MODULE FOUR
BIPOLAR JUNCTION TRANSISTORS
A transistor is a sandwich of one type of semiconductor (P-type or n-type) between two layers of
other types.
Transistors are classified into two types;
1. PNP transistor: It is obtained when a n-type layer of silicon is sandwiched between two
p-type silicon material.
2. NPN transistor: it is obtained when a p-type layer of silicon is sandwiched between two
n-type silicon materials.
Figure3.1 below shows the schematic representations of a transistor which is equivalent of two
diodes connected back to back.
The three portions of transistors are named as emitter, base and collector. The junction between
emitter and base is called emitter-base junction while the junction between the collector and base
is called collector-base junction.
The base is thin and tightly doped, the emitter is heavily doped and it is wider when compared to
base, the width of the collector is more when compared to both base and emitter.
In order to distinguish the emitter and collector an arrow is included in the emitter. The direction
of the arrow depends on the conventional flow of current when emitter base junction is forward
biased.
In a pnp transistor when the emitter junction is forward biased the flow of current is from
emitter to base hence, the arrow in the emitter of pnp points towards the base.
3.1 Operating regions of a transistor
A transistor can be operated in three different regions as
a. active region
b. saturation region
c. cut-off region
Active region
The transistor is said to be operated in active region when the emitter-base junction is forward
biased and collector base junction is reverse biased. The collector current is said to have two
current components one is due to the forward biasing of EB junction and the other is due to
reverse biasing of CB junction. The collector current component due to the reverse biasing of the
collector junction is called reverse saturation current and it is very small in magnitude.
Saturation region
Transistor is said to be operating in saturation region when both EB junction and CB junction
are forward biased as shown. When transistor is operated in saturation region I C increases
rapidly for a very small change in VC .
Cut-off region
When both EB junction and CB junction are reverse biased, the transistor is said to be operated
in cut-off region. In this region, the current in the transistor is very small and thus when a
transistor in this region it is assumed to be in off state.
3.2 Working of a transistor ( NPN )
Consider a npn transistor operated in active region as shown in Figure 3.6
Since the EB junction is forward biased large number of holes present in the emitter as majority
carriers are repelled by the +ve potential of the supply voltage VEB and they move towards the
base region causing emitter current IE.
Since the base is thin and lightly doped very few of the holes coming from the emitter
recombine with the electrons causing base current I B and all the remaining holes move towards
the collector. Since the CB junction is reverse biased all the holes are immediately attracted by
the positive potential of the supply VCB . Thereby giving rise to collector current I C .
Thus I E I B I C
Since the CB junction is reversed biased a small minority carrier current Ico flows from the base
to collector.
Fig 3.7 above shows a transistor operated in active region. It can be noted from the diagram the
battery VEB forward biases the EB junction while the battery VCB reverse biases the CB
junction. As the EB junction is forward biased the holes from emitter region flow towards the
base causing a hole current IPE. At the same time, the electrons from base region flow towards
the emitter causing an electron current INE. Sum of these two currents constitute an emitter
current IE = IPE +INE.
The ratio of hole current IPE to electron current INE is directly proportional to the ratio of the
conductivity of the p-type material to that of n-type material. Since, emitter is highly doped when
compared to base; the emitter current consists almost entirely of holes.
Not all the holes, crossing EB junction reach the CB junction because some of them combine
with the electrons in the n-type base. If IPC is the hole current at (Jc) CB junction. There will be
a recombination current IPE - IPC leaving the base as shown in figure 3.7.
If emitter is open circuited, no charge carriers are injected from emitter into the base and hence
emitter current IE =o. Under this condition CB junction acts a a reverse biased diode and
herefore the collector current ( IC = ICO) will be equal to te reverse saturation current. Therefore
when EB junction is forward biased and collector base junction is reverse biased the total
collector current IC = IPC +ICO.
Transistor Configuration
There are three possible ways of connecting bipolar transistor in an electronic circuit with each
method of connection responding differently to its input signal as the static characteristics of the
transistor vary with each circuit arrangement.
1. Common Base Configuration - has Voltage Gain but no Current Gain.
2. Common Emitter Configuration - has both Current and Voltage Gain.
3. Common Collector Configuration - has Current Gain but no Voltage Gain.
1. Common Base Configuration
A simple circuit arrangement of CB configuration for pnp transistor is shown below.
