Bipolar Junction Transistor

What is a Transistor?
• Semiconductors: ability to change fromconductor to insulator• Can either allow current or prohibit
current to flow

• Useful as a switch, but also as an amplifier• Essential part of many technologicaladvances

A transistor is a semiconductor device
used to amplify and switch electronic
signals and electrical power.

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

3

Currents in a TransistorEmitter current is the sum of the collector andbase currents:
IE  IC  IB

The collector current is comprised of twocurrents:
IC  IC I COmajority
minority4

Transistor

SYMBOLS(BJTS)

Bipolar Junction Transistor

Transistor Operation• A single p-n junction has two different types of bias:- Forward Bias
- Reverse Bias
• Thus, a two pn-jnuction device has four types of bias.

BC junction(BCJ)

BE junction Mode of operation(BEJ)

Reverse

ReverseCut-off

Forward

ReverseCut-off

ReverseForward

ActiveForward

ForwardSaturation

8BJT Construction• The emitter layer is heavily doped, the base lightly
doped and collector only lightly doped.
• The doping of the sandwiched layer is considerably lessthan the outer layers (typically 10:1 or less).
• Lower doping level decreases the conductivity of thematerial by limiting the number of free electrons.
• The outer layers have widths much greater than thesandwiched p- or n-type material. The ratio of the total
width to that of the central layer is 150:1.

JUNCTIONSBASE-EMITTER JUNCTION(BEJ):
The pn junction joining the base region and the emitter region iscalled the base-emitter junction

COLLECTOR-BASE JUNCTION(CBJ):
The pn junction joining the base region and the collector region
is called the collector-base junction

Transistor as Two Diodes

Transistor Currents(NPN)

𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵

Transistor Currents(PNP)

𝐼𝐸 = 𝐼𝐶 + 𝐼𝐵

NPN

PNP

BJT Modes of Operation
MODE
CutOff
ActiveSaturation

IB or VCE Char.
IB = Very small
VCE = Moderate
VCE = Small

BEJ
ReverseForward
Forward

CBJ
Reverse
ReverseForwardCut-off –The transistor is “Fully-OFF” operating as a switch. There is voltage, but littlecurrent
Active Region –The transistor operates as an amplifier

Saturation –The transistor is “Fully-ON” operating as a switch. There is current, but littlevoltage.

DC BIAS VOLTAGESWhen a transistor is connected to dc bias voltages, for both npn and
pnp types,
VBB forward-biases the base-emitter junction
VCC reverse-biases the base-collector junction

DC Beta ( β )The dc current gain of a transistor is the ratio of the dc collector
current (𝐼𝐶 ) to the dc base current (𝐼𝐵 )𝑰𝑪𝜷=𝑰𝑩𝑰𝑪 = 𝜷𝑰𝑩
𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩DC Alpha ( α )The ratio of the dc collector current (𝐼𝐶 ) to the dc emitter current(𝐼𝐸 ) is the dc alpha (αDC).

𝑰𝑪
𝜶=𝑰𝑬

𝑰𝑪 = 𝜶𝑰𝑬
𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩

BJT Current–Voltage Relationships
𝜷𝜶=𝜷+𝟏

𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩
𝑰𝑪 = 𝑰𝑬 − 𝑰𝑩𝜶𝜷=𝟏−𝜶

𝑰𝑩 = 𝑰𝑬 − 𝑰𝑪𝑰

𝜷 = 𝑰 𝑪 , 𝑰𝑪 = 𝜷𝑰𝑩
𝑩𝑰𝑪
𝜶=, 𝑰𝑪 = 𝜶𝑰𝑬
𝑰𝑬
𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩
𝑰𝑬 = 𝑰𝑪 + 𝑰𝑩
𝑰𝑬 = 𝜷𝑰𝑩 + 𝑰𝑩

𝑰𝑬 = 𝜶𝑰𝑬 + 𝑰𝑩

𝑰𝑬 = 𝜷 + 𝟏 𝑰𝑩

𝑰𝑩 = (𝟏 − 𝜶)𝑰𝑬Modes of Operation(NPN)BJT configurations
1. Common Emitter Configuration –has both Current and Voltage Gain.
2. Common Base Configuration –has Voltage Gain but no Current Gain.
3. Common Collector Configuration –has Current Gain but no Voltage Gain.

