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. Ii Io 2 1
Two port network
Output port
Input port 2 1
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5. Common base configuration Common emitter configuration
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:
Common base configuration
Common emitter configuration
Common collector configuration
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6. Common Base (CB) Configuration
a) NPN transistor b) PNP transistor
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7. The output current IC is given by IC = IC(INJ) + ICBO
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. For CB configuration, we can write IC = αdc IE + ICBO
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 3.
Output current IC 4.
Current gain
αdc = IC / IE
βdc = IC / IB
γdc = IE / IB 5.
Input voltage
VEB
VBE
VBC 6.
Output voltage
VCB
VCE 7.
Voltage gain
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 = (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 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:
Fixed bias circuit
Base bias with emitter feedback
Base bias with collector feedback
Voltage divider biasing
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:
Temperature increases
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.
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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:
If IC increases due to change in temperature or βdc
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.
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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. This will increase the temperature of C-B junction.
Out of them the internal heating process is cumulative as explained below:
An increase in collector current IC increases the power dissipated in the collector-base junction of the transistor.
This will increase the temperature of C-B junction.
As the transistor has a negative temperature coefficient of resistivity., increased junction temperature reduces the resistance.
The reduced resistance will increase the collector current further.
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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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 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 +Vdc RS IO Ii Amplifier (Voltage gain AV) RL VS Vi Vo Ri Ro
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33. If transistor is used then it should be in the active region.
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:
Common Emitter (CE) amplifier
Common Collector (CC) amplifier or emitter follower.
Common Base (CB) amplifier
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37. Single stage RC coupled CE Amplifier
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38. Fig. shows the a single stage RC coupled CE amplifier.
Circuit Components and their Functions:
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. This will increase the voltage gain of the amplifier.
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:
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.
As ac input signal VS is applied, the base current varies above and below IBQ.
Hence output current IC varies above and below ICQ, because IC = βIB and this variation will be large.
As the IC varies, voltage across RC will also varies, because VRC = IC x RC.
Hence collector voltage VC varies above and below VCEQ as VC = VCC – ICRC.
Through C2 only the ac part is coupled to the load. Hence Vo is of same shape as VS but of large size.
Thus amplification has taken place.
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42. Common Collector or Emitter Follower Amplifier Circuit
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43. Why is CC amplifier called as emitter follower?
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. In this region, gain and output voltage remain constant.
The practical frequency response can be divided into three regions as follows:
Low frequency region.
Mid frequency region.
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. Lower cutoff frequency (f1 or fL):
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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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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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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56. In public address (P.A.) amplifier system Tape recorders
Applications:
In public address (P.A.) amplifier system
Tape recorders
TV, VCR and CD players
Stereo amplifier
RC coupled amplifier are basically voltage amplifier.
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57. Transformer Coupled Amplifier
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58. For impedance matching
Peaking due to resonance
Applications:
For impedance matching
For amplification of radio frequency (RF) signal.
In power amplifier
For transferring power to a low impedance load such as a loud speaker
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59. Direct Coupled Amplifier
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60. In the operational amplifiers (OP-AMPS). In the analog computations.
Applications:
In the operational amplifiers (OP-AMPS).
In the analog computations.
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.
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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