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ECE 342 – Jose Schutt-Aine 1
ECE 342Solid-State Devices & Circuits
6. Bipolar Transistors
Jose E. Schutt-AineElectrical & Computer Engineering
University of [email protected]
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ECE 342 – Jose Schutt-Aine 2
• Bipolar Junction Transistor (BJT)– First Introduced in 1948 (Bell labs)– Consists of 2 pn junctions– Has three terminals: emitter, base, collector
Bipolar Junction Transistor
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ECE 342 – Jose Schutt-Aine 3
BJT – Modes of Operation
Mode EBJ CBJ
Cutoff Reverse Reverse
Forw. Active Forward Reverse
Rev. Active Reverse Forward
Saturation Forward Forward
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ECE 342 – Jose Schutt-Aine 4
BJT in Forward Active Mode (NPN)
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ECE 342 – Jose Schutt-Aine 5
Electrons are minority carriers in the base (p-type)/(0) BE TV V
p pon n e
( ) (0)p pn E n E n
dn x nI A qD A qD
dx W
iC is independent of vCB
Collector current:
Minority electrons will diffuse in the p-type base
Longitudinal Current Flow
/BE Tv VC n Si I I e
2E n i
SA
A qD nI
N W
AE: cross section area of BEJW: Effective width of baseNA: doping concentration baseDn: electron diffusivityq: electron charge
5
ECE 342 – Jose Schutt-Aine 6
Base Current
/2
1
BE Tv VE p i
BD p
A qD n ei
N L
Dp: hole diffusivity in emitterLp: hole diffusion length in emitterND: doping concentration of emitter
• Base current: Two components– Hole injection into emitter iB1
– Electron recombination in base iB2
2n
Bb
Qi
Qn: minority carrier charge in basetb: minority carrier lifetime
2/1
(0)2 2
BE Tv VE in E p
A
A qWnQ A q n W e
N
Basepnp(0)
np(ideal)
effectivebase width
From area under triangle
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ECE 342 – Jose Schutt-Aine 7
• Base current has two functions
BJT Operation: Longitudinal and Base Currents
• Base current is small because
• Longitudinal current
Support reverse injection
Feed recombination that occur in the base
Has large lifetime
Base is thin
Emitter is much more heavily doped than base
Depends (exponentially) on emitter junction voltage Is independent of collector junction voltage
Field due to collector-base voltage attracts carriers but has no effect on rate of attraction
ECE 342 – Jose Schutt-Aine 8
BJT Operation: Current Gain
1 2B B Bi i i • Total Base current:
Define a current gain b such that
2/1
2BE Tp v VA
B Sn D p n b
D N W Wi I e
D N L D
C
B
i
i
2
1
1
2p A
n D p n b
D N W W
D N L D
Using previous relation for iC
b is the common-emitter current gain
In order to achieve a high gain b we need
Dn: largeLp: largeND: largeNA: smallW: small
Typically 50 < b < 200
In special transistors, b can be as high as 1000
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ECE 342 – Jose Schutt-Aine 9
Current Gain Temperature Dependence
ECE 342 – Jose Schutt-Aine 10
BJT Operation: Emitter Current
E C Bi i i • Emitter current:
Define a such that
/1 1BE Tv V
E C Si i I e
C Ei i
Using previous relation for iC
a is the common-base current gain
1
1
a 0.99
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ECE 342 – Jose Schutt-Aine 11
B
C
E
Structure of BJT’s
Collector surrounds emitter region electrons will be collected
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ECE 342 – Jose Schutt-Aine 12
Ebers-Moll Model
NPN Transistor
// 1 1BC TBE T v Vv V SC S
R
Ii I e e
// 1 1BC TBE T v Vv VSE S
F
Ii e I e
// 1 1BC TBE T v Vv VS SB
F R
I Ii e e
1F
FF
1
RR
R
Describes BJT operation in all of its possible modes
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ECE 342 – Jose Schutt-Aine 13
Common-Emitter Large-Signal Model
Common terminal is common to input and output
Common terminal is used as reference or ground
ECE 342 – Jose Schutt-Aine 14
BJT – Common-Emitter Characteristics
ECE 342 – Jose Schutt-Aine 15
BJT – Voltage-Current Characteristics
ECE 342 – Jose Schutt-Aine 16
Common Emitter Configuration
E B CI I I
C EI I
1C B BI I I
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ECE 342 – Jose Schutt-Aine 17
Common Emitter I-V Characteristics
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ECE 342 – Jose Schutt-Aine 18
Early Voltage
• Early Voltage VA– Dependence of collector current on collector voltage– Increasing VCE increases the width of the depletion region
ECE 342 – Jose Schutt-Aine 19
Output Resistance
/ 1BE Tv V CEC S
A
vi I e
V
1
tanBE
Co
CE V cons t
ir
V
/' BE TV VC SI I e
ro is output resistance seen from collector terminal
A CEo
C
V Vr
I
Alternatively, neglecting the Early effect on the collector current, we define
'A
oC
Vr
I
The output resistance then becomes
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ECE 342 – Jose Schutt-Aine 20
A transistor has = 100, vBE= 0.7V with IC = 1 mA. Design a circuit such that a current of 2 mA flows through the collector and a voltage of 5V appears at the collector.
