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8/9/2019 Lect 12 Feb 24
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8/9/2019 Lect 12 Feb 24
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8/9/2019 Lect 12 Feb 24
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Lecture 12-3
BE Diode Characteristic
We can effectively use a simplified model for the diode if we know theapproximate operating range of the BE diode characteristic
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0
1
2
3
mA
IE
Q1Default
+ 0VVBE
0.000 pA
8/9/2019 Lect 12 Feb 24
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Lecture 12-4
BE Diode Characteristic
Note that VON changes if were analyzing an order of magnitude less
current
So how do we know what the real VON is?
Q1Default
+ 0VVBE
0.000 pA
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0.0
0.1
0.2
0.3
mA
IE
8/9/2019 Lect 12 Feb 24
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Lecture 12-5
Simplified BJT Circuit Analysis
Assuming VBE is 0.78 volts, we can approximate this circuit solution by hand
analysis
Q1
RB100E3
+2VVIN
RC
1E3
+ 5VVCC
IS=1e-16
100=
8/9/2019 Lect 12 Feb 24
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Lecture 12-6
Simplified BJT Circuit Analysis
What happens as RC is decreased?
Will it remain in the active region?
Q1
RB100E3
+2VVIN
RC500
+ 5VVCC
IS=1e-16 100=
8/9/2019 Lect 12 Feb 24
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Lecture 12-7
Simplified BJT Circuit Analysis
What happens as RC is increased?
Will it remain in the active region?
Q1
RB100E3
+2VVIN
RC5000
+ 5VVCC
IS=1e-16 100=
8/9/2019 Lect 12 Feb 24
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Lecture 12-8
Saturation
When both the EBJ and CBJ are forward biased, the transistor is no longer inthe active region, but it is in the saturation region of operation
We can easily solve for the maximum iC that we can have before we reach
saturationfor this circuit
Q1
RB
+VIN
RC
+ 5VVCC
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Lecture 12-9
Saturation
With both diodes forward biased, the collector-to-emitter voltage, vCE,saturates toward a constant value
_
VBC
+
_
VBE
+
~0.4 volts
~0.7-0.9 volts
VBC
VBE
_
+
_
+ _
+
vCEsat
0.3volts
0 1 2 3 4 5
-1
0
1
2
mA
IC
VCE
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Saturation
In saturation, increasing iC shows little increase in iB. Why?
0.0 0.2 0.4 0.6 0.8 1.0
-1
0
1
2
mA
IC
VCE
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Lecture 12-
Regions of Operation
The complete i-v characteristic is:
0 1 2 3 4 5
-1
1
3
mA
VCE
IB=1A
IB=5A
IB=10A
IB=15A
IB=20A
IC
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Regions of Operation
VCE
IB=1A
IB=5A
IB=10A
IB=15A
IB=20A
0.0 0.2 0.4 0.6 0.8 1.0
1
3
mA saturation active
cut-off
IC
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Temperature Variations
The collector current vs. the base-emitter voltage follows a diodecharacteristic, which like a diode, is temperature dependent
0.6 0.7 0.8
0
2
4mA
VBE
T=32CT=27C
T=22C
Q1
Default
+ 0VVBE
R4
1E3
+ 15VVCC
Does this value of RC significantly impact the values for iC in this example?
IC
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Temperature Variations
In saturation, the collector current no longer increases with increasing VBE.
Why not?
VBE
T=32CT=27C
T=22C
Q1
Default
+ 0VVBE
R4
100E3
+ 15VVCC
0.6 0.7 0.8
0.0
0.1
0.2
mA
IC
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Base Width Variation
n-typep-typen-typeCE
BVBE VCB
x
In the active region, iC does vary somewhat with VCB (hence RC in our
previous examples) due to the variation it causes in the base width.
Effective base width, W*, decreases with increasing VCB
What do you expect would happen to iC as W* decreases?
W*
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Early Voltage
The IC vs. VCE curves in the active region have a finite slope to them due tothis iC dependence on VCB
Early showed that these slopes all converge to one negative voltage point
VCE
IB
=1A
IB=5A
IB=10A
IB=15A
IB=20A
-20 -10 0 10
-1
1
3
mA VAF=20 in SPICE(VA in the book)
-VA
IC
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Early Voltage
The finite slope in the active region due to decreasing base width can beapproximated by
ic
Ise
vbe
VT
1
vce
VA
-------+
=
This means that the output resistance between the collector and emitter is notinfinite --- very important for analog design
vce
iC
go
0=
0 1 2 3 4 5 6
-1
1
3
iC (mA)
at some fixedib point
vce
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Early Voltage
The output conductance, or resistance, at a fixed ib point represents the slopeof the line tangent to that point on the curve:
ic
Ise
vbe
VT
1
vce
VA
-------+
=
Generally not considered for dc bias point calculations, but ro can have a
significant impact on a transistor amplifier gain
8/9/2019 Lect 12 Feb 24
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Lecture 12-
Early Voltage
ie
ie
Is
----e
vbe
VT
=
E
B
C
ib
The equivalent circuit models can be modified accordingly:
ro
VAiC
-------=
ib
E
B
C
ib
Is
----e
vbe
VT
= ro
VAiC
-------=
or
8/9/2019 Lect 12 Feb 24
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Lecture 12-
dc Bias Point Calculations
ro is generally not considered for hand calculations of dc bias point -- why?
For hand calculations: use VBE=0.7 and assume that the transistor is in the
active region; Later verify that your assumptions were correct.
4V
10V
3.3k
RC
Whats the maximum value that RC can be without reaching saturation? Assume =100.
8/9/2019 Lect 12 Feb 24
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Lecture 12-
dc Bias Point Calculationsdc Bias Point Calculations
4V
10V
3.3k
RC
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Lecture 12-
What value of RC saturates the transistor?
10V
2k
RC
100=
-10V
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Lecture 12-
dc Bias Point Calculations
What value of VCC saturates the transistor for this same circuit?
10V
2k
1k
100=
VCC