In this configuration, base is used as common to both input and output. It can be noted that the
input section has an a.c. source Vi along with the d .c. source VEB . The purpose of including VEB is
to keep EB junction always forward biased (because if there is no VEB then the EB junction is
forward biased only during the positive half-cycle of the input and reverse biased during the
negative half cycle. In CB configuration, I E is the input current, I C is the output current.
Current Relations
1. Current amplification factor ( )
It is defined as the ratio of d.c. collector current to d.c. emitter current
I
C
IE
2. Total input-output o / p current
We know that CB junction is reverse biased and because of minority charge carriers a small
reverse saturation current I C flows from base to collector.
I E I B IC
IC I E
I E I E IC
Transistor Characteristics
1. Input characteristics
The input characteristic is a curve between I E and emitter base voltage VEB keeping VCB
constant. I E is taken along y-axis and VEB is taken along x-axis. From the graph following
points can be noted.
1. For small changes of VEB there will be a large change in I E . Therefore input resistance is very
small.
2. I E is almost independent of VCB .
VEB
3. Input resistance, Ri , with VCB constant
IE
2. Output characteristics
Out put characteristics is the curve between I C and VCB at constant I E . The collector current
I C is taken along y-axis and VCB is taken along x-axis. It is clear from the graph that the output
current I C remains almost constant even when the voltage VCB is increased.
i.e, a very large change in VCB produces a small change in I C . Therefore, output resistance is
very high.
VEB
Output resistance Ro , with I E constant
IC
Region below the curve I E 0 is known as cut-off region where I C is nearly zero. The region to
the left of VCB 0 is known as saturation region and to the right of VCB 0 is known as active
region.
2. CE configuration
In this configuration the input is connected between the base and emitter while the output is
taken between collector and emitter. For this configuration I B is input current and I C is the
output current. The common emitter is an inverting amplifier circuit resulting in the output signal
being 180o out of phase with the input voltage signal.
1. Current amplification factor ( )
It is the ratio of d .c. collector current to d .c. base current.
I
i.e., C
IB
2. Relationship between and
IC
We know
IE
IC
I E IC
Divide both numerator and denominator of RHS by I C , we get.
1
IE
IC 1
1
1
1
IC
IB
1
Also
1
1
1
Transistor characteristic
Input characteristic curve
Input characteristics is a curve between VBE and base current I B at constant VCE . From the graph
following can be noted.
1. The input characteristic resembles the forward characteristics of a p-n junction diode.
2. For small changes of VBE there will be a large change in base current I B . i.e., input resistance
is very small.
3. The base current is almost independent of VCE .
VBE
4. Input resistance, Ri with VCE constant
IB
It is the curve between VCE and I C at constant I B . From the graph we can see that,
1. Very large changes of VCE produces a small change in I C i.e. output resistance is very high.
VCE
2. Output resistance Ro with I B constant
IC
Region between the curve I B 0 is called cut-off region where I B is nearly zero. Similarly the
active region and saturation region is shown on the graph.
Common Collector Configuration.
In this configuration the input is connected between the base and collector while the output is
taken between emitter and collector. Here I B is the input current and I E is the output current.
Current relations
1. Current amplification factor ( )
2. Relationship between , and
IE
IB
I B IC
IB
Dividing both Numerator and denominator by I B
1 IICB
1
IC
1
IB 1
1
1
Comparison of CB, CE, CC
Characteristics Common Base Common Emitter Common Collector
Input resistance Ri Low Low High
Output resistance Ro High High Low
Current amplification factor
1 1 1
Phase relationship between In phase Out of phase In phase
input and output
Application High frequency Audio frequency Impedance matching
applications application
Current gain Less than Unity Greater than unity Very high
Voltage gain High Greater than unity Less than unity
Worked Example
1. An NPN Transistor has a DC current gain, 200 . Calculate the base current I B required
switch a resistive load of 4 mA .
IC
IB
IC 4 103
IB 20 A
200
Therefore, 200 , IC 4mA and I B 20 A .
Note: The collector voltage, VC must be greater than the emitter voltage, VE to allow current to
flow through the device between the collector-emitter junction. Then the base voltage, VBE of an
Transistor must be greater than this 0.7 V otherwise the transistor will not conduct with the base
current given as.
VB VBE
IB
RB
Worked Example.
2. An NPN Transistor has a DC base bias voltage, VB 10V and an input base resistor, RB of
100kΩ. What will be the value of the base current into the transistor?
VB VBE 10 0.7
IB 93 A
RB 100 103
The AC-DC Load Lines.
