Transistors and Amplifiers (1)

AMPLIFIERS

TRANSISTORS AND

I. TRANSISTOR FUNDAMENTALS

Transistor Developed in December 23, 1947

in Bell Laboratories

By John Bardeen, William Shockley, and Walter Brattain

Basically a resistor that amplifies electrical impulses as they are from its input to its output terminals

Basic Types

1. Bipolar Junction Transistor (BJT)

It is a three layer semiconductor device consisting of either two N-type and one P-type layers of materials or two P-type and one N-type layers of semiconductor materials.

Three Regions of BJT

Base Region to which carriers flow from emitter to collector.

1017 dopants/ cm3 Moderately doped


Emitter Region from which carriers flow 1019 dopants/ cm3 Heavily doped


Collector Region to which carriers flow 1015 dopants/ cm3 Lightly doped Largest

BJT Structure and Construction

Metal contacts

Epitaxial Planar Structure

Substrate Collector

Base Emitter

BJT Structure and Construction collector collector

base base

emitter emitter

npn-type pnp-type

n

n

n p p

p

Transistor Currents and Configuration

Common Base Configuration

In this circuit, the input signal is applied at the emitter, the output is taken at the collector and the base is the common terminal.

This has very low input impedance.

Vi Vo

VEE VCC

E C

B Ie Ic

RE RC


Alpha () In the dc mode, the levels of IC and IE due to

majority carriers are related by a quantity called

alpha and defined by the following equation:

= Ic Ie

Transistor Currents and Configuration Common Emitter

Configuration The input is applied to

the base, the amplified output is taken from the collector and the emitter is the common terminal.

The circuit is the one generally used for transistors because this has the best combination of current and voltage gains.

Vi

Vo

Ic

Ie

Ib

RE

RB

VBB

VCC

Ie = Ib + Ic


Beta () the ratio of collector current to the base current .

= Ic Ib


Common Collector Configuration

This circuit has the input applied to the base, the output taken at the emitter terminal and the collector is the common terminal.

Impedance matching.

Vi

Vo

Ic

Ie

Ib

RE

RB

VBB

VCC


Gamma () the ratio of collector current to the base current .

= Ie Ib

Comparison of Amplifier Configurations

Characteristic Common Base Common Emitter Common Collector

Power Gain moderate highest moderate

Voltage Gain highest moderate less than 1

Current Gain lowest than1 moderate highest Input

Impedance lowest moderate highest

Output Impedance highest moderate highest

Phase Inversion none

180o out of phase none

Application RF amplifier universal isolation

Transistor Biasing

Bias An electrical, mechanical or magnetic force

applied to a device to establish a desired electrical or mechanical reference level for its operation.

Is a DC voltage or current that sets the operating point for amplifying the AC signal

Transistor Biasing Fixed Bias

Is taken from a battery or power supply

Vi Vo RB

VCC

C C

RC

Transistor Biasing Self Bias

The amplifier produces its own DC voltage from an IR drop across a resistor in the return circuit of the common terminal.

Self bias is probably the type of bias used most often because it is economical and has stabilizing effect on the DC level of the output current.

Can be emitter stabilized or collector stabilized.


Vi Vo RB

VCC

C C

RC

RE Emitter

Stabilized


Vi Vo

RB

VCC

C C

RC

Collector Stabilized

Transistor Biasing Voltage-Divider Bias

The most stable type of circuit biasing.

Vi Vo RL

VCC

C C

RC

RE R2

Transistor Biasing Signal Bias

Vo C C

RC

RE

RB

VCC

Regions of Transistor Action SA

TUR

ATIO

N

RL VCC

ACTIVE

CUT- OFF

BRE

AKDO

WN

Q-POINT

LOADLINE

VCE VCC

IB

IB

IB

IB

IB

IC

Regions of Transistor Action Active region Base-emitter junction is forward biased and the

collector-base junction is reversed biased.

Transistors active operation as an amplifier. Saturation region Both junctions are forward biased. Switch on operation for the transistor.

Cut off region Both junctions are reverse biased. Switch off operation for the transistor.

Loadline and Q-Point

Loadline - Is a straight line drawn on the collector

curves between the cut-off and saturation points of the transistor.

Q-point (Quiescent point ) - Is the operating point of the transistor with

the time varying sources out of the circuit.

