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DC Biasing - BJTs

Chapter 4

Boylestad

Electronic Devices and Circuit Theory

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Electronic Devices and Circuit TheoryBoylestad

© 2013 by Pearson Higher Education, IncUpper Saddle River, New Jersey 07458 • All Rights Reserved

Ch.4 Summary

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

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Ch.4 Summary

Operating Point

The DC input

establishes anoperating orquiescent point called the Q-point .

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Ch.4 Summary

DC Biasing Circuits Fixed-bias circuit

Emitter-stabilized bias circuit

Voltage divider bias circuit

DC bias with voltage feedback

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Ch.4 Summary

Fixed Bias

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Ch.4 Summary

The Base-Emitter Loop From Kirchhoff’s voltagelaw:

Solving for base current:

+V CC – I BR B – V BE = 0

B

BE CC B

R

V V I

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Ch.4 Summary

Load Line Analysis

ICsat

I C = V CC / R CV CE = 0 V

VCEcutoff

V CE = V CC

I C = 0 mA

The Q-point is the operating point where the value of R B sets thevalue of I B that controls the values of V CE and I C .

The load line end points are:

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Ch.4 Summary

The Effect of V CC on the Q-Point

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Ch.4 Summary

The Effect of R C on the Q-Point

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Ch.4 Summary

The Effect of I B on the Q-Point

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Ch.4 Summary

Emitter-Stabilized Bias Circuit

Adding a resistor(R

E ) to the emitter

circuit stabilizesthe bias circuit.

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Ch.4 Summary

Base-Emitter Loop From Kirchhoff’s voltage law:

01 E B BE B BCC R )I ( β V R I V

0 R I V R I V E E BE B BCC

E B

BE CC B )R ( βR

V V I

1

Since I E = ( + 1) I B:

Solving for I B:

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Ch.4 Summary

Collector-Emitter Loop From Kirchhoff’s voltage law:

0V R I V R I CC E E CE C C

Since I E I C :

)R (R – I V V E C C CC CE

Also:

E BE B BCC B

C C CC E CE C

E E E

V V R – I V V

R I V V V V

R I V

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Ch.4 Summary

Improved Biased Stability Stability refers to a condition in which the currents andvoltages remain fairly constant over a wide range oftemperatures and transistor Beta ( ) values.

Adding R E to the emitter improvesthe stability of a transistor.

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Ch.4 Summary

Saturation Level

VCEcutoff : ICsat :The endpoints can be determined from the load line.

mA0 I

V V

C

CC CE

E RC

R

CC V

C I

V CE

V0

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Ch.4 Summary

Voltage Divider Bias

The currents andvoltages are nearlyindependent of anyvariations in .

This is a very s table b ias c i rcu i t .

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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:

2 1

CC 2 B

R R

V R V

E

E E R

V I

BE BE V V V

E E C C CC CE R I R I V V

)R (R I V V

I I

E C C CC CE

C E

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Ch.4 Summary

Voltage Divider Bias Analysis Transistor Saturation Level

E C

CC Cmax Csat

R R

V I I

Cutoff: Saturation:

mA0 I

V V

C

CC CE

V0 V CE

E R

C R

CC V

C I

Load Line Analysis

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Ch.4 Summary

DC Bias With Voltage Feedback Another way to improvethe stability of a biascircuit is to add afeedback path fromcollector to base.

In this bias circuit the

Q-point is only slightlydependent on thetransistor beta, .

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Ch.4 Summary

Base-Emitter Loop

)R β(R R V V

I E C B

BE CC B

From Kirchhoff’s voltage law:

0E E BE BBC C CC R –I –V R –I R I –V

Where I B << I C : C BC C I I I I'

Knowing I C = I B and I E I C , theloop equation becomes:

0 E B BE B BC BCC R βI V R I R βI V

Solving for I B:

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Applying Kirchoff’s voltage law:

I E R 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

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Ch.4 Summary

Base-Emitter Bias Analysis Transistor Saturation Level

E C

CC Cmax Csat

R R

V I I

Cutoff Saturation

mA0I

V V

C

CC CE

V 0V CE

E R

C R

CC V

C I

Load Line Analysis

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Ch.4 Summary

Switching Circuit Calculations

C

CC Csat

R

V I

dc

Csat B

β

I I

Csat

CEsat sat I

V R

CEO

CC cutoff I

V R

Saturation current:

To ensure saturation:

Emitter-collectorresistance atsaturation and cutoff:

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Ch.4 Summary

Switching Time

Transistor switching times:

d r on t t t

f soff t t t

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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 transistoroperation.

VBE .7 V for silicon transistorsVCE 25% to 75% of V CC

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Electronic Devices and Circuit Theory © 2013 by Pearson Higher Education, Inc

Ch.4 Summary

PNP Transistors

The analysis for pnp transistor biasing circuits is

the same as that for npn transistor circuits. Theonly difference is that the currents are flowing inthe opposite direction.

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