PART 4

PART 3TRANSISTORS and AMPLIFIERS

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 transferred 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 and one P-type layers of materials or two P and one N-type layers of semiconductor materials. Three regions of BJT a. Base

Region to which carriers flow from emitter to collector

1017 dopants/cm3 Moderately doped b. Emitter

Region from which carriers flow

1019 dopants/cm3 Heavily doped c. Collector

Region to which carriers flow

1015 dopants/cm3 Lightly doped

Largest

BJT Structure and Construction

Transistor Currents and Configurations a. 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

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:

( =

b. Common Emitter Configuration The input is applied at the base, the amplified output is taken from the collector, and the emitter is the common terminal. This circuit is the one generally used for transistors because the CE amplifier has the best combination of current and voltage gains

Ie =

Beta () Ratio of the collector current to the base current =

c. Common Collector Configuration

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

Impedance matching

Gamma (() Forward current gain for common collector configuration

( =

Comparison of Amplifier ConfigurationsCharacteristicCommon BaseCommon EmitterCommon

Collector

Power Gainmoderatehighestmoderate

Voltage Gainhighestmoderate less than 1

Current Gainlowest less than1moderatehighest

Input Impedancelowestmoderatehighest

Output Impedancehighestmoderatelowest

Phase Inversionnone180 Out of phasenone

ApplicationsRF ampuniversalisolation

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 a. Fixed Bias

is taken from a battery or power supply

b. 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

c. Voltage-Divider Bias

The most stable type of circuit biasing

d. Signal Bias

Regions of Transistor Action

a. Active Region Base-emitter junction is forward biased and the collector-base junction is reverse biased.

Transistors active operation as an amplifier

b. Saturation Region

both junctions are forward biased

switch on operation for the transistor

c. Cut off Region

both junctions are reverse biased

switch off operation for the transistorLoadline 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

BJT Small Signal Analysis

H-Parameters:1. hi - short circuit input impedance

; (Vo = 0)2. hr - open circuit reverse voltage gain (voltage feedback ratio)

hr = (Ii = 0)3. hf - short circuit forward current gain

hf = ( Vo = 0)4. ho - open circuit output admittance

ho = (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 VGSID = JFET/D-MOSFET transfer characteristics Can be n-channel or p-channel

JFET Symbols

Operation of JFET JFET is always operated with the gate-source PN junction reversed biased Reverse biasing of the gate-source junction with 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.

DC Biasing for JFET

1. Fixed Bias a separate power source.

2. Self Bias

3. Source Bias

4. Voltage Divider

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 the substrate material and connected by a narrow channel adjacent to the insulated gate

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

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.

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 a switch, this device is normally off

LD MOSEFT, VMOSFET and TMOSFET are EMOSFET technologies developed for higher power dissipation

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 or level of an electronic signal. Used as boosters

Classifications of Amplifiers:

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

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

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 3. According to Class of Operation

a. Class A Amplifier

b. Class B Amplifier

c. Class C Amplifier

d. Class AB AmplifierClass AClass BClass CClass AB

Efficiency50%78.5%100%Between A & B

ConductionAngle360(180(Below 180(Slightly greater than 180(

DistortionLowHighextrememoderate

Bias

(base emitter)Linear portionAbove cut-offBelow

cut-offcut-off

inputoutput

output

outputoutputoutput

Comparison of Amplifier Classes 4. According to Frequency

a. DC Amplifier Amplifies DC signal(zero frequency)

b. Audio Amplifier Amplifies signal whose frequency is within the audio range(20 Hz-20KHz)

c. RF Amplifier Amplifies signal whose frequency is within the radio frequency range

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 process5. According to the Signal being amplified

a. Small Signal Amplifiers Amplifier that utilizes only the very linear portion of the active device b. Large Signal Amplifiers Amplifier that utilizes almost the full rated output power of the active device6. According to Method of Coupling

a. Direct Coupling Amplifiers connected or coupled without any passive component in between.

b. Capacitive Coupling Amplifiers are connected or coupled by the used of capacitor termed as coupling-capacitor

c. Inductive Coupling Amplifiers are connected or coupled by the use of inductor or 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 opposition.

