BJT 1sdsdsa

Transistor can be grouped into

two families: 1. Bipolar Junction Transistor (BJT)

2. Unipolar Junction Transistor (UJT)

1

The First BJT

2

Transistor Size (3/8L X 5/32W X 7/32H) No Date Codes. No Packaging.

Modern Transistors

3

Example of BJT Specification Sheet

4

Fig. 3.32 Type Q2T2905 Texas Instruments quad pnp silicon transistor: (a) appearance; (b) pin

connections. (Courtesy Texas Instruments Incorporated.)

5 Robert L. Boylestad Electronic Devices and Circuit Theory, 9e

Copyright 2006 by Pearson Education, Inc.

Upper Saddle River, New Jersey 07458

All rights reserved.

6

Transistor Terminal Identification

Definition

A three terminal device in which current flowing between two terminals can be

controlled by a signal on the third terminal.

There are two types of transistors: pnp npn

7

Symbol & Construction

8

Transistor Operation

Forward bias, base to emitter

narrows the BE

depletion region.

9

Reverse bias, base to collector

widen the BC

depletion region.

10

Transistor Operation

11

Forward and Reverse bias of a BJT.

Transistor currents.

12 Thomas L. Floyd Electronic Devices, 6e and Electronic

Devices: Electron Flow Version, 4e




Currents in a Transistor

By Kirchhoffs current law; ( 4.1)

The current gain of the transistor;

( 4.2)

The ratio of the collector current to the emitter current;

( 4.3)

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BCE III

B

Cdc

I

I

E

Cdc

I

I

Currents in a Transistor

Relationship of and

(4.4)

Dividing the Eq 4.1 by IC; Substitute Eq.4.2 and Eq.4.3 into Eq.4.4

Rearrange the above expression;

and

14

BI

CI

EI

dc

dc

C

B

C

C

C

E

I

I

I

I

I

I

dcdc

11

1

dc

dcdc

1dc

dcdc

1

General Bias Modes

15

Forward active mode

VBE > 0v ( BE at FB) ; VBC < 0v ( BC at RB)

General Bias Modes

16

Reverse active mode

VBE < 0v ( BE at RB ) ; VBC > 0v ( BC at FB )

General Bias Modes






Saturation mode

VBE > 0v ( BE at FB ) ; VBC > 0v ( BC at FB )

General Bias Modes

18

Cut off mode

VBE < 0v ( BE at RB ) ; VBC < 0v ( BC at RB )

Operation Mode

19

Operation Mode

Active: Most importance mode, e.g. for amplifier

operation.

The region where current curves are practically flat.

Saturation: Barrier potential of the junctions cancel each

other out causing a virtual short.

Ideal transistor behaves like a closed switch.

Cutoff: Current reduced to zero

Ideal transistor behaves like an open switch. 20

Operation Mode

21

Transistor currents and voltages.

22

DC load line on a family of collector characteristic curves illustrating the

cutoff and saturation conditions.

23

TRANSISTOR CONFIGURATION

CIRCUIT.

Common Emitter Configuration

Common Base Configuration

Common Collector Configuration

The way a transistor is connected with the specific

terminal become common terminal for both input and

output loops.

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Circuit Configuration

25

26

Common 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.

Common-Emitter Configuration

It is called common-emitter configuration since :

- emitter is common or reference to both input and

output terminals.

- emitter is usually the terminal closest to or at

ground potential.

Almost amplifier design is using connection of CE due

to the high gain for current and voltage.

Two set of characteristics are necessary to describe

the behavior for CE ;input (base terminal) and output

(collector terminal) parameters.

27

Common-Emitter Configuration

28

29

Characteristics of Common-Emitter

Collector characteristics = output characteristics.

Base characteristics = input characteristics.

IB is microamperes compared to miliamperes of IC.

IB will flow when VBE > 0.7V

for silicon and 0.3V for germanium

Before this value IB is very small and no IB.

Base-emitter junction is forward bias

Increasing VCE will reduce IB

for different values.

30

Characteristics of Common-Emitter

Collector characteristics = output

characteristics.

Base characteristics = input

characteristics.

VCE < VCESAT, IC increase linearly with increasing of VCE VCE > VCESAT, IC not totally depends on VCE constant IC

IB(uA) is very small compare to IC (mA). Small increase in IB cause big increase in IC

IB=0 A ICEO occur.

Noticing the value when IC=0A. There is still some value of current flows.

32

Beta ()

In DC mode: [Formula 3.10]

In AC mode: [Formula 3.11]

indicates the amplification factor of a transistor. ( is sometimes referred to as hfe, a

term used in transistor modeling calculations)

B

C

I

Idc

constantVI

Iac CE

B

C

Example

From output characteristics of common

emitter configuration, find ac and dc with an

Operating point at IB=25 A and VCE =7.5V.

33

34

Solution:

Determining ac and dc from the collector characteristics.

35 Robert L. Boylestad Electronic Devices and Circuit Theory, 9e




36

Common Base Configuration

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

transistor.