Review Question: Given the circuit below, draw the DC loadline

Analysis: At cut-off, IC = 0 thus VCE = VCC At saturation, VCE = 0 thus IC = VCC / RC

VBB = 3V

VCC = 25V

10K

1K

Ic

VCE

DC Loadline

25 mA

25 V

BJT Small Signal Analysis

Vo Vi

Ii

hi

hr Vo

hi Ii hi

Io

Transistor Hybrid Equivalent Circuit


H - Parameters 1. hi short circuit input impedance

hi = Vi

Ii

2. hr open circuit reverse voltage gain (voltage feedback ratio)

(Vo = 0)

hr = Vi

Vo

(Ii = 0)


H - Parameters 3. hf short circuit forward current gain

hf =

Io

Ii

(Vo = 0)

4. ho open circuit output admittance

ho = Io

Vo

(Ii = 0)

2. Field Effect Transistor (FET)

Unipolar device because they operate only with one type of charge carrier.

Voltage controlled device where the voltage between two of the terminals (gate and source) controls the current through the device.

Major feature is very high input resistance.

a. Junction Field Effect Transistor (JFET)

Operates with a reverse-biased PN junction to control current in the channel .

Square law device because of the relation of ID and VGS

ID = IDSS VGS

VGS(OFF) JFET/ D-MOSFET transfer characteristics

1 - 2

can be n-channel or p-channel

n-ch

anne

l

p-ch

anne

l

p p n n

drain

gate

source

drain

gate

source

D D G G

S S

Types of JFET, its structure

and parts

JFET Symbol

n-channel p-channel

Operation of JFET

JFET is always operated with the gate-source PN junction reversed biased.

Reverse biasing of the gate source junction with the negative voltage produces a depletion region along the PN junction which extends into the n-channel and thus increases its resistance by restricting the channel width as shown in the preceding figure.

Operation of JFET

n-ch

ann

el

p p

drain

gate

source

VGS

VDS

Operation of JFET

Pinch off Region

Bre

akdo

wn

Reg

ion

Ohm

ic R

egio

n

Vp pinch off voltage Va avalanche breakdown voltage

DC Biasing for JFET

1. Fixed Bias - a separate power source.

Vin

RG

RL

VDD

VGS

VGG

-

+

-

+

ID

DC Biasing for JFET

2. Self Bias

Vin

RG

RL

VDD

VGS

-

+

RS VS

+

ID

DC Biasing for JFET

3. Source Bias

Vin

RG

RL

VDD

VGS RS

+

ID

- VSS

-

+

DC Biasing for JFET

4. Voltage Divider

Vin

R2

RL

VDD

VGS RS

+

ID

VS

-

+

R1

b. Metal Oxide Semiconductor Field Effect Transistor (MOSFET)

Second category of the field effect transistor

Because of the presence of an insulated gate, then it is sometimes called IGFETs

MOSFETs differs from JFET in that it has no PN junction structure.

It has two basic types: D MOSFET and E - MOSFET

Depletion MOSFET (D - MOSFET)

The drain and source are diffused into substrate material and connected by a narrow channel adjacent to the insulated gate

It can be operated two in modes, the depletion mode or the enhancement mode and sometimes called depletion/enhancement mode MOSFET

Depletion MOSFET (D - MOSFET)

It can be operated with a zero, positive or negative gate-source voltage.

Normally operated in the depletion mode. When configured as switch, it is normally-

on.

D D

G G

S S

n-channel p-channel

p-substrate n-substrate

drain

gate

source

gate

drain

source

n-channel D-MOSFET p-channel D-MOSFET

SiO2 SiO2

Depletion Mode Negative gate to

source voltage is applied

n-channel is depleted of some electrons hence decreasing channel conductivity.

Enhancement

Mode Positive gate voltage

is applied.

More conduction electrons are attracted to the channel thus enhancing channel conductivity.

Enhancement MOSFET (E - MOSFET)

Operates only in the enhancement mode Has no depletion mode It has no structural channel It has no IDSS parameter For an n-channel type of this device, a positive

gate voltage above threshold induces a channel by creating a layer of negative charges (inversion layer) in the substrate portion that is adjacent to the SiO2 layer.

Enhancement MOSFET (E - MOSFET)

An n-channel E-MOSFET has a positive VGS while a p-channel E-MOSFET has a negative VGS.

The conductivity of its channel is enhanced by increasing the gate to source voltage.

For gate voltage below the threshold, there is no channel to be formed.

