Radio Shack SSM-1750 User Manual download pdf (Page 44)

its

input

when triggered

and

con-

tinues

to "hold"

that

level until an-

other command

is

issued.

When the

control

line

goes

high, the

ICl OTA

acts

as

a closed

switch

and charges

CI

until

the

input and

output

voltages are equal.

With the

line

low,

the "switch"

opens

so

that the

volt-

age

is stored

in Cl.

To

prevent

leak-

age

currents

from

introducing

er-

rors, use a

low

input bias operational

amplifier

like the CA3140

or

LM351

for

IC2 and polyester

or

polystyrene

capacitor

for Cl.

There

are quite

a

few uses

for the

sample

/hold

circuit

in electronics.

For

example,

it

can

be

used

to create

a random

music box,

as

shown

in

Fig. 6.

Here

IC3

is

a

simple

volt-

age-

controlled

oscillator

(vco)

that

acts as

a tone

source

and serves

as

an

input for

the

Fig. 5 sample

/hold

cir-

cuit.

Trigger

generator

IC4 can

be

varied

from 0.5

Hz to

beyond

10

kHz.

The sample

/hold

circuit

picks

voltages off

the triangle

-wave

output

of

IC3 and

feeds them

back

to

the

vco to give

the

familiar

Hollywood

"computer

language"

sound

used

in

motion

pictures

and

TV shows.

You

are

likely

to find

this

neat little

toy

quickly

appropriated

by your

chil-

dren,

however.

You

may

have noticed

that

the cir-

cuits

in Figs.

4 and

5 lack

the

input

attenuators

that

were

necessary

in

the

vca circuits.

This

is

possible

be-

cause

here the

10- to -40

-mV

level ap-

plies

the difference

between

the -

and +

inputs.

Since

one of

the

vca's

inputs

in Figs.

2 and 3

was grounded,

the

other

input

had to be

near

ground

potential.

In a switching

circuit,

however,

when the switch

is open,

no

signal

can

get through.

Also, distor-

tion

is meaningless.

When

the switch

is closed,

the output

buffer

sends

the

input

signal

back

to the

remaining

in-

put.

With

both

inputs

at the

same

level,

distortion

is

not a problem.

Some

Available

OTAs

In this

article,

we showed

representa-

tive

circuits

built

around

RCA's

Output

T047

Trigger

I

I I

I

I

Output

Fig. S.

A typical

OTA

sample

/hold

circuit.

CA3080

because

this OTA

is readily

available

and

inexpensive

to pur-

chase.

It costs about

$1.00

from

mail -order

suppliers.

RCA (Solid

State

Div.,

P.O.

Box

3200,

Sommer-

ville, NJ 08876)

has been

marketing

the

CA3080

since

1971.

It is

still

used

in

new designs

today.

It may suffer

from

a severely

limited

input

level

that compromises

its signal

-to -noise

ratio, but the CA3080

is still useful

where this

is not an

important

con-

sideration.

Note, too,

that

RCA also

manufactures

the CA3280,

a dual

version of the CA3080.

National

Semiconductor's

(2900

Semiconductor

Dr., Santa

Clara,

CA

95051)

very

interesting

and

ver-

satile

LM13600

dual

OTA

has sever-

al

features

not

found elsewhere.

Among these

are

linearizing

diodes

that

reduce

signal

distortion

and

an

internal

buffer

for each

OTA.

Be-

cause

the

buffers

are

not

internally

wired, you

can substitute

an external

buffer

if desired.

National's

applica-

tions

booklet

for the

LM13600

has a

variety of

circuits

for

the experi-

menter,

including

a stereo

volume

control,

voltage- controlled

filters

(vcfs), and a two

-chip

sine -wave

vco

with

a range

of 5

Hz to 50

kHz,

the

last at

less

than

1% distortion.

Since

audio signal

processing

and

electronic

music

make

up a signifi-

cant portion

of

the

OTA

market,

it

should

come

as no surprise

that

man-

ufacturers

have

designed

ICs espe-

cially

for audio

applications

.While

these

devices

are

much

more expen-

sive

than

the

CA3080,

their

im-

(Continued

on

page

83)

O

Fig. 6.

Adding

these

two

IC circuits

and a few

other

parts

to

the sample

/hold

cir-

cuit

in Fig.

5 produces

an entertaining

random

music

machine.

10K

+v

.1pF

_.0013F

1K

2 2K

To

audio

amplifier

1p F

100K

AI

10K

From

output

of

Fig. 5

To

input of

Fig. 5

10K

NOTE:

_

D =

1N914

2pF

I

IBI

1K

To

trigger

of

Fig.

5

50 / MODERN

ELECTRONICS / September

1986

1 2 ... 39 40 41 42 43 44 45 46 47 48 49 ... 83 84

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