One amplifies half of the cycle and the remaining half is being amplified by the other amplifier

b. Complementary Symmetry Amplifiers

Push-pull amplifiers using complementary transistors such as pair of pnp and npn c. Quasi-Complementary Amplifier

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 = Av1Av2Av3.. AvnAv(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 the Darlington connection is that the composite transistor acts 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

(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.TEST YOURSELF 3Review Questions

1. A PNP transistor is made of

a. silicon

b. germanium

c. carbon

d. either silicon or germaniumAnswer d. either silicon or germanium2. The transistor is usually encapsulated in

a. graphite powder

b. enamel paint

c. epoxy raisin

d. black plasticAnswer c. epoxy raisin3. Power transistors are invariably provided with

a. solder connections

b. heat sink

c. metallic casing

d. screw bolt

Answer b. heat sink

4. The transistor specification number 2N refers to a

a. diode

b. junction transistor

c. FET with one gate

d. SCRAnswer b. junction transistor5. Which if the following is necessary for a transistor action?

a. the base region must be very wide

b. the base region must be very narrow

c.the base region must be made from insulating materials

d. the collector region must be heavily dopedAnswer b. the base region must be very narrow

6. As compared to a CB amplifier a CE amplifier has

a. low current amplification

b. higher current amplification

c.lower input resistance

d. higher input resistanceAnswer b. higher current amplification7. It is the most stable type of circuit biasing

a. self-bias

b. signal bias

c. voltage-divider bias

d. fixed biasAnswer c. voltage-divider bias8. The quiescent state of a transistor implies

a. zero bias

b. no output

c. no distortion

d. no input signalAnswer d. no input signal9. Each of the 2 cascaded stages has a voltage gain of 30. What is the overall gain?

a.3

b. 9

c. 30

d. 900Answer d. 900

Solution

Gtotal = (30)(30) = 90010. Which class of amplifiers operates with the least distortion?

a.Class A

b. Class B

c. Class C

d. Class DAnswer a. Class A11. Which of the following circuit is the fastest switching device?

a. JFET

b. BJT

c. MOSFET

d. TriodeAnswer c. MOSFET12. Which of the following device is a unipolar?

a. FET

b. BJT

c. Zener diode

d. LEDAnswer a. FET

13. The cascaded amplifier which is often used in the IC is

a. inductively coupled

b. capacitively coupled

c. direct coupled

d. transformer coupledAnswer c. direct coupled14. Highest operating frequency can be expected in case of

a. bipolar transistor

b. JFET

c. MOSFET

d. IGFETAnswer a. bipolar transistor15. Which of the following is expected to have the highest input impedance?

a. MOSFET

b. JFET amplifier

c. CE bipolar transistor

d. CC bipolar transistorAnswer a. MOSFET16. The ______is quite popular in digital circuits especially in CMOS which require very low power consumption.

a. JFET

b. BJT

c. D-type MOSFET

d. E-type MOSFETAnswer d. E-type MOSFET

17. What is the amplification factor in FET transistor amplifiers?

a. Zi

b. gm

c. ID

d. IGAnswer b. gm18. The E-MOSFET is quite popular in what type of applications.

a. digital circuitry

b. high frequency

c.buffering

d. a, b and cAnswer d. a, b and c

19. A JFET just operates with specifically

a. the drain connected to ground

b. gate to source PN junction forward biased

c. gate connected to the source

d. gate to source PN junction reverse biasedAnswer d. gate to source PN junction reverse biased

20. The main difference of a MOSFET from a JFET is that

a. JFET has PN junction

b. of the power rating

c. MOSFETS has two gates

d. MOSFETs do not have physical channelAnswer a. JFET has PN junction

21. A small signal amplifier

a. uses only a small portion of its loadline

b. always has an output signal in the mV range

c. goes into saturation once on each input cycle

d. is always a common emitter amplifierAnswer a. uses only a small portion of its loadline

22. The parameter hfe corresponds to

a. DC

b. AC

c. re

d. rcAnswer b. AC23. If the DC emitter current in a certain transistor amplifier is 3 mA, the approximate value of re is

a. 3K

b. 3

c. 8.33

d. 0.33 KAnswer c. 8.33 Solution

re =

24. The input resistance of a common base amplifier is

a. very low

b. very high

c. the same as CE

d. the same as CCAnswer a. very low25. Each stage of a four stage amplifier has a voltage gain of 15. The overall voltage gain is

a. 60

b.15

c. 50625

d. 3078Answer c. 50625

Solution

Vover-all = (15)(15)(15)(15) = 50 62526. The maximum efficiency of a transformer coupled Class A amplifier is__.

