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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.