is called output resistance ro.

The value of output resistance of CE configuration is more than that of CB

COMMON BASE CONNECTION (CB CONFIGURATION)
Definition: The configuration in which the base of the transistor is common between emitter
and collector circuit is called a common base configuration. The common base circuit
arrangement for NPN and PNP transistor is shown in the figure below. In common base-
emitter connection, the input is connected between emitter and base while the output is taken
across collector and base.

CURRENT AMPLIFICATION FACTOR (Α)

The ratio of output current to input current is known as a current amplification factor. In
the common base configuration, the collector current IC is the output current, and the emitter
current IE is the input current. Thus, the ratio of change in emitter current to the collector at
constant collector-base voltage is known as a current amplification factor of a transistor in
common base configuration. It is represented by α (alpha).
Where ΔIC is the change in the collector and ΔIE is changed in emitter current at constant VCB.
Now,

The value of current amplification factor is less than unity. The value of the amplification
factor (α) reaches to unity when the base current reduces to zero. The base current becomes
zero only when it is thin and lightly doped. The practical value of the amplification factor
varies from 0.95 to 0.99 in the commercial transistor.

COLLECTOR CURRENT
The base current is because of the recombination of the electrons and holes in the base
region. The whole emitter current will not flow through the current. The collector current
increase slightly because of the leakage current flows due to the minority charge carrier. The
total collector current consists;

The large percentage of emitter current that reaches the collector terminal, i.e., αI E.

The leakage current Ileakage. The minority charge carrier is because of the flow of minority
charge carrier across the collector-base junction as the junction is heavily reversed. Its value
is much smaller than αIE.

Total collector current,

The above expression shows that if IE = 0 (when the emitter circuit is open) then still a small
current flow in the collector circuit called leakage current. This leakage current is represented
by as ICBO, i.e., collector-base current with emitter circuit is open.

The leakage current is also abbreviated as ICO i.e., the collector current with emitter circuit
open.
CHARACTERISTICS OF COMMON BASE (CB) CONFIGURATION

The characteristic diagram of determining the common base characteristic is shown in the
figure below.

The emitter to base voltage VEB can be varied by adjusting the potentiometer R1. A series
resistor RS is inserted in the emitter circuit to limit the emitter current I E. The value of the
emitter change to a large value even the value of a potentiometer slightly change. The value
of collector voltage changes slightly by changing the value of the potentiometer R2. The input
and output characteristic curve of the potentiometer explains below in details.

INPUT CHARACTERISTIC
The curve plotted between emitter current IE and the emitter-base voltage VEBat constant
collector base voltage VCB is called input characteristic curve. The input characteristic curve
is shown in the figure below.

The following points are taken into consideration from the characteristic curve. For a specific
value of VCB, the curve is a diode characteristic in the forward region. The PN emitter
junction is forward biased. When the value of the voltage base current increases the value of
emitter current increases slightly. The junction behaves like a better diode. The emitter and
collector current is independent of the collector base voltage V CB. The emitter current
IE increases with the small increase in emitter-base voltage V EB. It shows that input resistance
is small.

INPUT RESISTANCE

The ratio of change in emitter-base voltage to the resulting change in emitter current at
constant collector base voltage VCB is known as input resistance. The input resistance is
expressed by the formula

The value of collector base voltage VCB increases with the increases in the collector-base
current. The value of input resistance is very low, and their value may vary from a few ohms
to 10 ohms.

OUTPUT CHARACTERISTIC CURVE

In common base configuration, the curve plotted between the collector current and collector
base voltage VCB at constant emitter current IE is called output characteristic. The CB
configuration of PNP transistor is shown in the figure below. The following points from the
characteristic curve are taken into consideration.

The active region of the collector-base junction is reverse biased, the collector current I C is
almost equal to the emitter current IE. The transistor is always operated in this region.

The curve of the active regions is almost flat. The large charges in VCBproduce only a tiny
change in IC The circuit has very high output resistance ro.
 When VCB is positive, the collector-base junction is forward bias and the collector current
decrease suddenly. This is the saturation state in which the collector current does not
depend on the emitter current.

When the emitter current is zero, the collector current is not zero. The current which flows
through the circuit is the reverse leakage current, i.e., ICBO. The current is temperature
depends and its value range from 0.1 to 1.0 μA for silicon transistor and 2 to 5 μA for
germanium transistor.