Common-Base Configuration

Common-base terminology is derived from the fact that

the :

- base is common to both input and output of the

configuration.

- base is usually the terminal closest to or at

ground potential.

All current directions will refer to conventional (hole)

flow and the arrows in all electronic symbols have a

direction defined by this convention.

Note that the applied biasing (voltage sources) are such

as to establish current in the direction indicated for each

branch. 37

Common-Base Configuration

38

39

Input Characteristics for a Common-Base Amplifier

This demonstrates the input current IE to input voltage VBE for various levels of output

voltage VCB.

Ic(mA)

In the dc mode the level of IC and IE due to the majority carriers are related by a quantity called alpha

=

IC = IE + ICBO It can then be summarize to IC = IE (ignore ICBO due

to small value)

For ac situations where the point of operation moves on the characteristics curve, an ac alpha defined by

Alpha a common base current gain factor that shows the efficiency by calculating the current percent from current flow from emitter to collector.The value of is typical from 0.9 ~ 0.998.

E

C

I

I

E

C

I

I

41

Alpha ()

Biasing

Proper biasing CB configuration in active region by

approximation IC IE (IB 0 uA)

42

Transistor as an amplifier

43

Simulation of transistor as an amplifier

44

45


The input on the Base and the output is on the Emitter.

Common Collector Configuration Also called emitter-follower (EF).

It is called common-emitter configuration since both the

signal source and the load share the collector terminal

as a common connection point.

The output voltage is obtained at emitter terminal.

The input characteristic of common-collector

configuration is similar with common-emitter.

configuration.

Common-collector circuit configuration is provided with

the load resistor connected from emitter to ground.

It is used primarily for impedance-matching purpose

since it has high input impedance and low output

impedance.

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47

Limits of Operation

Many BJT transistor used as an amplifier. Thus it is

important to notice the limits of operations.

At least 3 maximum values is mentioned in data sheet.

There are:

a) Maximum power dissipation at collector: PCmax

or PD

b) Maximum collector-emitter voltage: VCEmax

sometimes named as VBR(CEO) or VCEO.

c) Maximum collector current: ICmax

There are few rules that need to be followed for BJT

transistor used as an amplifier. The rules are:

i) transistor need to be operate in active region!

ii) IC < ICmax

ii) PC < PCmax 48

49

Limitations of Operation for Each Configuration

Note: 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.

50

Power of Dissipation

Common Base: [Formula 3.18]

Common Emitter: [Formula 3.16]

Common Collector: a

CCBC IVmaxP

CCEC IVmaxP

ECEC IVmaxP

51

Refer to the fig. Step1: The maximum collector power dissipation, PD=ICmax x VCEmax (1) = 18m x 20 = 360 mW Step 2: At any point on the characteristics the product of and must be equal to 360 mW. Ex. 1. If choose ICmax= 5 mA, subtitute into the (1), we get VCEmaxICmax= 360 mW VCEmax(5 m)=360/5=7.2 V Ex.2. If choose VCEmax=18 V, subtitute into (1), we get VCEmaxICmax= 360 mW (10) ICmax=360m/18=20 mA

Derating PDmax

PDmax is usually specified at 25C.

The higher temperature goes, the less is PDmax

Example;

A derating factor of 2mW/C indicates the power

dissipation is reduced 2mW each degree centigrade

increase of temperature.

52

Example

Transistor 2N3904 used in the circuit with

VCE=20 V. This circuit used at temperature

1250C. Calculate the new maximum IC.

Transistor 2N3904 have maximum power

dissipation is 625 mW. Derating factor is

5mW/0C.

53

Solution

Step 1:

Temperature increase : 1250C 250C = 1000C

Step 2:

Derate transistor : 5 mW/0C x 1000C = 500 mW

Step 3:

Maximum power dissipation at 1250C = 625 mW500

mW=125 mW.

Step 4:

Thus ICmax = PCmax / VCE=125m/20 = 6.25 mA.

Step 5:

Draw the new line of power dissipation at 1250C .

54

Example

The parameters of transistor 2N3055 as follows:

- Maximum power dissipation @ 250C=115 W

- Derate factor=0.66 mW/0C.

This transistor used at temperature 780C.

Find the new maximum value of power dissipation.

Find the set of new maximum of IC if VCE=10V,

20V and 40 V.

55

Solution

Step 1:

Temperature increase : 780C 250C = 530C

Step 2:

Derate transistor : 0.66mW/0C x 530C = 35 mW

Step 3:

Maximum power dissipation at 780C = 115W 35W=80

mW.

Step 4:

ICmax = PCmax / VCE=80m/10 = 8 mA (point C)

ICmax = PCmax / VCE=80m/20 = 4 mA. (point B)

ICmax = PCmax / VCE=80m/40 = 2 mA (point A)

56

57

Step 5:

Draw the new line of power dissipation at 780C .

58

Transistor Testing

1. Curve Tracer

Provides a graph of the characteristic curves.

2. DMM

Some DMMs will measure DC or HFE.

3. Ohmmeter

Partial transistor data sheet.






BJT 1sdsdsa

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