Massive Audio DB8000 Manuel d'utilisateur télécharger pdf (Page 48)

DRAIN

DRAIN

GATE w

GATE

SOURCE

(A)

SOURCE

(8)

DRAIN

(C)

sembled as tetrode devices, with the

sub-

strate

body ( often identified as

gate

2)

connected to a

separate

electrode.

Since

the drain and

source are isolated from

the substrate,

any drain -to- source

cur-

rent

in

the absence of gate voltage is

extremely low because, electrically,

the

structure

is

equivalent to

two

diodes

connected back

to

back.

Insulated

-gate

FET's

have

extremely

high

input

impedances- higher, in

fact,

than many vacuum tubes -but are very

sensitive to

stray

electrical

charges and

can

be

destroyed

by body static. Input

impedances higher

than

10

million meg-

ohms

are

not uncommon.

Manufacturers

generally wrap

IGFET

leads

in metallic

foil,

or supply them with the leads held

together by a metal eyelet as a protec-

tive

measure. Extra

care

must be

taken

during

installation,

wiring,

and testing

of the

IGFET

to prevent its destruction.

The

junction

field

-effect transistor

(JFET) shown

in Figs.

7 and 8 can be

made

as an n- channel or a p- channel

device. As with

conventional junction

transistors, JFET's are identified

by the

slightly modified schematic

symbols

shown

in Figs. 10

(

a)

and

10

( b) . With

the

source

considered common,

an

n-

channel

FET

requires

a positive

drain

voltage and a

negative

gate bias

; the

INPUT

GND

Fig. 10.

Schematic symbols currently

used for field -effect

transistors

include

(a)

n-

channel JFET, (b) p- channel

JFET,

and (c) one

form

of

p-

channel

IGFET.

p- channel

FET is operated with a

nega-

tive drain

voltage and a positive gate

bias.

As

shown

in

Fig. 10

(c)

,

the

IGFET is

identified by an entirely different sym-

bol. This general type

of FET

is offered

in

two

basic

forms

and

in many individ-

ual types with different

electrical speci-

fications

and

operating characteristics.

Unlike

the JFET, however,

a given

IGFET may require either

a positive

or

negative

gate bias, with respect

to its

source,

depending

on mode

of operation.

In

addition to

regular FET's,

light -

sensitive

FET's

are being produced

by

a

number

of

manufacturers.

Called

photo -

FET's,

they are similar to

conventional

FET's

but are equipped

with transparent

lenses that focus

external light on

their

sensitive surface

areas.

The

photoFET

can

be

up to ten times as sensitive

as a

junction phototransistor,

and

has

a

bet-

ter

gain

bandwidth factor,

in

addition to

offering exceptional isolation between

input and output circuitry.

Terminology.

As

with

any new

tech-

nology,

a

number

of terms

are

used to

describe

FET

devices,

and their charac-

teristics. Some terms are used

primarily

by manufacturers,

others

chiefly

by

cir-

cuit

designers. Unfortunately,

the

terms

Fig.

11.

This FET

volt-

meter,

featuring

a

matched

pair of Sili-

conix U112 FET's

in

a

differential

ampli-

fier

arrangement, has

a sensitivity

of 0.5-

1.0 volt full

scale.

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