This document discusses various transistor configurations and their characteristics. It begins with a quote by Albert Einstein. It then discusses the common-base, common-emitter, and common-collector configurations. For each configuration, it describes the input and output characteristics, showing how the input and output currents and voltages relate. It notes that the common-emitter configuration is most commonly used and describes how to properly bias a common-emitter amplifier. The document also briefly discusses the early effect in transistors.

1. Transistor Configuration
Quote of the day
“Try not to become a man of success,
but rather try to become a man of
value.”
― Albert Einstein

3. Modes of Operation
3
Modes EBJ CBJ Application
Cutoff Reverse Reverse
Switching application
in digital circuits
Saturation Forward Forward
Active Forward Reverse Amplifier
Reverse
active
Reverse Forward
Performance
degradation

4. • Relationship between the
collector current and the
base current in a bipolar
transistor
– characteristic is
approximately linear
– magnitude of collector
current is generally
many times that of the
base current
– the device provides
current gain
B
C
I
I
CEV
ac




constant
dc = IC / IB
BJT Amplification(Current)

5. The output voltage Vo can
be calculated as
B
C
v
V
V
A



BJT Amplification (Voltage)
RIVV co 
RIV cC 
When input voltage Vi change the VB changes
which cause IB to change. The change in base
current produces change in collector current
BdcC IIei  .. The change in Collector
voltage Vc is
i
o
v
V
V
A



The voltage amplification Av is
Ic   Ic
VB   VB
= V0   Vc

6. Example. Determine the dc collector voltage for the
circuit shown below if the transistor has the input
characteristics shown in side figure and  = 80.
Calculate the circuit voltage gain when input
variation is 50mV
+50mV
-50mV
Ic =?mA
=18V
=10K
=?V
Solution: From side fig IB=15 for VBE= 0.7 V
From fig2 V=18V & R-10K, Given =80, VB = 50mV
AII Bc  1580  mAIc 2.1
RIVV co   33
1010102.118  
oV
VVo 6
From above fig for IB=  3A, Vi= 50mV
AII Bc  380 AIc 240
RIV cC   KAVC 10240
i
o
v
V
V
A


VVC 4.2
mv
V
Av
50
4.2


 48vA

7. Transistor Configurations
• Common-Base Configuration
• Common-Emitter Configuration
• Common-Collector Configuration

8. Common-Base Configuration

9. Common-Base Configuration
• The common-base configuration with pnp and
npn transistors are shown in the figures in the
previous slide..
• The term common-base is derived from the fact
that the base is common to both the input and
output sides of the configuration.
• The arrow in the symbol defines the direction of
emitter current through the device.
• The applied biasing are such as to establish
current in the direction indicated for each branch.
• That is, direction of IE is the same as the polarity
of VEE and IC to VCC .
• Also, the equation IE = IC + IB still holds.

10. Input characteristics
• The driving point or input
parameters are shown in
the figure.
• An input current (IE) is a
function of an input voltage
(VBE) for various of output
voltage (VCB ).
• This closely resembles the
characteristics of a diode.
• In the dc mode, the levels of
IC and IE at the operation
point are related by:
αdc = IC / IE
• Normally, α 1.
• For practical devices, α is
typically from 0.9 to 0.998.
Figure: Input characteristics for common-base
transistor

11. Input characteristics contd..
• As an approximation, the change due to changes in
VCB can be ignored.
• The characteristics can be shown in orange curve.
• If piecewise-linear approach is applied, the
magenta green curve is obtain.
• Furthermore, ignoring the slop of the curve and the
resistance results the magenta curve.
• It is this magenta curve that is used in the dc
analysis of transistors.
• Once a transistor is in “on” state, the B-E voltage is
assumed to be 0.7V.
• And the emitter current may be at any level as
controlled by the external network.

12. Figure: Output characteristics for common-base transistor
Cutoff
Region
Saturation
Region
Active Region
Output characteristics

13. Figure: Output characteristics for common-base transistor
Output characteristics

14. Output characteristics
• The output set relates an output current (IC ) to an output voltage (VCB)
for various of level of input current (IE ).
There are three regions of interest:
Active region
• In the active region, the b-e junction is forward-biased, whereas the c-b
junction is reverse-biased.
• The active region is the region normally employed for linear amplifier.
Also, in this region,
I C  IE
Cutoff region
• The cutoff region is defined as that region where the collector current
is 0A.
• In the cutoff region, the B-E and C-B junctions of a transistor are both
reverse-biased.
Saturation region:
• It is defined as that region of the characteristics to the left of VCB= 0 V.
• In saturation region, the B-E and C-B junctions of a transistor are both
forward biased.

