This document discusses transistor configurations and biasing. It describes the common base, common emitter, and common collector configurations. It explains how biasing circuits like fixed bias, collector feedback bias, and voltage divider bias can stabilize the operating point. Thermal runaway and the use of heat sinks are also covered. Single-stage amplifiers using common emitter configuration with input and output coupling capacitors are discussed. Finally, the concepts of frequency response and bandwidth are introduced.

1. Transistor Circuits
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2. Course outcome
• C302.2:
Select a proper biasing and designing a
transistor as an amplifier.
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3. TRANSISTOR CONFIGURATIONS
AND BIASING
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4. Transistor configurations
• A general two port network is
• This network has input port and output port.
Therefore the total number of terminals are
four.
Two port network
Input
port
Output
port
1
1
2
2
Ii Io
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5. • But transistor have only 3 terminals, hence we
treat one of the three terminals “common” to
input and output port.
• Depending on which terminal is made common
to input and output port, there are three possible
configurations of transistor, they as follows:
1. Common base configuration
2. Common emitter configuration
3. Common collector configuration
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6. Common Base (CB) Configuration
a) NPN transistor b) PNP transistor
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7. • In CB configuration, base acts as common terminal
between the input and output ports.
• The input voltage VEB is applied between emitter and base
while output voltage VCB is taken between collector and
base.
• Current relations:
 The output current IC is given by
IC = IC(INJ) + ICBO
where IC(INJ) = injected collector current
and ICBO = reverse saturation current of CB junction
 As ICBO flows due to minority carriers, it is negligible as
compared to IC(INJ),
∴ IC ≈ IC(INJ)
• Current amplification factor (αdc):
αdc = IC / IE
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8. Common Emitter (CE) Configuration
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9. • In CE configuration, emitter acts as common
terminal between input and output poets.
• The input voltage VBB is applied between base
and emitter while output voltage VCC is taken
between collector and emitter.
• Current relations:
For CB configuration, we can write
IC = αdc IE + ICBO
Similarly for CE configuration, we can write
IC = βdc IB + ICEO
• Current gain (βdc):
βdc = IC / IB
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10. Common Collector (CC) Configuration
a) NPN transistor b) PNP transistor
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11. • In CC configuration, collector acts as a
common terminal between input and output.
• The input voltage VEE or VBB is applied
between base and collector while output
voltage VCC taken between collector and
emitter.
• Current gain (γdc):
γdc = IE / IB
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12. Comparison of configurations
Sr.
No.
Parameter CB CE CC
1. Common terminal
between input and
output
Base Emitter Collector
2. Input current IE IB IB
3. Output current IC IC IE
4. Current gain αdc = IC / IE βdc = IC / IB γdc = IE / IB
5. Input voltage VEB VBE VBC
6. Output voltage VCB VCE VBC
7. Voltage gain Medium Medium Less than 1
8. Input resistance Very low (20 Ω) Low (1kΩ) High (500 kΩ)
9. Output resistance Very high (1MΩ) High (40 KΩ) Low (50 Ω)
10. Applications As preamplifier Audio amplifier For impedance
matching
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13. DC Load Line
• To understand the concept of dc load line,
consider the CE configuration of npn transistor
and its output curcuit.
a) CE configuration b) Collector circuit
+
-
VCE
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14. • Apply KVL to collector circuit to write,
VCC – VCE – IC RC = 0 -----(1)
• Rearranging this equation we get,
IC = [-1 / RC] VCE + VCC/RC ------(2)
• Compare this equation with the general equation of a
straight line,
y = mx + C ------(3)
• From eq. (2) and (3), we get
y = IC x = VCE
m = -1/ RC C = VCC / RC
• This shows that eq. (2) represents a straight line. This
straight line is called as the dc load line.
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16. • Quiescent point (Q point) or bias point or operating
point:
 It is the point on the load line which represents the dc
current through a transistor (ICQ) and the voltage across
it (VCEQ), when no ac signal is applied.
 The dc load line is a set of infinite number of such
operating points.
 If the transistor is being used for “amplification”
purpose, then Q point should be exactly at the center
of load line.
