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DC Biasing - BJTs

Chapter 4

Boylestad

Electronic Devices and Circuit Theory

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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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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.