Using the common emitter configuration shown below, a family of curves known commonly as
the Output Characteristics Curves, relates the output collector current, ( I c ) to the collector
voltage, ( Vce ) when different values of base current, ( I b ) are applied to the transistor for
transistors with the same β value. A DC Load Line can also be drawn onto the output
characteristics curves to show all the possible operating points when different values of base
current are applied. Initial Vce is correctly set to allow the output voltage to vary when
amplifying AC input signals. This is normally referred to as setting the operating point or
Quiescent Point (Q-point).
Output Characteristics Curves for a Typical Bipolar Transistor
The effect of Vce on the collector current I c when Vce is greater than about 1.0 volts is that I c is
largely unaffected by changes in Vce above this value and instead it is almost entirely controlled
by the base current, I b .
It can also be seen from the emitter current I e is the sum of the collector current, I c and the base
current, I b , added together so we can also say that I e I c I b .
By using the output characteristics curves and Ohm´s Law, the current flowing through the load
resistor, ( RL ), is equal to the collector current, I c entering the transistor which in turn
corresponds to the supply voltage, ( Vcc ) minus the voltage drop between the collector and the
emitter terminals, ( Vce ) and is given as:
VCC VCE
IC
RL
Also, a Load Line can be drawn directly onto the graph of curves above from the point of
saturation when VCE 0 to the point of Cut-off when I C 0 giving us the Operating or Q-point
of the transistor. These two points are calculated as:
VCC 0 VCC
When VCE 0 , I C
RL RL
VCC VCE
When I C 0 , 0 and VCC VCE
RL
Identifying Transistor using Resistance Values.
The table below gives the summary of the simplest way to identify resistors and their terminals.
Between Transistor Terminals PNP NPN
Collector Emitter RHIGH RHIGH
Collector Base RLOW RHIGH
Emitter Collector RHIGH RHIGH
Emitter Base RLOW RHIGH
Base Collector RHIGH RLOW
Base Emitter RHIGH RLOW
The Transistor as a Switch
BJT can be made to operate as a switch. If the circuit uses the Transistor as a Switch, then the
biasing is arranged to operate in Saturation (ON) and Cut-off (OFF).
Cut-off Region - Both junctions are Reverse-biased, Base current is zero or very small resulting
in zero Collector current flowing, and the device is switched fully OFF.
VCE is approximately equal to VCC
VCE cut off VCC
Saturation Region - Both junctions are Forward-biased, Base current is high enough to give a
Collector-Emitter voltage of 0V resulting in maximum Collector current flowing, and the device
is switched fully ON.
VCC
I C saturation
RC
An example of an NPN Transistor as a switch being used to operate a relay is given below. The
diode to dissipate the back EMF generated by the inductive load when the transistor switches
OFF and also protects the transistor from damage.
Example No1.
For example, using the transistor values from the previous tutorials of: 200 , I c 4 mA and
I b 20 A , find the value of the Base resistor ( Rb ) required to switch the load ON when the
input terminal voltage exceeds 2.5 V .
Vin Vbe 2.5 0.7
Rb 90 k
Ib 20 10 6
Example No2.
Again using the same values, find the minimum Base current required to turn the transistor fully
ON (Saturated) for a load that requires 200 mA of current.
Ic200 103 A
Ib 1mA
200
Example 3
Use the circuit below to answer the questions that follows.
a. For the trasnistor switch circuit below what is VCE , Vin 0V
b. What is the minimum value of I B will saturate of this transistor if the 200 .
c. Calculate the maximumvalue of RB when Vin 5V
Solution:
Transistor as an Amplifier
In order to use a transistor as an amplifier it should be operated in active region i.e. emitter
junction should be always forward bias and collector junction should be reversed bias. Therefore
in addition to the a.c. input source Vi , two d .c. voltages VBE and VCE are applied as shown. This
d .c. voltage is called bias voltage.
As the input circuit has low resistance, a small change in the input signal causes a large change
in the base current thereby causing the same change in collector current. Thus a weak signal
applied at the input circuit appears in the amplified form at the output.
C1 and C2 are coupling capacitors used to pass the signal into and out of the amplifier such that
it will not affect the bias voltages. C3 is a bypass capacitor that shorts the emitter signal voltage
(ac) to ground without disturbing the emitter voltage. Because of the bypass of capacitor, the
emitter is at signal (ac) ground (but not do ground), thus making the circuit a common emitter
amplifier. The bypass capacitor increases the signal voltage gain.
A standard DC analysis of four –resistor bias circuit provides the Q-point, and from that we
obtain the value load lines.
