Lecture 15 17 BJT

Electronic Devices and Circuit Theory Boylestad

Bipolar Junction Transistors Chapter 3

Ch.3 Summary

Transistor Construction There are two types of transistors:

pnp

pnp and npn

The terminals are labeled: E - Em Emit itte ter r B - Base C - Co Coll llec ecto tor r

Electronic Devices and Circuit Theory

npn

© 2013 by Pearson Higher Education, Inc

Ch.3 Summary

Transistor Operation With the external sources, V E E and V C C , connected as shown: The emitter-base junction is forward

biased

The base-collector

junction is reverse biased Electronic Devices and Circuit Theory


Ch.3 Summary

Currents in a Transistor Emitter current is the sum of the collector and base currents: I E



I  I C B

The collector current is comprised of two currents:  I I  I C C (majority ) CO (minority)



Ch.3 Summary

Common-Base Configuration

The base is common to both input (emitter –base) – base) junction and output (collector –base) – base) junction of the transistor. transistor. Electronic Devices and Circuit Theory


Ch.3 Summary

Common-Base Amplifier Input Characteristics This curve shows the relationship between of input current (I (I E ) to input voltage (V (V BE ) for three output voltage (V CB) levels.



Ch.3 Summary

Common-Base Amplifier Output Characteristics This graph demonstrates the output current (I (I C ) to an output voltage (VCB) for various levels of input current (I (I E ). Electronic Devices and Circuit Theory


Ch.3 Summary

Operating Regions Active Operating range of the amplifier.

Cutoff The amplifier is basically off. There is voltage, but little current.

Saturation The amplifier is fully on. There is current, but little voltage. Electronic Devices and Circuit Theory


Ch.3 Summary

Approximations Emitter and collector currents: I C



I E

Base-emitter voltage: V BE




0.7

V (for Silicon)


Ch.3 Summary

Alpha ( ) Alpha ( ) is the ratio of I C to I E : α

dc

Ideally: In reality: reality:

I C 

I E

=1 falls falls somewher somewhere e between between 0.9 and 0.998

Alpha ( ) in the AC mode: α ac


ΔI C 

ΔI E © 2013 by Pearson Higher Education, Inc

Ch.3 Summary

Transistor ransistor Amplifier Amplifier

I E

Currents and Voltages:



I i



V i

200 20 0 m V 

R i



20Ω

10 m A

Voltage Gain:

I  I C E I L V L





I i



10 m A

I R  (10 mA )(5 k Ω ) L




50 V

Av



V L V i

50 V 



250 25 0

200 mV


Ch.3 Summary

Common-Emitter Configuration The emitter is common to both input (base-emitter) and output (collectoremitter) circuits. The input is applied to the base and the output is taken from the collector.



Ch.3 Summary

Common-Emitter Characteristics

Collector Characteristics Electronic Devices and Circuit Theory

Base Characteristics © 2013 by Pearson Higher Education, Inc

Ch.3 Summary

Common-Emitter Amplifier Amplifier Currents Ideal Currents

Actual Currents

I E = I C + I B I C =  I I E + I CBO

I C =  I I E where ICBO = minority collector current

I CBO is usually so small that it can be ignored, except in high power transistors and in high temperature environments. When I B = 0  A the transistor is in cutoff, but there is some minority current flowing called I CEO. Electronic Devices and Circuit Theory

I CEO



I CBO 1



α

I B 0 μA 


Ch.3 Summary

Beta ( ) represents the amplification factor of a transistor transistor..

In DC D C mod ode: e:

In AC A C mod ode e:

βdc

ac 



I C I B

I C I B

V CE constant

 ac is sometimes referred to as h fe, a term used in transistor modeling calculations Electronic Devices and Circuit Theory


Ch.3 Summary

Beta ( ) Determining

( 3.2 m A  2.2 mA) ( 30  A  20  A)

β AC  

from a Graph

1 mA 10  A

V CE 7.5 V

 100

βDC 

2.7 mA 25  A

V CE  7 .5 V

10 8  108



Ch.3 Summary

Beta ( ) Relationship between amplification factors α 

β

β

β  1

and

:

α 

α



1

Relationship Between Currents: I C




βI B

I E



( β  1)I B


Ch.3 Summary

Common-Collector Configuration The input is on the base and the output is on the emitter.



Ch.3 Summary

Common-Collector Configuration The characteristi characteristics cs are similar to those of the commonemitter amplifier, except the vertical axis is I E .



Ch.3 Summary

Operating Limits V CE is maximum and I C is minimum in the cutoff region. I C (max)



I CEO

I C is maximum and V CE is minimum in the saturation region. V CE (max)



V CE ( sat )



V CEO

The transistor operates in the active region between saturation and cutoff. Electronic Devices and Circuit Theory


Ch.3 Summary

Power Dissipation Common-base: P Cmax



V CBI C

P Cmax



V CE I C



V CE I E

Common-emitter :

Common-collector: P Cmax



Ch.3 Summary

Transistor Specification Sheet



Ch.3 Summary




Ch.3 Summary




Ch.3 Summary

Transistor Testing Curve Tracer

Provides a graph of the characteristic curves.

DMM

Some DMMs measure DC or hFE.

Ohmmeter:



Ch.3 Summary

Transistor Terminal Identification



Lecture 15 17 BJT

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