 The factors affetcing the stability of Q points are:
1. Changes in temperature 2. changes in the value of
βdc
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17. Biasing circuits
• Biasing circuits required to stabilize the position
of the Q point or bias point.
• Types of biasing circuits:
1. Fixed bias circuit
2. Base bias with emitter feedback
3. Base bias with collector feedback
4. Voltage divider biasing
5. Emitter bias
• Out of these, voltage divider biasing circuit is
most popularly used.
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18. Fixed bias circuit
• Fixed bias circuit is simplest bias circuit.
• In this circuit, single power supply is used to
supply power collector as well as base.
+
-
VCE
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19. • As we know, for CE configuration,
IC = βdc IB + ICEO
• Therefore, as temperature increases, ICEO
increases, so IC will increase.
• The fixed bias circuit cannot automatically
keep IC constant and stabilize the Q point.
• Thus no stabilization is provided by the fixed
bias circuit.
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20. Collector to Base Bias Circuit
(Base Bias with Collector Feedback)
• Collector to base bias circuit is an
improvement over fixed bias circuit.
• In this circuit base resistance Rb is connected
to collector and not to sypply.
• As we know, for CE configuration,
IC = βdc IB + ICEO
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21. • Stabilization of Q point by collector to base
bias circuit:
βdc and ICEO increases
Therefore IC increases
Drop across RC i.e. ICRC increases
VCE decreases as VCE = VCC – (IC + IB) RC
IB decreases as IB = (VCE – VBE) / RB
This reduces IC because IC = βdc IB,
this compensateing for the initial increas in IC.
Temperature increases
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23. Voltage Divider Bias or Self Bias or
Potential Divider Bias
• The resistor R1 and R2 form a potential divider
to apply a fixed voltage VB to the base.
• The resistor RE is connected to the emitter.
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24. • Stabilization of Q point by voltage divider bias
circuit:
Then IE increases
Hence drop across RE increases (VE = IE RE)
But VB is constant. Hence VBE decreases.
Hence IB decreases.
Hence IC also decreases. Thus compensation
for increase in IC is achieved.
If IC increases due to change in temperature or βdc
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25. Thermal Runaway
• The maximum power that a transistor can
dissipate without getting damaged, depends
largely on the maximum temperature that
collector- base junction can withstand.
• The rise in collector- base junction takes place
due to two reasons:
1. Due to increase in the ambient temperature
2. Due to the internal heating
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26. • Out of them the internal heating process is cumulative
as explained below:
1. An increase in collector current IC increases the
power dissipated in the collector-base junction of the
transistor.
2. This will increase the temperature of C-B junction.
3. As the transistor has a negative temperature
coefficient of resistivity., increased junction
temperature reduces the resistance.
4. The reduced resistance will increase the collector
current further.
5. This becomes a cumulative process which will finally
damage the transistor due to excessive internal
heating. This process is known as “Thermal Runaway ”
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27. Ms. A. A. Lande E & TC Dept

28. Heat Sink
• Heat sinks are large metal pieces of different
shapes.
• The power transistors are mounted on some form
of heat sink but there is no electrical contact
between the transistor and heat sink.
• When heat sink is used, due to large area of heat
sink the heat produced by the transistor is
radiated into the air more quickly and easily.
• Due to efficient heat radiation by heat sink, the
case temperature of the transistor is held to a
much lower value.
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29. • The heat sinks are painted black because black
coloured objects can radiate more heat as
compared to the objects of other colours.
• Heat sinks are made from aluminium because
aluminium is a very good conductor of heat.
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30. BJT Circuits
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31. Amplification and Amplifier
• Amplification:
Amplification is a process of adding strength
to the input signal or it is a process of
“magnifying” the input signal without changing
its shape.
• Amplifier:
The circuit which amplifies a small input signal is
called as an “amplifier”.
An amplifier is required to amplify weak signals
and it is used in radio, TV, telephones, mobile
phones, music system etc.
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32. Amplifier
Amplifier
(Voltage gain AV)
RS
RL
Ri Ro
VS
+Vdc
IO
Ii
Vi Vo
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33. • In order to magnify the input signal VS all the
amplifier need a source of energy which is
provided by battery or DC supply.
