LARGE-SIGNAL BEHAVIOR OF BJTS

1
LARGE-SIGNAL BEHAVIOR OF BJTS

• Large-signal models in the forward-active region
  
• Effects of collector voltage in the
  forward-active region
• Ohmic and inverse active regions
• Transistor breakdown voltages
• Dependence of transistor current gain on
  operation conditions

(saturation)
2
Bipolar Junction Transistor (BJT)
One diode...
Why not another diode
3
Bipolar Junction Transistor (BJT)
What happens when we combine them?
4
NPN Bipolar Junction Transistor
Emitter (E)
Collector (C)
Base (B)
BJT Bipolar Junction Transistor
5
Carrier Transport in a BJT
6
BJT Band Diagram
Forward-Active Region of Operation
Base
Collector
Emitter
Base Width (W)
Ef
7
BJT Current Equations
Base Width (W)
No recombination
Electrons and Holes recombine in the base region
at a rate t.
Recombination
Typically, b is limited by this recombination
(looks like a saturated subthreshold MOSFET)
8
Forward-Active Region of a BJT
Carrier Concentration
E-B Depletion Region
C-B Depletion Region
0
W
Emitter
Base
Collector
VBE gt 0 (Forward Biased), VBC lt 0 (Reversed
Biased)
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Forward-Active Region of a BJT
Carrier Concentration
E-B Depletion Region
C-B Depletion Region
0
W
Emitter
Base
Collector
W ltlt Ln
10
Gummel Plots
-5
Ic n1, Is 5.52fA
10
-6
10
Currents
-7
10
-8
10
-9
10
Ib n1.019, Is 0.048fA
-10
10
-11
10
-12
10
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Base-Emitter Voltage (V)
11
Gummel Plot b depends on Ic
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Current gain changes
Region I Low current region where bF
decreases as iC decreases
Region II Midcurrent region where bF is
approximately constant
Region III High current region where bF
decreases as iC increases
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Large-Signal Modeling (sort of)
Does not incorporate the Early effect
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Reverse-Active Region of a BJT
Carrier Concentration
E-B Depletion Region
C-B Depletion Region
0
W
Emitter
Base
Collector
VBC gt 0 (Forward Biased), VBE lt 0 (Reversed
Biased)
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Ohmic Region of a BJT
Carrier Concentration
E-B Depletion Region
C-B Depletion Region
0
W
Emitter
Base
Collector
VBC gt 0 (Forward Biased), VBE gt 0 (Forward
Biased)
This region is often called saturation
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Ebers-Moll Model of a BJT
Carrier Concentration
E-B Depletion Region
C-B Depletion Region
0
W
Emitter
Base
Collector
VBE/UT
VBC/UT
IC IC0 ( e - 1 ) - IE0 ( e -
1 ) IC0 e ( ( e - (IE0
/ IC0) e ) IC0 - IE0
VB/UT
-VE/UT
-Vc/UT
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Ohmic and Active BJT Regions
5
4.5
4
VBE/UT
IC (IC0 - IE0 ) IC0 e (
1 - (IE0 / aR IC0) e )
3.5
-VCE/UT
3
2.5
Collector current (mA)
2
1.5
1
0.5
0
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
Collector voltage (V)
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Simplified Ohmic Model
In ohmic, both junctions are forward biased
and the impedance levels looking into
the emitter or collector is very low.

VBE(on) 0.6 to 0.7V and VCE(sat) 0.2V
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TRANSISTOR BREAKDOWN VOLTAGES
Common-Base Transistor Breakdown Characteristics
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BJT Breakdown Characteristics