Bradley 233 - Page 21

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4.1.

7

Differentiator

(Waveforms

;

Fig,

5)

Pin

2

of

ICSOb

goes

to

the

high

state

if

both

pin

4

and

5

are

in

the

low

state.

This

condition

happens

for

a

short

time

after

a

negative

edge

at

pins

6

and

4,

due

to

the

propagation

delay

through

ICSOa,

and

the

delay

caused

by

R36

and

the

input

capacitance

at

pin

5.

4.1.

8

Catching

Diode

The

emitter

follower

output

of

IC30

pin

14

prevents

the

voltage

at

pin

12

from

going

below

the

low

logic

level,

thus

enabling

it

to

be

latched

up

by

a

positive

logic

swing

at

pin

9,

through

C31.

The

low

impedance

output

of

the

emitter

follower

also

provides

a

short

recovery

time.

ICs

20d,

40d,

50d

and

60b

work

similarly.

107Ob

keeps

positive

going

edges

centred

around

the

threshold

whether

the

period

is

long

or

short

compared

with

the

differentiation

time

of

075.

4.1.

9

Lamp

Driver

Because

the

conduction

drop

of

D60

is

less

than

the

low

logic

level,the

output

of

1060c

pin

3

can

successfully

drive

the

light-emitting

diode.

In

the

low

state

the

emitter

follower

of

pin

3

is

off.

In

the

high

state

pin

3

supplies

all

the

current

through

R63

because

there

is

insufficient

voltage

drop

across

the

diode

to

make

it

conduct.

DELAY

A

OUTPUT

10

30

PIN

6

S

4

PIN

5

PIN

2

&

13

FIG.

5

DIFFERENTIATOR

WAVEFORMS

-

17

-