How will measure the common emitter mode input, output & transfer charactaristics of a n-p-n
transistor? Sketch the circuit diagram and charactaristics.
Common Emitter Configuration:
For the common-emitter configuration of Fig. 7.21a, the input terminals are the base and emitter terminals, but
the output set is now the collector and emitter terminals. In addition, the emitter terminal is now common
between the input and output ports of the amplifier. Substituting the re equivalent circuit for the npn transistor
will result in the configuration of Fig. 7.21b. Note that the controlled-current source is still connected
between the collector and base terminals and the diode between the base and
Question: Sketch the circuit diagram of a common emmitter of npn transistor & hence explain the
mechanics of current flow through the different parts.
Emitter current: The above figure shows a biased transistor the minus (-) sign represents free electrons
in the emitter region. The heavily doped emitter emits or injects free electron in the base when external
voltage source .
forward biases the emitter base junction. By convectional flow of current, the
emitter current flows in the opposite direction of electron flow which has shown in the previous circuit.
Base Current: When the forward bias is applied to the emitter base junction the electron in the emitter
will not reach the biased region until the applied voltage overcomes the emitter base potential barrier.
Once when
is the greater than the emitter base barrier potential electrons will reach into the base
region. Theoretically these electrons can flow in either or two directions.
1. They can flow to the left and out of the base through
.
to the positive terminal of source voltage
2. The free electrons can flow into the collector region.
Collector Current: Almost all the electrons go into the collector as shown above. Once they are in the
collector, the free electrons flow through the collector & through
until they reach positive terminal
of collector supply
. The conventional flow of current has shown above.
Question: Obtain expression for current gain, voltage gain, power gain, input resistance &
output resistance for the common emitter configuration of a transistor.
The current flowing between emitter and collector of a transistor is much greater than that flowing
between base and emitter. Thus a small base current is controlling the emitter collector current. The
ratio of the two currents, ICE/IBE is constant, provided that the collector emitter voltage
is constant.
Therefore, if the base current rises, so does collector current.
This ratio is the CURRENT GAIN of the transistor and is given the symbol hfe. A fairly low gain transistor
might have a current gain of 20 to 50, while a high gain type may have a gain of 300 to 800 or more. The
spread of values of
for any given transistor is quite large, even in transistors of the same type and
batch.
Characteristic curves (graphs) can be drawn to show other parameters of a transistor and are used both
to detail the performance of a particular device and as an aid to the design of amplifiers.
Voltage Gain:
Ib 
Vi
Zb
Vo  I o R C  βI b R C
V 
  β i R C
 Zb 
V
 βR C
 AV  o 
Vi
Zb
substituting Z b  β(re  R E ) gives
 AV 
Vo
 RC

Vi re  R E
and for the approximation Z b  βR E
 AV 
Vo  R C

Vi
RE
Current Gain:
The magnitude of R B is often too close to Z b to permit the approximation
I b  I i . Applyingthe current - divider rule to the input circuit will result in :
Ib 
R B Ii
R B  Zb
Ib
RB

Ii R B  Zb
I o  I b
Io

Ib
 Ai 
Io Io Ib
RB


Ii I b Ii
R B  Zb
 A i  A v
Zi
RC
Common-Base Input Resistance
Apply test current, with load resistor
Present at output.
:
Question: What is D.C load line? How will draw a D.C load line on the output characteristics
curve of a transistor & importance.
IC(sat) = VCC/(RC+RE)
DC Load Line
IC
(mA)
VCE(off) = VCC
VCE
•
•
The straight line is known as the DC load line
Its significance is that regardless of the behavior of the transistor, the collector
current I and the collector-emitter voltage V must always lie on the load
C
line, depends ONLY on the V , R and R
•
CC
C
CE
E
(i.e. The dc load line is a graph that represents all the possible combinations
of I and V for a given amplifier. For every possible value of I , and
C
CE
amplifier will have a corresponding value of V .)
•
CE
C
It must be true at the same time as the transistor characteristic. Solve two
condition using simultaneous equation
 graphically  Q-point !!
Importance Of D.C load line: Load line is important tools in electronics because it contains every
possible operating point for the transistor circuit. Stated another way when the base resistance varies
from zero to infinite, it causes to vary that makes &
to vary over entire ranges. If we plot the
&
a values for every possible
value. We’ll get the load line. Therefore the load line is a visual
summery of all the possible transistor operating points.
Question: What is operating point? How it is determined? What is saturation point & Cut-off
point? Explain briefly.
The "Q point" for a transistor amplifier circuit is the point along its operating region in a
"quiescent ", where no input signal gets amplified.
The currents and voltages in an electronic circuit when the input signal is replaced by its average value,
so that all currents and voltages can be approximated by series expansions around this point. Also
known as Q point.
When the signal of applied variation of collector current and collector emitter voltage take place
about this point that is : why it is called silent point.
Saturation Point: The point at which the load line intersects the characteristic curve near the
collector current axis is referred to as the saturation point. At this point of time, the current
through the transistor is maximum and the voltage across collector is minimum for a given value
of load. Therefore saturation current for the fixed bias circuit,
Ic (sat) =Vcc/Rc
Cutoff Point: The point where the load line intersects the cutoff region of the collector curves is referred
as the cutoff point (i.e. end of load line). At this point, collector current is approximately zero and
emitter is grounded for fixed bias circuit. Therefore,
Vce (cut) = Vc = Vcc