Electronic Devices and Circuit Theory

heartlustElectronics - Devices

Nov 2, 2013 (3 years and 7 months ago)

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DC Biasing
-

BJTs

Chapter 4

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Electronic Devices and Circuit Theory

Electronic Devices and Circuit Theory


Boylestad

© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Biasing

Biasing:

Applying DC voltages to a transistor in order
to turn it on so that it can amplify AC signals.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Operating Point

The DC input
establishes an
operating or
quiescent point


called the
Q
-
point
.

Electronic Devices and Circuit Theory


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Ch.4 Summary

The Three Operating Regions

Active or Linear Region Operation



Base

Emitter junction is forward biased



Base

Collector junction is reverse biased


Cutoff Region Operation



Base

Emitter junction is reverse biased


Saturation Region Operation



Base

Emitter junction is forward biased



Base

Collector junction is forward biased

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

DC Biasing Circuits

Fixed
-
bias circuit

Emitter
-
stabilized bias circuit

Collector
-
emitter loop

Voltage divider bias circuit

DC bias with voltage feedback

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Fixed Bias

Electronic Devices and Circuit Theory


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Ch.4 Summary

The Base
-
Emitter Loop

From Kirchhoff’s voltage
law:

Solving for base current:

+
V
CC



I
B
R
B



V
BE

= 0

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Collector
-
Emitter Loop

Collector current:

From Kirchhoff’s voltage law:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Saturation

When the transistor is operating in
saturation
, current
through the transistor is at its
maximum
possible value.

Electronic Devices and Circuit Theory


Boylestad

© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Load Line Analysis

I
Csat


I
C

= V
CC

/ R
C


V
CE

= 0 V

V
CEcutoff


V
CE

= V
CC


I
C

= 0 mA


The
Q
-
point is the operating point where the value of
R
B

sets the
value of
I
B

that controls the values of
V
CE

and
I
C
.

The load line end points are:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

The Effect of
V
CC

on the Q
-
Point

Electronic Devices and Circuit Theory


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Ch.4 Summary

The Effect of
R
C

on the Q
-
Point

Electronic Devices and Circuit Theory


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Ch.4 Summary

The Effect of
I
B

on the Q
-
Point

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Emitter
-
Stabilized Bias Circuit

Adding a resistor
(
R
E
) to the emitter
circuit stabilizes
the bias circuit.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Base
-
Emitter Loop

From Kirchhoff’s voltage law:

Since
I
E

= (


+ 1)
I
B
:

Solving for I
B
:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Collector
-
Emitter Loop

From Kirchhoff’s voltage law:

Since
I
E



I
C
:

Also:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Improved Biased Stability

Stability

refers to a condition in which the currents and
voltages remain fairly constant over a wide range of
temperatures and transistor Beta (

) values.

Adding
R
E

to the emitter improves
the stability of a transistor.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Saturation Level

V
CEcutoff
:

I
Csat
:

The endpoints can be determined from the load line.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Voltage Divider Bias

The currents and
voltages are nearly
independent of
any
variations in

.

This is a very stable bias circuit.

Electronic Devices and Circuit Theory


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Ch.4 Summary

Approximate Analysis

Where
I
B

<<
I
1

and
I
1



I
2
:

Where

R
E

> 10
R
2
:

From Kirchhoff’s voltage law:

Electronic Devices and Circuit Theory


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Ch.4 Summary

Voltage Divider Bias Analysis

Transistor Saturation Level

Cutoff:



Saturation:

Load Line Analysis

Electronic Devices and Circuit Theory


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Ch.4 Summary

DC Bias With Voltage Feedback

Another way to improve
the stability of a bias
circuit is to add a
feedback path from
collector to base.


In this bias circuit the
Q
-
point is only slightly
dependent on the
transistor beta,

.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Base
-
Emitter Loop

From Kirchhoff’s voltage law:

Where
I
B

<<
I
C
:

Knowing
I
C

=

I
B

and
I
E



I
C
, the
loop equation becomes:

Solving for
I
B
:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Applying Kirchoff’s voltage law:


I
E

+ V
CE

+ I’
C
R
C



V
CC

= 0


Since
I

C



I
C

and
I
C

=

I
B
:


I
C
(R
C

+ R
E
) + V
CE



V
CC

=0


Solving for V
CE
:


V
CE

= V
CC



I
C
(R
C

+ R
E
)

Ch.4 Summary

Collector
-
Emitter Loop

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Base
-
Emitter Bias Analysis

Transistor Saturation Level

Cutoff



Saturation

Load Line Analysis

Electronic Devices and Circuit Theory


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Ch.4 Summary

Transistor Switching Networks

Transistors with only the DC source applied can be
used as electronic switches.

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

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Ch.4 Summary

Switching Circuit Calculations

Saturation current:

To ensure saturation:

Emitter
-
collector
resistance at
saturation and cutoff:

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

Switching Time

Transistor switching times:

Electronic Devices and Circuit Theory


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Ch.4 Summary

Troubleshooting Hints


Approximate voltages


Test for opens and shorts with an ohmmeter.

Test the solder joints.

Test the transistor with a transistor tester or a curve tracer.

Note that the load or the next stage affects the transistor
operation.

V
BE



.7 V for silicon transistors

V
CE



25% to 75% of V
CC

Electronic Devices and Circuit Theory


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© 2013 by Pearson Higher Education, Inc

Upper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

PNP Transistors

The analysis for
pnp

transistor biasing circuits is
the same as that for
npn

transistor circuits. The
only difference is that the currents are flowing in
the opposite direction.