20.7 ln 0.717 at 2 mA
1BE Tv V V
Problem
CBJ reversed biased FAR
Voltage drop across RC = 15-5 =10VIC = 2mA RC = 10V/2mA = 5kW
Since vBE=0.7V at IC = 1 mA
Since base is at 0V, emitter voltage is at –0.717 volts =VE
For b = 100, a = 100/101=0.99 IE = IC/a = 2/0.99 = 2.02 mA ( 15) 0.717 15
7.07 2.02
EE
E
VR k
I
WNow,
This order of accuracy is not necessary
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ECE 342 – Jose Schutt-Aine 21
Forward active region can be maintained for negative vCB down to about -0.4V
Operation in the Saturation ModeIV Characteristics Minority Carrier Profile
Beyond that point, the transistor enters the saturation mode and iC decreases with decreasing vCB
ECE 342 – Jose Schutt-Aine 22
// 1 1BC TBE T v Vv V SC S
R
Ii I e e
// 1 1BC TBE T v Vv VS SB
F R
I Ii e e
Operation in the Saturation Mode
If vBC increases, iC will decrease, as described by
The base current iB will decrease, as described by
The current gain will decrease to a value lower than bF described as:
Cforced F
B saturation
i
i
We will also have: CEsat BE BCV V V
ECE 342 – Jose Schutt-Aine 23
Operation in the Saturation Mode
Blue: Gradient that gives rise to diffusion currentGray: Minority carriers driving transistor deeper into saturation
ECE 342 – Jose Schutt-Aine 24
NPN in Saturation Mode
ECE 342 – Jose Schutt-Aine 25
Biasing Bipolar Transistors
ECE 342 – Jose Schutt-Aine 26
BJT Bias
1. Base Current Bias
CC BEBQ
B
V VI
R
0.6CCBQ
B
VI
R
CCBQ
B
VI
R
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ECE 342 – Jose Schutt-Aine 27
2. Emitter Bias
BJT Bias
Provides good stability with respect to changes in b with temperature
27
Thevenin Equivalent
ECE 342 – Jose Schutt-Aine 28
BJT Emitter Bias
th th B BE E EE R I V R I
1E B C BI I I I
( 1)th BE
B BQth E
E VI I
R R
2
1 2th CC
RE V
R R
1 21 2
1 2th
R RR R R
R R
28
Thevenin Equivalent
1th BE th B E BE V R I R I
ECE 342 – Jose Schutt-Aine 29
• Methods– First method is to find R1 & R2 from Eth and Rth and IBQ
– Second method is to select R2 to be 10 times to 20 times RE to provide good stability & then select R1 to give proper IBQ
Bipolar Biasing Approach
Remark: To keep collector voltage at the middle of the forward active region, use:
min max 12 2
C C CC ECQ
E C
V V V RV
R R
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ECE 342 – Jose Schutt-Aine 30
Stability Considerations
Objective: Minimize effect of variations in b. Circuit must be stable with respect to changes in .b
CQ CC CQ C CC BQ CV V I R V I R
– Need to examine quiescent point in variations for interchanged BJT’s
( 1)th BE
BQth E
E VI
R R
( 1)
th BE CCQ CC
th E
E V RV V
R R
1th BQ BQ E th BER I I R E V
30
ECE 342 – Jose Schutt-Aine 31
Stability Considerations
CCQ CC th BE
th
RV V E V
R
Changes in b lead to significant changes in VCQ
6060 0.983
61
(A) If Rth>> (b+1) RE
100100 0.99
101
(B) If (b+1) RE >> Rth
1
CCQ CC th BE
E
RV V E V
R
a varies only 1% to 2% for large b variations (B) is good choice.
CCQ CC th BE
E
RV V E V
R
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ECE 342 – Jose Schutt-Aine 32
The circuit shown below has RC = 8.2 kW , RE = 1 kW , R2=20 kW , VCC = 12 V, b = 100, VBE = 0.7V
- Select R1 to place VCQ at midpoint of the (forward) active region.- Find maximum symmetrical peak-to-peak output voltage that
can be obtained before saturation or cutoff occurs.
Bias Example
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ECE 342 – Jose Schutt-Aine 33
Bias Example - Solution
Minimum: min
12 121.3043
1 8.2 9.2E
CQ CCE C
RV V V
R R
maxCQ CCV VMaximum:
Midpoint: min max 13.30436.65
2 2CQ CQ
CQ
V VV V
12 6.650.652 CQ
C
I mAR
0.6520.00652 mA 6.52
100BQI A
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ECE 342 – Jose Schutt-Aine 34
Bias Example (con’t)
0.7 0.652 0.7 1.35 VBQ E EV R I
2
1.35 V0.0676 mA 67.6
20 kI A
W
1 2 67.6 6.82 74.1 BI I I A A A
11
1.35 12 1.35143.6
74.1 CCV
R kI A
W
max 12 6.65 5.35V V 1 143.6 R k W
max 5.35V V
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ECE 342 – Jose Schutt-Aine 35
PNP
NPN
BJT Transistor Polarities