• The dc supply is also essential for biasing the BJT
used in amplifier circuits.
• The amplifier should contain atleast one active
device such as transistor or FET or OPAMP.
• If transistor is used then it should be in the active
region.
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34. Amplifier characteristics
• 1. voltage gain AV and current gain AI :
The gain of an amplifier is defined as the ratio of
output quantity to the input quantity.
∴ AV = Vo/ Vi
And AI = Io/ Ii
The gain of amplifier should be as large as possible.
• Input resistance (Ri):
It is the resistance seen looking into the input
terminals of an amplifier.
Ideally Ri should be infinite.
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35. • Output resistance (Ro):
It is the resistance seen looking into the
output terminals of an amplifier when the
input signal Vi = 0 and output circuit is open
circuited.
Ro should be equal to zero ideally.
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36. Single Stage Amplifier
• Depending on which terminal of transistor is
made common between input and output, the
amplifiers are classified into three types as
follows:
1. Common Emitter (CE) amplifier
2. Common Collector (CC) amplifier or emitter
follower.
3. Common Base (CB) amplifier
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37. Single stage RC coupled CE Amplifier
C1
CE
C2
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38. • Fig. shows the a single stage RC coupled CE amplifier.
• Circuit Components and their Functions:
1. Resistors:
 Resistors R1, R2 and RE are used to bias the transistor
in active region by using voltage divider bias circuit.
 RC is collector resistor used to control collector
current.
2. Input coupling capacitor C1:
 The input coupling capacitor C1 is used to couple the
ac input voltage VS to the base of the transistor.
 As capacitor block dc, C1 couples only the ac
component of the input signal.
 This capacitor also ensures that the dc biasing
conditions of transistor remain unchanged even after
applications of the input signal.
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39. 3. Bypass capacitor CE:
 As CE is connected in parallel with RE is called emitter
bypass capacitor CE.
 This capacitor offer a low reactance to the amplified ac
signal, therefore RE gets bypassed through CE for only the ac
signals.
 This will increase the voltage gain of the amplifier.
4. Output coupling capacitor C2:
 This capacitor couples the amplifier output to the load or to
the next stage amplifier.
 It is used for blocking the dc part and passing only the ac
part of the amplified signal to the load.
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40. • Operation of the RC coupled amplifier:
1. In the absence of ac input signal current IB = IBQ, IC =
ICQ and voltage VCE = VCEQ. The Q point is selected to
be in the active region of transistor.
2. As ac input signal VS is applied, the base current varies
above and below IBQ.
3. Hence output current IC varies above and below ICQ,
because IC = βIB and this variation will be large.
4. As the IC varies, voltage across RC will also varies,
because VRC = IC x RC.
5. Hence collector voltage VC varies above and below
VCEQ as VC = VCC – ICRC.
6. Through C2 only the ac part is coupled to the load.
Hence Vo is of same shape as VS but of large size.
7. Thus amplification has taken place.
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41. Ms. A. A. Lande E & TC Dept

42. Common Collector or Emitter Follower
Amplifier Circuit
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43. • In CC amplifier, input signal is applied at base and
output is obtained at emitter.
• Why is CC amplifier called as emitter follower?
The voltage gain of CC amplifier is almost equal to
1 . Therefore input and output voltages are equal
and in phase with each other.
Hence it is said that output (emitter) follows the
input voltage. Hence the name is emitter
follower.
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44. Common Base Amplifier
In CB amplifier, input signal is applied at emitter and amplified
output is taken at the collector with respect to ground.
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45. Frequency Response and Bandwidth
• The frequency response is graph of amplifier
output voltage (or gain) versus the frequency
of input signal.
• Ideally frequency response should be flat over
the entire frequency range.
• Practically the frequency response of an
amplifier is not flat over the entire operating
frequency region.
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46. • The practical frequency response can be divided into three
regions as follows:
1. Low frequency region.
2. Mid frequency region.
3. High frequency region.
1. Low frequency region:
In low frequency region, the gain or output voltage
decreases due to the increased reactance of the coupling
and bypass capacitor.
2. Mid frequency region:
In this region, gain and output voltage remain constant.
