Chapter 3: Bipolar Junction Transistors

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Chapter 3:

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Transistor Construction Transistor Construction

There are two types of transistors:

• pnp

• npn

The terminals are labeled:

• E - Emitter

pnp pnp

• E - Emitter

• B - Base

• C - Collector

npn

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Transistor Operation Transistor Operation

With the external sources, V EE and V CC , connected as shown:

• The emitter-base junction is forward biased

• The base-collector junction is reverse biased

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Currents in a Transistor Currents in a Transistor

The collector current is comprised of two

I B I C

I E = = = = + + + +

Emitter current is the sum of the collector and base currents:

The collector current is comprised of two currents:

minority I CO

majority I C

I C = +

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Common

Common--Base Configuration Base Configuration

The base is common to both input (emitter–base) and

output (collector–base) of the transistor.

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Common

Common--Base Amplifier Base Amplifier

Input Characteristics Input Characteristics

This curve shows the relationship

between of input current (I E ) to input

voltage (V BE ) for three output voltage

(V CB ) levels.

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This graph demonstrates the output current (I C ) to an output voltage (V CB ) for

Common

Common--Base Amplifier Base Amplifier

Output Characteristics Output Characteristics

an output voltage (V CB ) for various levels of input

current (I E ).

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Operating Regions Operating Regions

• Active – Operating range of the amplifier.

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

current.

•• Saturation – The amplifier is full on.

There is current, but little voltage.

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I E I C ≅

Approximations Approximations

Emitter and collector currents:

Base-emitter voltage:

Silicon) (for

V BE 0.7

V =

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Ideally: αααα = 1

Alpha (

Alpha ( αααααααα ))

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

I E I C α dc =

Ideally: αααα = 1

In reality: αααα is between 0.9 and 0.998

Alpha ( α α α α ) in the AC mode AC mode:

I E I C α

∆

ac =

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Transistor Amplification Transistor Amplification

Voltage Gain:

V 50 kΩ

5 ma 10 mA 10

10mA 20Ω

200mV

=

≅

=

) )(

( L R

L I V

I i I L

I E I C

R i V i I i

I E

Currents and Voltages:

250 200mV

50V =

=

V i

V L

A v

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Common

Common– –Emitter Configuration Emitter Configuration

The emitter is common to both input (base-emitter) and output (collector- emitter).

The input is on the base and the

output is on the collector.

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Common

Common--Emitter Characteristics Emitter Characteristics

Collector Characteristics Base Characteristics

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Common

Common--Emitter Amplifier Currents Emitter Amplifier Currents

Ideal Currents Ideal Currents

I E = I C + I B I C = α α α α I E

Actual Currents Actual Currents

I C = α α α α I E + I CBO where I CBO = minority collector current I C = α α α α I E + I CBO

When I B = 0 µ µ µA the transistor is in cutoff, but there is some minority µ current flowing called I CEO .

µA 0

− =

= I

B

CBO

CEO α

I I

1

where I CBO = minority collector current

I

_CBO

is usually so small that it can be ignored, except in high

power transistors and in high temperature environments.

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Beta ( Beta ( β β β β β β β β ))

In DC mode:

β β β

β represents the amplification factor of a transistor. ( β β β β is

sometimes referred to as h fe , a term used in transistor modeling calculations)

I C

β dc =

In AC mode:

I B

β dc =

constant

ac =

∆

= ∆

V

CE

B C

I

β I

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Determining β β β β from a Graph

Beta ( Beta (β β β β β β β β))

100 µA 10

mA 1

µA) 20 µA

(30

mA) 2.2

mA β (3.2

7.5 V

AC

CE

= =

=

−

= −

=

108 A 25

mA

β DC 2.7 V 7.5

CE

= =

µ µ µ µ

= =

= = = = = =

= 100

=

= =

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Relationship between amplification factors β β β β and α α α α

1 β α β

+ + +

= +

=

= α 1

β α

− − −

= −

=

Beta ( Beta (β β β β β β β β))

Relationship Between Currents Relationship Between Currents

B C βI

I = = = = I E = = = = (β + + + + 1)I B

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Common

Common– –Collector Configuration Collector Configuration

The input is on the

base and the output is

on the emitter.

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Common

Common– –Collector Configuration Collector Configuration

The characteristics are similar to those of the common-emitter

common-emitter

configuration, except the

vertical axis is I E .

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V CE is at maximum and I C is at

minimum (I Cmax = I CEO ) in the cutoff region.

I C is at maximum and V CE is at

minimum (V CE max = V CEsat = V CEO ) in

Operating Limits for Each Configuration Operating Limits for Each Configuration

minimum (V CE max = V CEsat = V CEO ) in the saturation region.

The transistor operates in the active

region between saturation and cutoff.

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Power Dissipation Power Dissipation

C CB Cmax V I P = = = =

C CE Cmax V I P = = = =

Common-base:

Common-emitter:

Common-collector:

E CE

Cmax V I

P = = = =

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Transistor Specification Sheet

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Transistor Specification Sheet

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Transistor Testing Transistor Testing

•• Curve Tracer Curve Tracer

Provides a graph of the characteristic curves.

•• DMM DMM

Some DMMs measure ββββ DC or h FE .

•• Ohmmeter Ohmmeter

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Transistor Terminal Identification