BJT Basics,BJT Amplifier Configurations and BJT Input_Output
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Transistor can be grouped into
two families: 1. Bipolar Junction Transistor (BJT)
2. Unipolar Junction Transistor (UJT)
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The First BJT
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Transistor Size (3/8L X 5/32W X 7/32H) No Date Codes. No Packaging.
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Modern Transistors
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Example of BJT Specification Sheet
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Fig. 3.32 Type Q2T2905 Texas Instruments quad pnp silicon transistor: (a) appearance; (b) pin
connections. (Courtesy Texas Instruments Incorporated.)
5 Robert L. Boylestad Electronic Devices and Circuit Theory, 9e
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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6
Transistor Terminal Identification
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Definition
A three terminal device in which current flowing between two terminals can be
controlled by a signal on the third terminal.
There are two types of transistors: pnp npn
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Symbol & Construction
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Transistor Operation
Forward bias, base to emitter
narrows the BE
depletion region.
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Reverse bias, base to collector
widen the BC
depletion region.
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Transistor Operation
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Forward and Reverse bias of a BJT.
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Transistor currents.
12 Thomas L. Floyd Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
Copyright 2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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Currents in a Transistor
By Kirchhoffs current law; ( 4.1)
The current gain of the transistor;
( 4.2)
The ratio of the collector current to the emitter current;
( 4.3)
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BCE III
B
Cdc
I
I
E
Cdc
I
I
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Currents in a Transistor
Relationship of and
(4.4)
Dividing the Eq 4.1 by IC; Substitute Eq.4.2 and Eq.4.3 into Eq.4.4
Rearrange the above expression;
and
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BI
CI
EI
dc
dc
C
B
C
C
C
E
I
I
I
I
I
I
dcdc
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1
dc
dcdc
1dc
dcdc
1
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General Bias Modes
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Forward active mode
VBE > 0v ( BE at FB) ; VBC < 0v ( BC at RB)
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General Bias Modes
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Reverse active mode
VBE < 0v ( BE at RB ) ; VBC > 0v ( BC at FB )
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General Bias Modes
17 Thomas L. Floyd Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
Copyright 2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
Saturation mode
VBE > 0v ( BE at FB ) ; VBC > 0v ( BC at FB )
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General Bias Modes
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Cut off mode
VBE < 0v ( BE at RB ) ; VBC < 0v ( BC at RB )
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Operation Mode
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Operation Mode
Active: Most importance mode, e.g. for amplifier
operation.
The region where current curves are practically flat.
Saturation: Barrier potential of the junctions cancel each
other out causing a virtual short.
Ideal transistor behaves like a closed switch.
Cutoff: Current reduced to zero
Ideal transistor behaves like an open switch. 20
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Operation Mode
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Transistor currents and voltages.
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DC load line on a family of collector characteristic curves illustrating the
cutoff and saturation conditions.
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TRANSISTOR CONFIGURATION
CIRCUIT.
Common Emitter Configuration
Common Base Configuration
Common Collector Configuration
The way a transistor is connected with the specific
terminal become common terminal for both input and
output loops.
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Circuit Configuration
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Common Emitter Configuration
The Emitter is common to both input (base-emitter) and output (collector-emitter).
The input is on the Base and the output is on the Collector.
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Common-Emitter Configuration
It is called common-emitter configuration since :
- emitter is common or reference to both input and
output terminals.
- emitter is usually the terminal closest to or at
ground potential.
Almost amplifier design is using connection of CE due
to the high gain for current and voltage.
Two set of characteristics are necessary to describe
the behavior for CE ;input (base terminal) and output
(collector terminal) parameters.
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Common-Emitter Configuration
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Characteristics of Common-Emitter
Collector characteristics = output characteristics.
Base characteristics = input characteristics.
IB is microamperes compared to miliamperes of IC.
IB will flow when VBE > 0.7V
for silicon and 0.3V for germanium
Before this value IB is very small and no IB.
Base-emitter junction is forward bias
Increasing VCE will reduce IB
for different values.
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Characteristics of Common-Emitter
Collector characteristics = output
characteristics.
Base characteristics = input
characteristics.
VCE < VCESAT, IC increase linearly with increasing of VCE VCE > VCESAT, IC not totally depends on VCE constant IC
IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC
IB=0 A ICEO occur.
Noticing the value when IC=0A. There is still some value of current flows.
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Beta ()
In DC mode: [Formula 3.10]
In AC mode: [Formula 3.11]
indicates the amplification factor of a transistor. ( is sometimes referred to as hfe, a
term used in transistor modeling calculations)
B
C
I
Idc
constantVI
Iac CE
B
C
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Example
From output characteristics of common
emitter configuration, find ac and dc with an
Operating point at IB=25 A and VCE =7.5V.
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Solution:
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Determining ac and dc from the collector characteristics.
35 Robert L. Boylestad Electronic Devices and Circuit Theory, 9e
Copyright 2006 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.
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36
Common Base Configuration
The base is common to both input (emitter base) and output (collector base) of the
transistor.
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Common-Base Configuration
Common-base terminology is derived from the fact that
the :
- base is common to both input and output of the
configuration.