Common-Base Configuration

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

23

Common Base Configuration
Common Base or grounded base configuration, the BASE connection
is common to both the input signal AND the output signal with theinput signal being applied between the base and the emitter
terminals. The corresponding output signal is taken from between
the base and the collector terminals
Input: Emitter
Output: Collector

Common-Base Amplifier

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

25

Common-Base Amplifier

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

26

Operating Regions•Active – Operating range of theamplifier.

•Cutoff – The amplifier is basicallyoff. There is voltage, but littlecurrent.

•Saturation – The amplifier is full on.
There is current, but little voltage.

27

Approximations
Emitter and collector currents:
ICIE

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

28

Alpha (a)Alpha (a) is the ratio of IC to IE :αdc ICIE

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

ΔI C
ΔI E

29

Transistor Amplification

Currents and Voltages:

Voltage Gain:

Vi200mVI E  Ii  10mA
Ri20ΩICIVIEL

 I  10 mAi

L I R  (10 ma )( 5 kΩ)  50 VL
30Av VLVi

50V200mV 250

Common–Emitter Configuration

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

31Common Emitter Configuration
Common Emitter or grounded emitter configuration, the input
signal is applied between the base, while the output is taken frombetween the collector and the emitter as shown.
Input: BaseOutput: Collector

Common-Emitter CharacteristicsCollector Characteristics

Base Characteristics

33

Common-Emitter Amplifier Currents
Ideal Currents
IC = a IE

IE = IC + IB

Actual Currents
IC = a IE + ICBO

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

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

I CEO I CBO
I  0 μA
1 α B34

Beta () represents the amplification factor of a transistor. (  issometimes referred to as hfe, a term used in transistor modelingcalculations)
In DC mode:

ICβdc IBIn AC mode: ac ICV constantIB CE35

Beta ()Determining  from a Graph(3.2 mA  2.2 mA)(30 μA  20 μA)
1 mA
V  7.5
10 μA C E
 100

β AC 

2.7 mA
VC E  7.525 A 108

β DC 36

Beta ()Relationship between amplification factors  and aαββ1αβα 1

Relationship Between Currents

I C  βI B

I E  (β  1)I B37Common–Collector Configuration

The input is on thebase and the output ison the emitter.

38

Common Collector Configuration
Common Collector or grounded collector configuration, thecollector is now common through the supply. The input signal isconnected directly to the base, while the output is taken from theemitter load as shown. This type of configuration is commonly
known as a Voltage Follower or Emitter Follower circuit.
Input: Base
Output: Emitter

Common–Collector Configuration

The characteristics are
similar to those of thecommon-emitter
configuration, except thevertical axis is IE.

40

Operating Limits for Each Configuration
VCE is at maximum and IC is atminimum (ICmax= ICEO) in the cutoff
region.
IC is at maximum and VCE is atminimum (VCE max = VCEsat = VCEO) inthe saturation region.

The transistor operates in the active
region between saturation and cutoff.

41

Transistor Configuration
There are 3 types of transistor configuration in electric circuit:a) CB (common base)b) CE (common emitter)c) CC (common collector)
This configuration is base on which the terminal is connected to the input signal and output
signal.
Table below shows the relationship between input terminal and output terminal with thetransistor configuration.

Configuration

Input terminal

Output terminal

CBECCE

BCCC

B

E

42Characteristics of the Transistor Configurations
CommonBaseCommonEmitter

CommonCollector

Low

MediumHigh

OutputImpedance

Very High

High

Low

Phase Angle

0o

180o

0o

Voltage Gain

HighMedium

Low

Current Gain

Low

MediumHigh

Power Gain

Low

Very High

MediumCharacteristic

Input ImpedancePower Dissipation
Common-base:PCmax  VCB I C
Common-emitter:

PCmax  VCE I C

Common-collector:

PCmax  VCE I E44

Transistor Specification Sheet

45

Transistor Specification Sheet

46

Transistor Testing•Curve Tracer
Provides a graph of the characteristic curves.

•DMM
Some DMMs measure  DC or hFE.

•Ohmmeter47

Transistor Terminal Identification

48