OUTPUT RESISTANCE

The ratio of change in collector-base voltage to the change in collector current at constant
emitter current IE is known as output resistance.

The output characteristic of the change in collector current is very little with the change in
VCB. With the change in collector-base voltage. The output resistance is very high of the order
of several kilometres.

COMMON COLLECTOR CONNECTION (OR CC CONFIGURATION)

Definition: The configuration in which the collector is common between emitter and base is
known as CC configuration. In CC configuration, the input circuit is connected between
emitter and base and the output is taken from the collector and emitter.
The collector is common to both the input and output circuit and hence the name common
collector connection or common collector configuration.

CURRENT AMPLIFIER FACTOR (Y)

The current amplification factor is defined as the ratio of the output current to the input
current. In common emitter configuration, the output current is emitter current I E, whereas the
input current is base current IB.
Thus, the ratio of change in emitter current to the change in base current is known as the
current amplification factor. It is expressed by the Y.

RELATION BETWEEN Υ and α

The Y is the current amplification factor of common collector configuration and the α is
current amplification factor of common base connection.

and,

Substituting the value of ΔIB in above first equation, we get,

The above relation shows that the value of Y is nearly equal to β. This circuit is mainly used
for amplification because of this arrangement input resistance is high, and output resistance is
very low. The voltage gain of the resistance is very low. This circuit arrangement is mainly
used for impedance matching.
COLLECTOR CURRENT
We know that,

INPUT CHARACTERISTIC CURVE

The input characteristic of the common collector configuration is drawn between collector
base voltage VCE and base current IB at constant emitter current voltage VCE. The value of the
output voltage VCE changes with respect to the input voltage VBC and IB With the help of these
values, input characteristic curve is drawn. The input characteristic curve is shown below.

OUTPUT CHARACTERISTIC CURVE

The output characteristic of the common emitter circuit is drawn between the emitter-
collector voltage VEC and output current IE at constant input current IB. If the input current
IB is zero, then the collector current also becomes zero, and no current flows through
the transistor.
The transistor operates in active region when the base current increases and reaches to
saturation region. The graph is plotted by keeping the base current IB constant and varying the
emitter-collector voltage VCE, the values of output current IE are noticed with respect to VCE.
By using the VCE and IE at constant IB the output characteristic curve is drawn.

COMPARISON OF TRANSISTOR CONFIGURATION

SMALL SIGNAL – LOW FREQUENCY H – PARAMETER MODEL

Let us consider transistor amplifier as a block box as shown in the Fig. 12.1.

Here, Ii : is the input current to the amplifier

Vi : is the input voltage to the amplifier
Io : is the output current to the amplifier and
Vo : is the output voltage to the amplifier
As we know transistor is a current operated device, input current is an independent variable.
The input current, Ii and output voltage Vodevices the input voltage Vi as well as the output
current Io. Hence input voltage Vi and outpur current Io are the dependent variables, whereas
input current Ii and output voltage Vo are independent variables. Thus we can write
 Vi = f1 (Ii, Vo) ... (1)
Io = f2 (Ii, Vo) … (2)
This can be written in the equation form as follows
Vi = h11 Ii + h12 Vo ... (3)
Io = h21 Ii + h22 Vo ... (4)
The above equations can also be written using alphabetic notations,
Vi = hi . Ii + hr . Vo ... (5)
Io = hf . Ii + ho . Vo ... (6)
Definitions of h – parameter
The parameters in the above equation are defined as follows:

h11 = = input resistance with output short – circuited, in ohms..

h12 = = Fraction of output voltage at input with input open circuited.

This parameter is ratio of similar quantities, hence unitless

h21 = = Forward current transfer ratio or current gain with output

short circuited.
This parameter is a ratio of similar quantities, hence unitless.

h22 = = Output admittance with input open-circuited, in mhos.

From the above discussion we can say that, 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. As we use small letter for ac analysis, these are commonly known as h-
parameters. The standard notations can be given as
i = 11= input o = 22 = output
f = 21 = forward transfer r = 12 = reverse transfer
Thus we can write h-parameters as follows.
a) With output short circuited :
h11 = hi : Input resistance
h21 = hf : Short circuit current gain
b) With input open circuited :
h12 = hr : Reverse voltage transfer ratio
h22 = ho : output admittance.
H- parameter equivalent circuit for transistor is shown in the following figure

In order to analyze transistorized amplifier circuit and calculate its input impedance, output
impedance, current gain and voltage gain, it is necessary to replace transistor circuit with its
equivalent. The equivalent circuit can be drawn with the help of two equations, as shown in
Fig. 1.10.
Vi = hi Ii + hr Vo Io = hf Ii + ho Vo
Many transistor models have been proposed, each one having its advantages and
disadvantages. The transistor model used in this text is in terms of h-parameters.
Benefits of h-parameters
Real numbers at audio frequencies.
Easy to measure.
Can be obtained from the transistor static characteristics curves.
Convenient to use in circuit analysis and design.
Most of the transistor manufacturers specify the h-parameters.

