Post on 19-Dec-2015
Bipolar JunctionBipolar Junction Transistors Transistors
EE314
Chapter 13: Bipolar Junction Transistors
1.History of BJT2.First BJT3.Basic symbols and features 4.A little bit of physics…5.Currents in BJT’6.Basic configurations7.Characteristics
Current Flow in BJT
pnp BJT
1. Injected h+ current from E to B2. e- injected across the forward-biased EB junction (current from
B to E)3. e- supplied by the B contact for recombination with h+
(recombination current)4. h+ reaching the reverse-biased C junction5,6.Thermally generated e- & h+ making up the reverse saturation
current of the C junction
iE
-iB
-iC
-VCE
Now, you can try…
npn BJT
BJTs – Basic configurations
npn BJTs – Operation Modes
Forward & reverse polarized pn junctions
Different operation modes:
npn BJTs – Operation Modes
•When there is no IB current almost no IC flows•When IB current flows, IC can flow•The device is then a current controlled current device
Operational modes can be defined based on VBE and VBC
BJT-Basic operation
npn BJTpnp BJT
(n+), (p+) – heavy doped regions; Doping in E>B>C
Operation mode: vBE is forward & vBC is reverse
The Shockley equation
1exp
T
BEESE V
vIi
IES–saturation I (10-12-10-16A); VT=kT/q -thermal V (26meV) D – diffusion coefficient [cm2/s] – carrier mobility [cm2/Vs]The Kirchhoff’s laws
BCE iii
EiIt is true regardless of the bias conditions of the junction
Useful parameter
B
C
i
i
the common-emitter current gainfor ideal BJT is infinite
0 CEBCBE VVV
BJTs – Current & Voltage Relationships
q
kTD
Einstein relation
1exp
T
BEESC V
vIi
E
C
i
iUseful
parameterthe common-base current gainfor typical BJT is ~0.99
The Shockley equationonce more
If we define the scale current
ESS II
T
BESC V
vIi
A little bit of math… search for iB
EB ii 1
1exp1
T
BEESB V
vIi
1B
C
i
i
BC ii Finally…
BJTs – Current & Voltage Relationships
BJTs – Characteristics
SchematicCommon-Emitter
Input
Output
VBC<0 or equivalently VCE>VBE
If VCE<VBE the B-C junction is forward bias and IC decreasesRemember VBE has to be greater than 0.6-07 V
BC ii
Example 13.1
BJTs – Load line analysisCommon-Emitter Amplifier
)()()( tvtiRtvV BEBBinBB Input loop
if iB=0 inBBBE vVv if vBE=0 BinBBE RvVi /)(
smaller vin(t)
BJTs – Load line analysis
CECCCC viRV Output loop
Common-Emitter Amplifier
Example 13.2
Circuit with BJTs
Our approach: Operating point - dc operating pointAnalysis of the signals - the signals to be amplified
Circuit is divided into: model for large-signal dc analysis of BJT circuitbias circuits for BJT amplifiersmall-signal models used to analyze circuits for signals being amplified
Remember !