Chapter 3: Bipolar Junction Transistors - web page for staff

Chapter 3: Bipolar Junction Transistors

Transistor Construction There are two types of transistors: • pnp • npn

pnp

The terminals are labeled: • E - Emitter • B - Base • C - Collector npn

Electronic Devices and Circuit Theory, 10/e Robert L. Boylestad and Louis Nashelsky

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Copyright ©2009 by Pearson Education, Inc. Upper Saddle River, New Jersey 07458 • All rights reserved.

Transistor Operation With the external sources, VEE and VCC, 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 Emitter current is the sum of the collector and base currents: IE = IC + IB

The collector current is comprised of two currents: IC = IC + I CO majority minority


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Common--Base Configuration Common

The base is common to both input (emitter–base) and output (collector–base) of the transistor.


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Common--Base Amplifier Common

Input Characteristics This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels.


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Common--Base Amplifier Common

Output Characteristics This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE).


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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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Approximations Emitter and collector currents: I

C

≅ I

E

Base-emitter voltage: VBE = 0.7 V (for Silicon)


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Alpha (α (α ) Alpha (α) is the ratio of IC to IE : αdc =

IC IE

Ideally: α = 1 In reality: α is between 0.9 and 0.998 Alpha (α) in the AC mode: mode αac =


∆I C ∆I E

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Transistor Amplification

Currents and Voltages:

Voltage Gain:

V 200mV I E = Ii = i = = 10mA Ri 20Ω I

C

I V

≅I

Av =

VL Vi

=

50V

= 250

200mV

E

L

≅ I = 10 mA i

L

= I R = (10 ma )(5 kΩ) = 50 V L


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Common––Emitter Configuration Common

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


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Common--Emitter Characteristics Common

Collector Characteristics


Base Characteristics

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Common--Emitter Amplifier Currents Common Ideal Currents IC = α IE

IE = IC + IB

Actual Currents IC = α IE + ICBO

where ICBO = minority collector current ICBO is usually so small that it can be ignored, except in high power transistors and in high temperature environments.

When IB = 0 µA the transistor is in cutoff, but there is some minority current flowing called ICEO.

ICEO =


ICBO 1− α

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I B =0 µA


Beta (β) β represents the amplification factor of a transistor. (β is sometimes referred to as hfe, a term used in transistor modeling calculations) In DC mode:

β dc

IC = IB

In AC mode:

β ac =


∆ IC ∆ IB

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VCE = constant


Beta (β (β ) Determining β from a Graph

(3.2 mA − 2.2 mA) (30 µA − 20 µA) 1 mA = V = 7.5 10 µA CE = 100

β AC =

2.7 mA V CE = 7.5 25 µ A = 108

β DC =


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Beta (β (β ) Relationship between amplification factors β and α

α=

β β+1

β=

α α −1

Relationship Between Currents

I C = βIB


I E = (β + 1)I B

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Common– Common –Collector Configuration

The input is on the base and the output is on the emitter.


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Common– Common –Collector Configuration

The characteristics are similar to those of the common-emitter configuration, except the vertical axis is IE.


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Operating Limits for Each Configuration VCE is at maximum and IC is at minimum (ICmax= ICEO) in the cutoff region. IC is at maximum and VCE is at minimum (VCE max = VCEsat = VCEO) in the saturation region. The transistor operates in the active region between saturation and cutoff.


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Power Dissipation Common-base:

PCmax = VCB I C Common-emitter:

PCmax = VCE I C

Common-collector:

PCmax = VCE I E


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Transistor Specification Sheet


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Transistor Specification Sheet


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Transistor Testing •

Curve Tracer Provides a graph of the characteristic curves.

•

DMM Some DMMs measure βDC or hFE.

•

Ohmmeter


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Transistor Terminal Identification


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