3. High frequency region:
In this region, the output voltage and gain will
decrease due to the transistor internal capacitances and
stray capacitance.
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47. • Bandwidth:
 Bandwidth is the band of frequencies in which the
magnitude of output voltage or gain is either equal or
relatively close to their mid frequency band value.
 The frequencies fL and fH are called cutoff frequencies or
half power frequencies.
 Bandwidth of the amplifier is defined as the difference
between the half power frequencies.
• Lower cutoff frequency (f1 or fL):
It is the frequency of the input signal at which the
amplifier gain or output voltage reduce to 70.7% of their
mid frequency range value. f1 is always less than f2.
• Upper cutoff frequency (f2 or fH):
It is the frequency of the input signal at which the
amplifier output voltage reduce to 70.7% of their mid
frequency range value. f2 is always higher than f1.
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48. Ms. A. A. Lande E & TC Dept

49. Multistage Transistor Amplifier
• The multistage amplifier is obtained by cascading
a number of amplifiers i.e. connecting a number
of amplifier stages to each other with the output
of the previous stage to the input of next stage.
• The most important parameters of an amplifier
are its input impedance, voltage gain, bandwidth
and output resistance which are dependent on
the particular applications.
• In general, a single stage amplifier is not capable
to fulfill all these requirements. Hence we have to
use a multistage amplifier.
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50. Ms. A. A. Lande E & TC Dept

51. Overall Gain of the Multistage
Amplifier
• Overall voltage gain:
Let AV1, AV2, AV3….AVn be the voltage gain of n number
stages of multistage amplifier. Then total voltage gain AV of
multistage amplifier is given by
AV = AV1 x AV2 x AV3 x ……. x Avn
• Overall current gain:
Similarly, overall current gain AI of multistage amplifier
having n number of stages is given by
AI = AI1 x AI2 x AI3 x …….. x AIn
• Overall input resistance (Ri):
the overall input resistance of a cascaded amplifier is
equal to the input resistance of the first stage.
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52. • Overall output resistance (Ro):
The overall output resistance of a cascaded
amplifier is equal to the output resistance of the last
stage.
• Gain in decibels:
The gain expressed as a ratio of output voltage and
input voltage is called as the linear gain.
On the logarithmic scale the gain is expressed in
decibels as follows:
1. Power gain in dB = 10 log10 [Po/ Pi]
2. Voltage gain in dB = 20 log10 [Vo/ Vi]
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53. Methods of Coupling Multistage
Amplifier
• In the multistage amplifier, the output signal
of preceding stage is to be connected to the
input of the next stage. This is called as
interstage coupling.
• To achieve interstage coupling, there are three
coupling techniques:
1. R-C coupling
2. Transformer coupling
3. Direct coupling
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54. R-C coupled Amplifier
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55. Ms. A. A. Lande E & TC Dept

56. • Applications:
1. In public address (P.A.) amplifier system
2. Tape recorders
3. TV, VCR and CD players
4. Stereo amplifier
• RC coupled amplifier are basically voltage
amplifier.
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57. Transformer Coupled Amplifier
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58. Peaking due to
resonance
•Applications:
1. For impedance matching
2. For amplification of radio frequency (RF) signal.
3. In power amplifier
4. For transferring power to a low impedance load such as a loud
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59. Direct Coupled Amplifier
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60. •Applications:
1.In the operational amplifiers (OP-AMPS).
2.In the analog computations.
3.In the linear power supplies (voltage regulators).
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61. Transistor as a Switch
• For switching applications, transistor is biased to
operate in the saturation (fully on) or cutoff (fully
off) regions.
1. Transistor in cutoff regions [open switch]:
 In cutoff region, both junctions are reverse
biased and very small reverse current flows
through transistor.
 Voltage drop across transistor (VCE)is high. Thus
transistor is equivalent to an open switch.
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62. IC = 0
VCE= VCC
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63. 2. Transistor in the saturation region [closed
switch]:
In saturation region, both junctions are
forward biased. The voltage drop across the
transistor is very small and collector current is
very large.
Thus in this region, transistor equivalent to a
closed switch.
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64. IC
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