15. • common-emitter
configurations
– Most common configuration
of transistor is as shown
– emitter terminal is common
to input and output circuits
this is a common-emitter
configuration
– we will look at the
characteristics of the device
in this configuration
– The current relations are still
applicable, i.e.,
– IE = IC + IB and IC =α IE
The common-emitter
configuration with npn and
pnp transistors are shown
in the figures.
Figure: Common-emitter configuration
of pnp transistor
Figure: Common-emitter configuration
of npn transistor

16. • Input characteristics
– the input takes the
form of a forward-
biased pn junction
– the input
characteristics are
therefore similar to
those of a
semiconductor
diode
An input current (IB) is a function of an
input voltage (VBE) for various of output
voltage (VCE ).

17. Figure: Output characteristics for common-emitter transistor
Cutoff
Region
Saturation
Region
Active RegionOutput characteristics

18. • Output characteristics
– The magnitude of IB is in μA and not as horizontal as IE
in common-base circuit.
– The output set relates an output current (IC) to an
output voltage (VCE) for various of level of input
current (IB ).
• There are three portions as shown:
Active region
 The active region, located at upper-right quadrant, has
the greatest linearity.
 The curve for IB are nearly straight and equally
spaced.
 In active region, the B-E junction is forward-biased,
whereas the C-B junction is reverse-biased.
 The active region can be employed for voltage,
current or power amplification.

19. Cutoff region
• The region below IB = 0μA is defined as cutoff
region.
• For linear amplification, cutoff region should be
avoided.
Saturation region:
• The small portion near the ordinate, is the
saturation region, which should be avoided for
linear application.
• In the dc mode, the levels of IC and IB at the
operation point are related by: Normally, 
ranges from 50 to 400.
dc = IC / IB
For ac situations,  is defined as B
C
I
I
tconsV
ac
CE




tan


20. Biasing•The proper biasing is essential
to place the device in the
active region.
•A common-emitter amplifier
of a pnp transistor is shown in
the figure.
•The first step is to indicate the
direction of IE as established by
the arrow in the transistor
symbol.
Figure: Biasing for common-
emitter pnp transistor
•The other current , IB and IC , are introduced, satisfying
IC + IB = IE .
•The supplies are introduced with polarities that will support
the resulting directions of IB and IC .
•If the transistor is a npn transistor, all the current and
polarities would be reversed.

21. Base Width Modulation: “Early” Effect
• When bias voltages change, depletion widths change and
the effective base width will be a function of the bias
voltages
• Most of the effect comes from the C-B junction since the
bias on the collector is usually larger than that on the E-
B junction
Base width gets smaller as applied voltages get larger

22. The Early Effect
Converge ~ at single point called "Early Voltage" (after James Early)
Large "Early Voltage" = Absence of "Base Width Modulation"
= Transistor ~ immune to operating voltage changes
Range is -100V to -200 V

23. Common-Collector Configuration
• The common-collector configuration with npn and
pnp transistors are shown in the figures.
Figure: Common-collector
configuration of npn transistor
Figure: Common- collector
configuration of pnp transistor

24. •It is used primarily for impedance-matching purpose
since it has a high input impedance and low output
impedance.
•The load resistor can be connected from emitter to
ground.
•The collector is tied to ground and the circuit resembles
common-emitter circuit.
•The output set relates an output current (IE) to an
output voltage (VCE) for various of level of input current
(IB ).
Common-Collector Configuration

25. Input characteristics
• It is a curve which shows the relationship between
the base current, IB and the collector base voltage VCB
at constant VCE This method of determining the
characteristic is as follows.
• First, a suitable voltage is applied between the emitter
and the collector.
• Next the input voltage VCB is increased in a number
of steps and corresponding values of IE are noted.
• The base current is taken on the y-axis, and the input
voltage is taken on the x-axis. Fig. shows the family of
the input characteristic at different collector- emitter
voltages.

26. Input characteristics
Figure: Common-collector
circuit used for impedance-
matching purpose
• The following points may be noted from
the family of characteristic curves.
• Its characteristic is quite different from
those of common base and common
emitter circuits.
• When VCB increases, IB is decreased.

27. • This is almost the same as the output
characteristics of common-emitter circuit,
which are the relations between IC and VCE for
various of level of input current IB.
Since that: IE  IC .
Output characteristics
Figure: Output
characteristics for
common-collector
transistor