TransistorsBipolar Junction Transistors

Engr. Katrina B. Acapulco

• Basically, it consists of two back-to-back PN junctions manufactured in a single semiconductor crystal.

• These two junctions give rise to three regions called emitter, base and collector.

Introduction

Emitter – more heavily doped than any of the other regions because its main function is to supply majority charge carriers to the base.

Base – forms the middle section of the transistor. It is very thin as either the emitter or collector and it is very lightly-doped.

Collector – its main function is to collect majority charge carriers coming from the emitter and passing through the base. – this region is made physically larger than the emitter region because it has to dissipate much greater power.

Introduction

Transistor Biasing

• For proper working of a transistor, it is essential to apply voltages of correct polarity across its two junctions.

1. Emitter-base junction is always forward biased.

2. Collector-base junction is always reverse-biased.

Important Biasing Rule

• For a PNP transistor, both collector and base are negative with respect to the emitter. Collector is more negative than the base.

• For NPN transistor, both collector and base are positive with respect to the emitter. Collector is more positive than the base.

Transistor Currents

• The three primary currents which flow in a properly-biased transistor are IE, IB and IC.

• IE = IB + IC

• A small part (about 1-2%) of emitter current goes to supply base current and the remaining major part (98-99%) goes to supply collector current.

• Moreover, IE flows into the transistor whereas IB and IC flow out of it.

Transistor Currents

Summing Up

• The four basic guideposts about all transistor circuits are:

1. Conventional current flows along the arrow whereas electrons flow against it.

2. E/B junction is always forward-biased.

3. C/B junction is always reverse-biased.

4. IE = IB + IC

Transistor Circuit Configurations

• Basically, there are three types of circuit connections for operating a transistor.

1. CB 2. CE 3. CC- the term “common” is used to denote the electrode that is

common to the input and output circuits.

CB Configuration

• In this configuration, emitter current IE is the input current and collector current IC is the output current.

• The input signal is applied between the emitter and base whereas output is taken out from the collector and base.

CB Configuration

• The ratio of the collector current to the emitter current is called dc alpha (dc)of a transistor.

dc = -IC / IE

• The negative sign is due to the fact that current IE flows into the transistor whereas IC flows out of it.

IC = - dc IE

• If we write dc simply as then = IE/IC

• It is also called forward current transfer ratio(-hFB).

• The of a transistor is a measure of the quality of the transistor: the higher the value the better the transistor in the sense that collector current more closely equals the emitter current.

• Its value ranges from 0.95 to 0.999.

• It applies only to CB configuration of a transistor.

IC = IE IB = IE - IE = (1- IE

CB Configuration

• Incidentally, there is also an ac for a transistor. It refers to the ratio of change in the collector current to the change in emitter current.

ac = - IC / IE

• It is also known as short-circuit gain of a transistor and it is written as –hfb.

• For all practical purposes: dc = ac =

CB Configuration

CE Configuration

• Here, input signal is applied between the base and emitter and output signal is taken out from the collector and emitter circuit.

• IB is the input current and IC is the output current.

CE Configuration

• The ratio of the collector current to the dc base current is called dc beta (dc) or just of a transistor.

= - IC / - IB = IC / IB or IC = IB

• It is also called common-emitter dc forward transfer ratio and it is written as hFE.

• It is possible for to have a value as high as 500.

• For ac analysis: ac = IC / IB

• It is also written as hfe.

• IE = IB + IC = IB + IB = (1 +

Relation Between and

= IC / IB and = IC / IE

• Therefore : • Now : IB = IE – IC

• Therefore: IC / IE – IC

C/IE) / (IE/IE) – (IC/IE)

• Cross-multiplying the previous equation:

or or

CC Configuration

• In this case, input signal is applied between base and collector and output signal is taken out from emitter-collector current.

• Conventionally speaking, here IB is the input current and IE is the output current.

• IE/IB = IE/IC x IC/IB =

• IE = IB + IC = IB +

• Therefore:

output current = (1+ x input current

CC Configuration

Relations Between Transistor Currents

While deriving various equations, following definitions should be kept in mind:

= IC / IE , = IC / IB , and

1.IC = x IE

2.C/ IE / 3. IE = IC/ x IC =

.The three transistor dc currents always bear the following ratio:

IE: IB : IC

1 : Incidentally, it may be noted that for ac currents, small

letters ie, ib and ic are used.

Relations Between Transistor Currents