Phy 4240 lec (9) and (10)

Lecture (9, 10)

Transistors

and transistor circuits

1

Introduction

Transistors are three terminal devices that replaced vacuum tubes.

They are solid state devices that are used for…

Amplification

Switching

Detecting Light

The three terminals are the…

Emitter, Base, Collector (BJT)

Source, Gate, Drain (FET)

2

The BJT

Bipolar Junction Transistors are made with

n-type and p-type semiconductors.

There are two types: npn and pnp.

Circuit Symbols

Transistor Analogy

3

The three layers of

the sandwich are

conventionally called

the Collector, Base,

and Emitter.

A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are

joined back-to-back.

This forms a sort of a sandwich where one kind of semiconductor is placed in

between two others.

There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties. 4

B

C

E

B

C

E

The BJT – Bipolar Junction Transistor

The Two Types of BJT Transistors:

npn pnp

n p nE

B

C p n pE

B

C

Cross Section Cross Section

B

C

E

Schematic

Symbol

B

C

E

Schematic

Symbol

• Collector doping is usually ~ 106

• Base doping is slightly higher ~ 107 – 108

• Emitter doping is much higher ~ 1015

5

BJT Relationships - Equations

B

CE

IE IC

IB

-

+

VBE VBC

+

-

+- VCE

B

CE

IE IC

IB-

+

VEB VCB

+

-

+ -VEC

npnIE = IB + ICVCE = -VBC + VBE

pnpIE = IB + ICVEC = VEB - VCB

Note: The equations seen above are for the transistor, not the circuit.6

Some of the basic properties exhibited by a

Bipolar Transistor are immediately

recognizable as being diode-like.

However, when the 'filling' of the sandwich is

fairly thin some interesting effects become

possible that allow us to use the Transistor

as an amplifier or a switch.

To see how the Bipolar Transistor works we

can concentrate on the NPN variety.

7

8

9

10

What are the primary

uses of transistors?

1. Switching (Logic Gates)

2. Amplification (Op-Amps)

11

The bipolar junction transistor (BJT) is constructed with

three doped semiconductor regions separated by two pn

junctions

Regions are called emitter, base and collector12

There are two types of BJTs, the npn and pnp

The two junctions are termed the base-emitter

junction and the base-collector junction

The term bipolar refers to the use of both holes and

electrons as charge carriers in the transistor

structure

In order for the transistor to operate properly, the

two junctions must have the correct dc bias

voltages

the base-emitter (BE) junction is forward

biased(>=0.7V for Si, >=0.3V for Ge)

the base-collector (BC) junction is reverse

biased

13

14

Basic circuits of BJT

15

Operation of BJTs

BJT will operates in one of following

four region

Cutoff region (for digital circuit)

Saturation region (for digital circuit)

Linear (active) region (to be an amplifier)

Breakdown region (always be a disaster)

16

Operation of BJTs

17

DC Analysis of BJTs

Transistor Currents:

IE = IC + IB

alpha (DC)

IC = DCIE

beta (DC)

IC = DCIBDC typically has a value between 20 and

200

18

Transistor Equations

Alpha Factor

Current Gain

Kirchhoff’s Current Rule

E

C

I

I

B

C

I

I

BCE III 19

DC Analysis of BJTs

DC voltages for the biased

transistor:

Collector voltage

VC = VCC - ICRC

Base voltage

VB = VE + VBE

for silicon transistors, VBE = 0.7 V

for germanium transistors, VBE = 0.3 V20

DC and DC

= Common-emitter current gain

= Common-base current gain

The relationships between the two parameters are:

Note: and are sometimes referred to as dc and dc

because the relationships being dealt with in the BJT are

DC.

E

C

B

C

I

I

I

I

11

21

BJT ExampleUsing Common-Base NPN Circuit Configuration

+_

+_

Given: IB = 50 A , IC = 1 mA

Find: IE , , and

Solution:

IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA

= IC / IB = 1 mA / 0.05 mA = 20

= IC / IE = 1 mA / 1.05 mA = 0.95238

could also be calculated using the value of

with the formula from the previous slide.

IC

IE

IB

VCB

VBE

E

C

B

954.021

20

1

22

BJT Transconductance Curve

Typical NPN Transistor 1

VBE

IC

2 mA

4 mA

6 mA

8 mA

0.7 V

Collector Current:

IC = IES eVBE/VT

Transconductance:

(slope of the curve)

gm = IC / VBE

IES = The reverse saturation current

of the B-E Junction.

