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

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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

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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=

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Lecture 12-6


What happens as RC is decreased?

Will it remain in the active region?

Q1

RB100E3

+2VVIN

RC500

+ 5VVCC

IS=1e-16 100=

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Lecture 12-7


What happens as RC is increased?

Will it remain in the active region?

Q1

RB100E3

+2VVIN

RC5000

+ 5VVCC

IS=1e-16 100=

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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

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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

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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

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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

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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

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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*

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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

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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

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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

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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

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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.

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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

Documents

Lect 12 Feb 24