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(ELEC101)tutorial13.pdf
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Tutorial 13 --
BJT Circuit
Fall 2006
ELEC101 BASIC ELECTRONICS
Outline
.BJT Circuit
CNPN BJT Basic
COperation Modes
.Cutoff Mode
.Saturation Mode
.Active Mode
CAC model of NPN BJT
CGraphical Method
BJT basic
NPN Bipolar Junction Transistor (BJT) is more commonly use due to fasterspeed.
IE= IB+ IC
VBE= VB-VE
ICFIE
ICFIB
Text Box: .F . 0.99 - 0.995.F . 50 - 200
Operation Modes
Cutoff Mode
In Cutoff ModeNPN BJT is like an OFF (open) switch.
B
C
E
IB= IC = IE = 0
VBE < VBE0.7V
Text Box: VB
Active Mode
In Active ModeNPN BJT is like an (current) Amplifier.
VCE> VCE(SAT) 0.2V
VBE = VBE0.7V
IC= FIE
IC= FIB
Text Box: Amplifier
Saturation Mode
VCE= VCE(SAT) 0.2V
IC= FORCED*IB
FORCED<
Text Box: In Saturation Mode NPN BJT is like an ON (closed) switch.
AC model of NPN BJT
r: input resistanceof BJT.
It is the input R of the BE diode.
Graphical Method
Load line : line described by VCC= VCE+ ICRC
Vout = 3.5V
Graphical Method
Q-point : value of ICand VCE
when Vinis a DC voltage (has no AC ).
IB(20k .)= 1.2V CVBE(ON)
IB= 25A
IC= IB
IC=1.25mA
Text Box: In this circuit, Q point is : IC = 1.25mA, VCE = 3.75V
Graphical Method
FindVoltage Gain.
Text Box: current gain
Text Box: voltage gain
DC and AC Analysis
I. DC Analysis
II. AC Analysis
Using Superposition
Text Box: DC analysis is actually to find the Q point of the circuit
Text Box: AC analysis is to find iC , iB and vCE Hence can find current gain iC /iB voltage gain vCE / vin etc. Can use graphical method or apply AC model of BJT.