H – parameters equivalent circuit for CE configuration in the following figure

Simple common emitter configuration

To see how we can derive a hybrid model for a transistor, let us consider the common emitter
configuration as shown in the above figure.. The variables Ib, Ic, Vb and Vc represent total
instantaneous currents and voltages.
Ib = input current
Ic = output current
Vbe = input voltage
Vce = output voltage.
The following figure shows the h-parameter equivalent circuit for the common emitter
configuration.
h-parameter equivalent circuit for the common emitter configuration

From the h-parameter equivalent circuit of the common emitter configuration we can
write,
Vbe = hie Ib + hre Vce …(7)
Ic = hfe Ib + hoe Vce …(8)
The quantities ∆VBE (Vbe), ∆VCE (Vce), ∆IB (Ib) and ∆IC (Ic) represent the small change in base
and collector voltages and currents.

H-PARAMETERS FOR ALL THREE CONFIGURATIONS

As mentioned earlier, transistor can be represented as a two port network by making any one
terminal common between input and output. Since there are three possible configurations in
which a transistor can be used, there is a change in terminal voltage and current for different
transistor configurations. For different configurations the relation between input parameters
and output parameters also differs. Therefore, one needs to define different set of h-
parameters for different configurations. To designate the type of configuration another
subscript is added to the h-parameters.
For example :
hie = h11e = input resistance in common emitter configuration.
hfb = h21b = short-circuit current gain in common base configuration.

The following table summarizes the h-parameters for all the three configurations.
parameter CB CE CC

Input resistance hib hie hic

Reverse voltage gain hrb hre hrc

Forward transfer current gain hfb hfe hfc

Output admittance hob hoe hoc

The Bipolar Transistor (Ebers Moll Model)

The bipolar transistor is an electronic device that originates a big evolution in the electronics
field. The basic features of the bipolar transistor are introduced on this topic. We’ll study the
basic models of these devices and their use in the analysis of circuits biasing.

Biasing a bipolar transistor is necesary for many linear and non linear applications, because it
establishes the voltage ranges and the direct current that is going to circulate on the transistor.

A bipolar transistor has two back to back PN junctions. Physically, the transistor consists of
three regions: emitter, base and collector. The base region is very thin (<1 µ m).
When the transistor operates in active mode, The emitter-base junction is forward biased and
the base-collector junction is reverse biased. In this case the directions of the currents and the
voltages are shown in the above pictures.

The relation between the currents is: IE = IB + IC, and …

VCE = VCB + VBE for an NPN transistor.

VCE = VEB + VBC for an PNP transistor.

Ebers Moll Model of a Bipolar Transistor

Ebers and Moll created a model between the current and voltages in the transistor terminals .
This model, known as the Ebers Moll model sets the following general equations, for
an NPN transistor:

IES and ICS represent saturation current for emitter and collector junctions, respectively. αaF
is the common base forward short circuit current gain (0.98 to 0.998) αR is the injection of
minority carriers fraction. In a PNP bipolar transistor, the Ebers Moll model equations are:

For the ideal transistor, the previous four parameters are related by the Reciprocity
Theorem. αFIES = αRICS. Tipical values of these parameters are: αF = 0.99, αR= 0.66, IES = 10-
15
A, ICS = 10-15A.
MULTIPLE-EMITTER TRANSISTOR

A multiple-emitter transistor is a specialized bipolar transistor mostly used at the

inputs of TTL NAND logic gates. Input signals are applied to the emitters. Collector current
stops flowing only if all emitters are driven by the logical low voltage, thus performing
a NAND logical operation using a single transistor. Multiple-emitter transistors
replace diodes of DTL and allow reduction of switching time and power dissipation.

cross section and symbol of a simple NPN bipolar transistor

cross section and symbol of a multiple emitter NPN bipolar transistor