a. 25

b. 50

c. 78.5

d. 100Answer b. 5027. In a MOSFET, the process of creating a channel by the addition of charge carrier is called.

a. inducement

b. improvement

c. balancing

d. enhancementAnswer d. enhancement28. What is the current gain of a common base circuit called?

a. gamma

b. delta

c. bravo

d. alphaAnswer d. alpha29. The name of the very first transistor

a. diode

b. junction transistor

c. point contact transistor

d. triodeAnswer c. point contact transistor30. Region in a transistor that is heavily doped.

a. collector

b. emitter

c. base

d. gateAnswer b. emitter

31. In a common base amplifier the voltage gain is_____.(April, 2003)

a. medium

b. low

c. zero

d. highAnswer d. high32. In a common collector amplifier, the input resistance is___.(Nov,2003)

a. high

b. zero

c. medium

d. lowAnswer a. high33. A depletion MOSFET (D-MOSFET) can operate with which of the following gate-source voltage? (November, 2003)

1. zero

2. positive

3. negative

a. 1 only

b. 2 only

c. 3 only

d. 1, 2 and 3

Answer b. 3 only34. What problem is caused by a loosely coupled transformer in an RF amplifier? (April, 2004)

a. a too narrow bandpass

b. over coupling

c. optimum coupling

d. a too-wide bandpassAnswer a. a too narrow bandpass35. Normally, how are high power tubes tested? (April, 2004)

a. visually

b. individually

c. in their circuit

d. use portable testersAnswer c. in their circuit

_1200198429.vsd+ VDD

RL

+

ID

Vin

RG

-

VGS

RS

VS

_1200286651.unknown

_1200505932.vsdC

Vo

RB

RC

VCC

C

RE

_1206785234.vsdp

p

N-channel

Drain

Source

Gate

n

n

P-channel

Drain

Source

Types of JFET, Its Structure and Parts

Gate

_1206785272.vsdp

p

N-channel

Drain

Source

Gate

VDS

VGS

_1206786623.unknown

_1200506244.vsd

_1200506777.vsdInversion layer

D

S

G

n

n

+

+

+

-

-

Operation

_1200506795.vsd

_1200506635.vsd

_1200506139.vsd

_1200505351.vsdVI

C

Vo

RB

RC

VCC

C

RE

Emitter Stabilized

_1200505570.vsdVI

C

Vo

RB

RC

VCC

C

RE

Collector Stabilized

_1200286688.unknown

_1200223445.vsd

_1200243304.vsd

_1200286590.unknown

_1200286625.unknown

_1200246670.vsd

_1200242880.vsd

_1200243174.vsd

_1200234539.unknown

_1200198550.vsd+ VDD

RL

+

ID

Vin

RG

-

VGS

RS

-VSS

_1200218903.vsdDrain

Source

Gate

Drain

Source

Gate

n-channel

p-channel

_1200198541.vsd+ VDD

RL

+

ID

Vin

R2

-

VGS

RS

VS

R1

_1200160833.vsdVbb = 3V

10K

_1200165025.unknown

_1200165062.unknown

_1200198227.vsd+ VDD

RL

+

- VGG

ID

Vin

RG

-

VGS

_1200165046.unknown

_1200161938.vsdVI

Ii

hi

hrVo

hf Ii

ho

VO

Io

Transistor Hybrid Equivalent Circuit

_1200164860.unknown

_1200161572.vsdIc

Vce

25 mA

25 V

DC loadline

_1200139556.vsdVi

Ib

Ie

Ic

Vo

VBB

RB

RE

VCC

_1200141454.vsdVI

C

Vo

RL

RC

VCC

C

RE

R2

_1200160424.vsdIB

IB

IB

IB

IB

VCE

IC

VCC

Load Line

ACTIVE

CUT-OFF

SATURATION

BREAKDOWN

Q Point

_1200141155.vsdVI

C

Vo

RB

RC

VCC

C

_1200133425.vsdVi

E

C

B

Vo

VEE

RE

RC

VCC

Ie

Ic

_1200138597.vsdVi

Vo

VBB

RB

RE

VCC

Ib

Ic

Ie

_1200131489.vsdEmitter

Base

Collector

Substrate

Metal Contacts

EPITAXIAL PLANAR STRUCTURE

n

p

n

collector

emitter

base

p

n

p

collector

emitter

base

npn-type

pnp-type