Lecture 8 Bipolar Junction Transistor (BJT)eng.staff.alexu.edu.eg/.../fall_2017/Lecture_8_BJT.pdf · Introduction to Bipolar Junction Transistor (BJT) ... Amplifiers Electronic switches

Lecture 8

Bipolar Junction Transistor (BJT)

BJT 1-1 Wednesday 18/10/2016

Outline Introduction to Bipolar Junction

Transistor (BJT) Definitions BJT principles and characteristics BJT structure BJT operation

• Operation modes

Currents in BJT

BJT 1-2

Introduction The transistor is the main building block of

electronics

It is a semiconductor device

It is a three terminal device

Two basic families of transistors Bipolar Junction Transistor (BJT)

Field Effect Transistor (FET)

Transistors are used in: Digital logic

Memory circuits

Amplifiers

Electronic switches

BJT 1-3

Bipolar Junction Transistor (BJT) There are two types of BJT

• pnp

• npn

The terminals are labeled

• E - Emitter

• B - Base

• C - Collector

npn and pnp BJT symbols

BJT 1-4

npn BJT Structure

BJT 1-5

The BJT is fabricated with three separately doped regions

The BJT has two junctions (boundaries between the n and the p regions) Junctions are similar to the pn junctions (diodes) where they

may be forward biased or reverse biased

Emitter-base junction (EBJ) and collector-base junction (CBJ)

These junctions have capacitance

BJT Operation

BJT 1-6

Depending on the biasing across each of the junctions, different modes of operation are obtained: cutoff, active, and saturation

BJT in linear amplifier circuits:

• 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.

BJT Operation (Active Mode)

BJT 1-7

Two external voltage sources set the bias conditions for active mode

The emitter-base junction (EBJ) is forward biased The base-collector junction (BCJ) is reverse biased

Currents in BJT

BJT 1-8

BJT operation in active mode

Currents in BJT The current through EBJ is related to the B-E

voltage as

Due to the large differences in the doping concentrations of the emitter and the base regions, the electrons injected into the base region (from the emitter region) results in the emitter current IE

BJT 1-9

Currents in BJT (cont’d) The number of electrons injected into the collector

region is directly related to the electrons injected into the base region from the emitter region So, the collector current is related to the emitter

current

BJT basic principle: The voltage between two terminals controls the

current through the third terminal

BJT 1-10

Currents in BJT (cont’d) Emitter current is the sum of the collector and base currents:

The collector current and the base current

are related by

β represents the amplification factor of a transistor (β = 100 in transistors of interest)

The relationship between the emitter and the base currents is

The relationship between the emitter and the collector currents is

Where is , called collector efficiency (α is between 0.9 and 0.998 in transistors of interest)

BJT 1-11

Directions of currents and voltage polarities

BJT 1-12

BJT Configurations

BJT 1-13

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

Common-Base configuration

BJT Configurations

BJT 1-14

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

Common-Base configuration

Emitter (or input)

Characteristics

BJT Configurations

BJT 1-15

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

Common-Base configuration

Collector (or output)

Characteristics

BJT 1-16

Lecture Summary Covered material Introduction to Bipolar Junction Transistor

(BJT) BJT principles BJT structure BJT operation modes BJT configurations Currents in BJT

Material to be covered next lecture

Continue BJT Configurations Biasing DC analysis