Bipolar Junction Transistors (BJT)
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• The Two Types of BJT TransistorsThe Two Types of BJT Transistors
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BJT Relationships - EquationsBJT Relationships - Equations
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= Common-emitter current gain= Common-emitter current gain = Common-base current gain= Common-base current gain = IC= IC = IC = IC IBIB IE IE
The relationships between the two parameters are:The relationships between the two parameters are: = = = = + 1+ 1 1 - 1 -
Note: Note: and and are sometimes referred to as are sometimes referred to as dc and dc and dc dc because the relationships being dealt with in because the relationships being dealt with in the BJT the BJT are DC.are DC.
DC DC and DC and DC
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Modes of OperationModes of Operation
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Three Types of BJT BiasingThree Types of BJT Biasing
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Electrostatic variables for a pnp BJT at equilibrium
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Circuit Diagram: NPN TransistorCircuit Diagram: NPN Transistor
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Although the Common-Base configuration is not the most Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of common biasing type, it is often helpful in the understanding of
how the BJT works. how the BJT works.
Emitter-Current CurvesEmitter-Current Curves
Active Active RegionRegion
IIEE = 0 = 0
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Eber-Moll BJT ModelEber-Moll BJT Model
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• Common-emitter transistor ampliers are so-called because the input and output voltage points share the emitter lead of the transistor in common with each other, not considering any power supplies.
• ² Transistors are essentially DC devices: they cannot directly handle voltages or currents that reverse direction. In order to make them work for amplifying AC signals, the input signal must be offset with a DC voltage to keep the transistor in its active mode throughout the entire cycle of the wave. This is called biasing.
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• Common-collector transistor ampliers are so-called because the input and output voltage points share the collector lead of the transistor in common with each other, not considering any power supplies.
• ² The output voltage on a common-collector amplier will be in phase with the input voltage, making the common-collector a non-inverting amplier circuit.
• ² The current gain of a common-collector amplier is equal to Ø plus 1. The voltage gain is approximately equal to 1 (in practice, just a little bit less).
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• Common-base transistor ampliers are so-called because the input and output voltage points share the base lead of the transistor in common with each other, not considering any power supplies.
• ² The current gain of a common-base amplier is always less than 1. The voltage gain is a function of input and output resistances, and also the internal resistance of the emitterbase junction, which is subject to change with variations in DC bias voltage.
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• Common-Base Configuration
Common-base BJT transistorre modelre equivalent cct.
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Common-base BJT transistor - pnp
ec I αI
re model for the pnp common-baseconfiguration
ec I αI
common-base re equivalent cct
Therefore, the input impedance, Zi = rethat less than 50Ω.For the output impedance, it will be as follows;
Determining Zo for common-base
26mVr I is the DC level of
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The hybrid parameters: hie, hre, hfe, hoe are developed and used to model the transistor. These parameters can be found in a specification sheet for a transistor.
Hybrid Equivalent Model
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Determination of parameter
, 0VV Solving
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General h-Parameters for any Transistor Configuration
hi = input resistancehr = reverse transfer voltage ratio (Vi/Vo)hf = forward transfer current ratio (Io/Ii)ho = output conductance
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Common emitter hybrid equivalent circuit
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Common base hybrid equivalent circuit
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Transistor as a Switch
CE CB BE CB CE BEV V V or V V V
, 0,CC CE CC BECM CB CM
V V V VI for V I
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Power Transistor Structure
n + 1019 cm - 3
p 1 0 16 cm - 3
n – 1014 cm - 3
n + 1019 cm - 3
50- 200 m
5- 20 m
(C ollector driftregion)
Base Th ickness
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Switching Characteristics of BJT
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