- base is usually the terminal closest to or at
ground potential.
All current directions will refer to conventional (hole)
flow and the arrows in all electronic symbols have a
direction defined by this convention.
Note that the applied biasing (voltage sources) are such
as to establish current in the direction indicated for each
branch. 37
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Common-Base Configuration
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Input Characteristics for a Common-Base Amplifier
This demonstrates the input current IE to input voltage VBE for various levels of output
voltage VCB.
Ic(mA)
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In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha
=
IC = IE + ICBO It can then be summarize to IC = IE (ignore ICBO due
to small value)
For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by
Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector.The value of is typical from 0.9 ~ 0.998.
E
C
I
I
E
C
I
I
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Alpha ()
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Biasing
Proper biasing CB configuration in active region by
approximation IC IE (IB 0 uA)
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Transistor as an amplifier
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Simulation of transistor as an amplifier
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Common Collector Configuration
The input on the Base and the output is on the Emitter.
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Common Collector Configuration Also called emitter-follower (EF).
It is called common-emitter configuration since both the
signal source and the load share the collector terminal
as a common connection point.
The output voltage is obtained at emitter terminal.
The input characteristic of common-collector
configuration is similar with common-emitter.
configuration.
Common-collector circuit configuration is provided with
the load resistor connected from emitter to ground.
It is used primarily for impedance-matching purpose
since it has high input impedance and low output
impedance.
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Common Collector Configuration
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Limits of Operation
Many BJT transistor used as an amplifier. Thus it is
important to notice the limits of operations.
At least 3 maximum values is mentioned in data sheet.
There are:
a) Maximum power dissipation at collector: PCmax
or PD
b) Maximum collector-emitter voltage: VCEmax
sometimes named as VBR(CEO) or VCEO.
c) Maximum collector current: ICmax
There are few rules that need to be followed for BJT
transistor used as an amplifier. The rules are:
i) transistor need to be operate in active region!
ii) IC < ICmax
ii) PC < PCmax 48
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Limitations of Operation for Each Configuration
Note: VCE is at maximum and IC is at minimum (ICmax=ICEO) in the cutoff region.
IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the
saturation region.
The transistor operates in the active region between saturation and cutoff.
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Power of Dissipation
Common Base: [Formula 3.18]
Common Emitter: [Formula 3.16]
Common Collector: a
CCBC IVmaxP
CCEC IVmaxP
ECEC IVmaxP
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Refer to the fig. Step1: The maximum collector power dissipation, PD=ICmax x VCEmax (1) = 18m x 20 = 360 mW Step 2: At any point on the characteristics the product of and must be equal to 360 mW. Ex. 1. If choose ICmax= 5 mA, subtitute into the (1), we get VCEmaxICmax= 360 mW VCEmax(5 m)=360/5=7.2 V Ex.2. If choose VCEmax=18 V, subtitute into (1), we get VCEmaxICmax= 360 mW (10) ICmax=360m/18=20 mA
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Derating PDmax
PDmax is usually specified at 25C.
The higher temperature goes, the less is PDmax
Example;
A derating factor of 2mW/C indicates the power
dissipation is reduced 2mW each degree centigrade
increase of temperature.
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Example
Transistor 2N3904 used in the circuit with
VCE=20 V. This circuit used at temperature
1250C. Calculate the new maximum IC.
Transistor 2N3904 have maximum power
dissipation is 625 mW. Derating factor is
5mW/0C.
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Solution
Step 1:
Temperature increase : 1250C 250C = 1000C
Step 2:
Derate transistor : 5 mW/0C x 1000C = 500 mW
Step 3:
Maximum power dissipation at 1250C = 625 mW500
mW=125 mW.
Step 4:
Thus ICmax = PCmax / VCE=125m/20 = 6.25 mA.
Step 5:
Draw the new line of power dissipation at 1250C .
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Example
The parameters of transistor 2N3055 as follows:
- Maximum power dissipation @ 250C=115 W
- Derate factor=0.66 mW/0C.
This transistor used at temperature 780C.
Find the new maximum value of power dissipation.
Find the set of new maximum of IC if VCE=10V,
20V and 40 V.
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Solution
Step 1:
Temperature increase : 780C 250C = 530C
Step 2:
Derate transistor : 0.66mW/0C x 530C = 35 mW
Step 3:
Maximum power dissipation at 780C = 115W 35W=80
mW.
Step 4:
ICmax = PCmax / VCE=80m/10 = 8 mA (point C)
ICmax = PCmax / VCE=80m/20 = 4 mA. (point B)
ICmax = PCmax / VCE=80m/40 = 2 mA (point A)
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Step 5:
Draw the new line of power dissipation at 780C .
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Transistor Testing
1. Curve Tracer
Provides a graph of the characteristic curves.
2. DMM
Some DMMs will measure DC or HFE.
3. Ohmmeter
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Partial transistor data sheet.
59 Thomas L. Floyd Electronic Devices, 6e and Electronic
Devices: Electron Flow Version, 4e
Copyright 2002 by Pearson Education, Inc.
Upper Saddle River, New Jersey 07458
All rights reserved.