Massive Audio DB8000 User Manual download pdf (Page 44)

TRANSISTOR

(from

TRANsfer and

re-

SISTOR), the device

was

nothing

than

a tiny cube of crystalline

semicon-

ductor material

with two

fine wire

cat's

whiskers.

A minute voltage applied

the base crystal

(thereafter called

the

base)

controlled

a much larger current

flowing

between the two whiskers, one

of which was

called

the emitter,

and

the

other

collector. The

early

transistors

were expensive, noisy, and

not

too

re-

liable. But these

disadvantages

were

off-

set

their extremely

small size, high

efficiency

and,

potentially

least, manu-

facturing

simplicity.

1951, long

before

this early

point

contact

transistor

posed even a mild

threat to the supremacy of

the

vacuum

tube,

radically new

type

of transistor,

the now

common and widely used

junc-

tion

transistor was

introduced.

Of Tubes and Transistors. Although

godsend in many ways,

transistors

brought a host of new problems to

cir-

cuit

designers.

Essentially

a current

amplifier,

the device could

not

be used

direct replacement for

the

vacuum

tube, which is

voltage

amplifier.

had a low -to-

moderate

input imped-

ance

contrast to

the

very high input

impedance

vacuum

tubes.

addition,

because the

transistor

has

direct re-

sistive connection between its

input

(base) and output

(collector) terminals,

a multiplicity of circuit feedback prob-

lems

had to be solved.

Improved

design

methods

were devel-

oped later, and transistorized receivers,

amplifiers,

transmitters,

hearing aids,

toys and

industrial

controls were pro-

duced

vast quantities.

But

there were

still many

circuit

requirements where

only

high- impedance

vacuum tubes could

fill

the bill, and

many

designers

yearned

for

miracle

-Iike device -a

transistor

with tube -like characteristics.

time went by, transistors

got

bet-

ter and better. Output voltage

and

cur-

rent ratings

were

being extended,

were

the upper operating frequency limits.

But

no matter how

the

newer

transistors

were improved,

they still had the basic

characteristics of

earlier types.

Meanwhile,

back

at the

laboratory,

scientists were experimenting

with a

new

solid -state device, based on

molec-

ular

principle

described by

Lilienfeld

far back as

1928.

Shockley, one

of the

co-

inventors

of the original transistor,

had proposed a practical transistor

-like

device based

Lilienfeld's

principle as

early

1948, but it was

not

until

the

mid 1950's

that

a workable device was

developed

the laboratories, and prac-

tical, reliable units

were

not

manufac-

tured

until

the

early

1960's.

The new

device

combined

the

most

de-

sirable features

of the

versatile

vacuum

tube and the efficient transistor.

It had

high input impedance

and offered

good

isolation between input and output elec-

trodes. Capable of

high

gain,

was,

the same time,

as small as conventional

transistors

and

extremely

efficient.

And,

oddly enough,

exhibited at

least one

of the

important operating

character-

istics of the

vacuum tube -the

control

of a current by

means

a varying elec-

tric

field

-in

solid

-state medium rather

than in a vacuum.

Identified by

variety

names

Fieldistor,

unipolar

transistor,

and so

on- during

its

gestation

period, the de-

vice is now known

as the field

-effect

transistor

(FET) . It is, indeed,

a tran-

sistor which

"thinks" and "acts"

like a

tube.

Meet Mr. FET.

Pictorial and

schematic

representations

of a triode

vacuum

tube,

junction transistor,

and field -effect

tran-

sistor are

illustrated in Figs. 1

through

3. Of the three schematic

symbols, the

FET

symbol

the

least

standardized

present.

In a vacuum

tube

(Fig.

1) ,

the plate

current

simply a

flow

free electrons

which are literally

"boiled" off of the

cathode

by the heated

filament

(in some

high -power tubes,

the

filament

is used

directly) and are attracted

the

posi-

tively-

biased plate.

The

electrons

leav-

ing

the cathode

must

travel through the

intervening grid.

A negative bias

on the

grid

establishes

an electric

field which tends

repel

the

electrons

flowing

from cathode

to plate,

limiting the plate current.

The plate

current can also be controlled, within

limits, by the plate voltage.

However,

since

the

grid is

much

closer

the

cathode than the plate, a smaller varia-

tion

in grid voltage has essentially

the

same

or greater effect on the plate cur-

rent as

a larger variation in plate volt-

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