If configured as switch, this device is normally off LD MOSFET, VMOSFET and TMOSFET are E-

MOSFET technologies developed for higher power dissipation.

D

G

S

p-substrate

SiO2 n

n

No permanent

channel

D

S

G n

n +

+

-

- Inversion

layer

Basic construction Operation drain

gate

source source

drain gate

n-channel p-channel

II. AMPLIFIERS

Electronic devices capable of amplification or increasing the amplitude of power, current or voltage at its output.

Circuits designed to increase the amplitude of level of an electronic signal.

Used as boosters.

AMPLIFIER input output

Classification of Amplifier

1. According to Function a. Voltage Amplifier

- Voltage controlled source

- Op-amps are voltage amplifier

b. Current Amplifier

- current controlled source

- BJTs are current amplifier c. Power Amplifier

- Boost the power level of the signal


2. According to Configuration a. Common Base Amplifier

- Transistor amplifier where input is applied at the emitter and output is taken from the collector terminal.

- The base is common to both input and output.

- maximum current gain is 1

- No phase inversion from input to output .


2. According to Configuration b. Common Collector Amplifier (emitter

follower)

- Transistor amplifier where input is applied at the base, output is taken from the emitter terminal.

- Maximum voltage gain is 1.

- Capacitors must have a negligible reactance at the frequency of operation.

- No phase inversion from input to output.


2. According to Configuration c. Common Emitter Amplifier

- Transistor amplifier wherein the input is applied at the base and the output is taken from the collector terminal.

- There is a phase inversion from input to output.

Classification of Amplifier 3. According to Class of Operation

Class A Class B Class C Class AB Efficiency 50 % 78.5 % 100 % Between A & B Conduction

Angle 360O 180O Below 360O

Slightly greater than

180O

Distortion Low High Extreme Moderate Bias (Base

Emitter) Linear portion

Above Cut-off

Below Cut-off Cut-off

Input Output Output Output Output


4. According to Frequency a. DC Amplifier

- amplifies DC signal.

b. Audio Amplifier

- amplifies signal whose frequency is within the audio range (20 Hz 20 KHz).

c. RF Amplifier

- amplifies signal whose frequency is within the radio frequency range.

Classification of Amplifier 4. According to Frequency d. IF Amplifier

- amplifies signal whose frequency is in between the carrier and the modulating frequency.

e. Video Amplifier

- a wide band amplifier that amplifies video signal.

- video signal refers to the frequency range of the picture information which arises from the television scanning process.


5. According to the signal being amplified a. Small Signal Amplifiers

- Amplifier that utilizes only the very linear portion of the

b. Large Signal Amplifiers

- Amplifier that utilizes almmost the full rated output power


6. According to method of coupling a. Direct Coupling

- Amplifiers connected or coupled without any passive

b. Capacitive Coupling

- Amplifiers are connected or coupled by the used


6. According to method of coupling c. Inductive Coupling

- Amplifiers are connected or coupled by the use of inductor transformer.

d. Transformer Coupling

- Most often, inductor is not used as coupling device instead transformer is used.


7. Power Amplifiers a. Push-Pull Amplifiers

- Amplifier with two similar circuits operating in phase.

- On amplifies the half of the cycle and the remaining half is being amplified by the other amplifier.


7. Power Amplifiers b. Complementary-Symmetry Amplifiers

- Push-pull amplifiers using complementary transistors such as pair of pnp and npn.

c. Quasi-Complementary Amplifiers

- Push-pull amplifiers using the same transistors at the output but the driver is using complementary transistors.

Compound Configurations

a. Cascade Connection - a cascade connection is a series

connection with the output of one stage then applied as input to the second stage.

- The cascade connection provides a multiplication of the gain of each stage for a larger overall gain.

AV = AV1AV2AV3AVn AV(dB) = 20Log(AV)


b. Cascode Connection - a cascode connection has one transistor on

top of (in series with) another.

- This arrangement is design to provide high input impedance with low voltage gain to ensure that the input Miller capacitance is minimum.


c. Darlington Connection - The main feature of Darlington connection

is that the composite transistor acts as a single unit with a current gain that is the product of the current gains of the individual transistors.

- It is a circuit meant to boost input resistance.


c. Darlington Connection

1 2 D = 1 2


d. Feedback Pair - The feedback pair connection is a two

transistor circuit that operates like the Darlington circuit.

- It uses a pnp transistor driving an npn transistor.

Transistors and Amplifiers (1)

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