VT = kT/q = 26 mV (@ T=300K)

= the emission coefficient and is

usually ~1

23

Modes of Operation

• Most important mode of operation

• Central to amplifier operation

• The region where current curves are practically flat

Active:

Saturation: • Barrier potential of the junctions cancel each other out

causing a virtual short

Cutoff:• Current reduced to zero

• Ideal transistor behaves like an open switch

* Note: There is also a mode of operation called

inverse active, but it is rarely used.

24

Three Types of BJT Biasing

Biasing the transistor refers to applying voltage to get the

transistor to achieve certain operating conditions.

Common-Base Biasing (CB) : input = VEB & IEoutput = VCB & IC

Common-Emitter Biasing (CE): input = VBE & IBoutput = VCE & IC

Common-Collector Biasing (CC): input = VBC & IBoutput = VEC & IE

25

Common-Base

Although the Common-Base configuration is not the most common

biasing type, it is often helpful in the understanding of how the BJT

works.

Emitter-Current Curves

Satu

rati

on

Reg

ion

IE

IC

VCB

Active Region

Cutoff

IE = 0

26

The Early Effect (Early Voltage)

VCE

IC

Note: Common-Emitter

Configuration

-VA

IB

Green = Ideal ICOrange = Actual IC (IC’)

A

CECC

V

VII

1

27

Early Effect Example

Given: The common-emitter circuit below with IB = 25A, VCC = 15V, = 100

and VA = 80.

Find: a) The ideal collector current

b) The actual collector current

Circuit Diagram

+_VCC

IC VCE

IB

A

CECC

V

VII

1

mA

IC

96.2

80

115105.2 3

A

A

II

I

I

BC

B

C

3

6

105.2

)1025(100

a)

b)

28

Breakdown Voltage

The maximum voltage that the BJT can withstand.

BVCEO = The breakdown voltage for a common-emitter

biased circuit. This breakdown voltage usually

ranges from ~20-1000 Volts.

BVCBO = The breakdown voltage for a common-base biased

circuit. This breakdown voltage is usually much

higher than BVCEO and has a minimum value of ~60

Volts.

Breakdown Voltage is Determined By:

• The Base Width

• Material Being Used

• Doping Levels

• Biasing Voltage

29

BJT as an amplifier

Class A Amplifiers

Class B Amplifiers

30

BJT Class A Amplifiers

In a class A amplifier, the transistor conducts for the full cycle of the input signal (360°) used in low-power applications

The transistor is operated in the active region, between saturation and cutoff saturation is when both junctions are forward biased

the transistor is in cutoff when IB = 0

The load line is drawn on the collector curves between saturation and cutoff

31


32


Three biasing mode for class A amplifiers

common-emitter (CE) amplifier

common-collector (CC) amplifier

common-base (CB) amplifier

33


A common-emitter (CE) amplifier

capacitors are used for coupling ac without disturbing dc

levels

34


A common-collector (CC) amplifier

The Common-Collector biasing circuit is basically equivalent to the

common-emitter biased circuit except instead of looking at IC as a function

of VCE and IB we are looking at IE.

Also, since voltage gain ~ 1, but current gain is greater than 1 and

= IC/IE that means IC~IE

35

Emitter-Current Curves


The third configuration is the common-

base (CB)

the base is the grounded (common)

terminal

the input signal is applied to the emitter

output signal is taken off the collector

output is in-phase with the input

voltage gain is greater than 1

current gain is always less than 136

BJT Class B Amplifiers

When an amplifier is biased such that it operates in the linear region for 180° of the input cycle and is in cutoff for 180°, it is a class B amplifier

A class B amplifier is more efficient than a class A

In order to get a linear reproduction of the input waveform, the class B amplifier is configured in a push-pull arrangement

The transistors in a class B amplifier must be biased above cutoff to eliminate crossover distortion

37

BJT Class B Amplifiers

38

The BJT as a Switch

When used as an electronic switch, a transistor

normally is operated alternately in cutoff and

saturation

A transistor is in cutoff when the base-emitter junction

is not forward-biased. VCE is approximately equal to

VCC

When the base-emitter junction is forward-biased and

there is enough base current to produce a maximum

collector current, the transistor is saturated

39

The BJT as a Switch

40

Education

Phy 4240 lec (9) and (10)