Discrete Schottky?

[Watson A.Name]

IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode
between base and collector to prevent the collector from going below a
certain voltage into saturation, thus speeding up the turn-on. I've
never seen a discrete circuit using this, but is it a workable
solution to put a 1N5711 schottky signal diode between the base and
collector of a 2N2369A or 2N3904 for example, to prevent it from
saturating, and help speed up its switching? I was thinking of
driving an IR LED with a signal that's fairly quick, and I don't think
that an open collector TTL chip would have enough current. But then
maybe parallel open collectors? Or what?

 

[Rene Tschaggelar]

What speed did you have in mind ?
Perhaps with a FET ?

Rene

Watson A.Name wrote:

IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode
between base and collector to prevent the collector from going below a
certain voltage into saturation, thus speeding up the turn-on. I've
never seen a discrete circuit using this, but is it a workable
solution to put a 1N5711 schottky signal diode between the base and
collector of a 2N2369A or 2N3904 for example, to prevent it from
saturating, and help speed up its switching? I was thinking of
driving an IR LED with a signal that's fairly quick, and I don't think
that an open collector TTL chip would have enough current. But then
maybe parallel open collectors? Or what?

 

[John Larkin]

On 3 Dec 2004 07:35:20 -0800, veryfree123@hotmail.com (Watson A.Name)
wrote:

IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode
between base and collector to prevent the collector from going below a
certain voltage into saturation, thus speeding up the turn-on. I've
never seen a discrete circuit using this, but is it a workable
solution to put a 1N5711 schottky signal diode between the base and
collector of a 2N2369A or 2N3904 for example, to prevent it from
saturating, and help speed up its switching? I was thinking of
driving an IR LED with a signal that's fairly quick, and I don't think
that an open collector TTL chip would have enough current. But then
maybe parallel open collectors? Or what?

How fast? How much current?

John

 

[Larry Brasfield]

"Jim Thompson" <thegreatone@example.com> wrote in message
news:m241r09bp2ub2iv5st4j8uvlg6c4dfjrq2@4ax.com...
....

(1) How "quick" does ANY circuit need to be when flashing an LED?

Maybe it is signaling to something not a person?

(2) A 2N2369A is gold-doped, thus does not need to be
Schottky-clamped to prevent charge storage.

Gold doped BJTs still exhibit charge storage. That doping
reduces carrier lifetime, so such transistors can switch off
in a few 10's of nS, but preventing saturation will still speed
up the turn-off.

If speed mattered, I would use a small MOSFET. They are
inexpensive and good for interface to logic.

....

...Jim Thompson

--
--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.

 

[clicliclic]

veryfree123@hotmail.com (Watson A.Name) wrote in message news:<18ebe27.0412030735.60f39e82@posting.google.com>...

IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode
between base and collector to prevent the collector from going below a
certain voltage into saturation, thus speeding up the turn-on. I've
never seen a discrete circuit using this, but is it a workable
solution to put a 1N5711 schottky signal diode between the base and
collector of a 2N2369A or 2N3904 for example, to prevent it from
saturating, and help speed up its switching? I was thinking of
driving an IR LED with a signal that's fairly quick, and I don't think
that an open collector TTL chip would have enough current. But then
maybe parallel open collectors? Or what?

Have a look the Hewlett-Packard Application Note 1066: "Fiber-Optic
Solutions for 125 MBd Data Communication Applications at Copper Wire
Prices". They suggest direct drive by three 74HCTQ00 gates in
parallel. They easily achive rise/fall times below 10ns; the problem
then is the LED, not the driver.

Martin.

 

[Nico Coesel]

veryfree123@hotmail.com (Watson A.Name) wrote:

IIRC, with the LSTTL chips, the schottky TTLs use a schottky diode
between base and collector to prevent the collector from going below a
certain voltage into saturation, thus speeding up the turn-on. I've
never seen a discrete circuit using this, but is it a workable
solution to put a 1N5711 schottky signal diode between the base and
collector of a 2N2369A or 2N3904 for example, to prevent it from
saturating, and help speed up its switching? I was thinking of
driving an IR LED with a signal that's fairly quick, and I don't think
that an open collector TTL chip would have enough current. But then
maybe parallel open collectors? Or what?

I usually put a diode parallel with the base resistor to discharge the
base capacitance more quickly. But like the others already asked,
what's the required speed? And why aren't you connecting the LED
directly to the open collector output?

--
Reply to nico@nctdevpuntnl (punt=.)
Bedrijven en winkels vindt U op www.adresboekje.nl

 

[Winfield Hill]

Jim Thompson wrote...

On Fri, 3 Dec 2004 11:02:14 -0800, "Larry Brasfield"
donotspam_larry_brasfield@hotmail.com> wrote:

"Jim Thompson" <thegreatone@example.com> wrote in message
news:m241r09bp2ub2iv5st4j8uvlg6c4dfjrq2@4ax.com...
...
(1) How "quick" does ANY circuit need to be when flashing an LED?

Maybe it is signaling to something not a person?

(2) A 2N2369A is gold-doped, thus does not need to be
Schottky-clamped to prevent charge storage.

Gold doped BJTs still exhibit charge storage. That doping
reduces carrier lifetime, so such transistors can switch off
in a few 10's of nS, but preventing saturation will still speed
up the turn-off.

If speed mattered, I would use a small MOSFET. They are
inexpensive and good for interface to logic.


The 2N2369 at 10mA load turns off in 30ns. With the 1N5711's CJO=2pF
I doubt that you will get much improvement... might even be worse.

...Jim Thompson

--
Thanks,
- Win

 

[Watson A.Name - \"Watt Su]

"Fred Bloggs" <nospam@nospam.com> wrote in message
news:41B1B66E.2020507@nospam.com...

Ken Smith wrote:



In discrete land, the voltage swing is often too large for just a
schottky. In those cases a "Baker clamp"[1] is used. I have used
the
schottky clamp method at 10V and had it work.


[1] Baker clamp is like this


Simple version:



D1
-------+------->!--------+----
! !
! D2 !/
------->!------!
!\ e
!

It takes away base drive when the collector gets down to one diode
drop.
D2 is often the E-B junction of another transistor. This circuit
works ok
but the speed and capacitance of D1 is an issue during turn off.



All that said, you want to use a MOSFET to drive the IR LED. They
switch
quite fast with no extra work. The Supertex TN0604 driven by HC
logic
like this:

+12V
+-------+
! !
\ \
/R1 /R2
! \
V !
--- !
! !
!!----+-------
-----!!
!!--
!
GND

R1 sets the diodes current.

R2 speeds up the discharging of capacitances during turn off. It
needs to
be about 100-1K depending on how fast you need.


There are plenty of drivers that avoid saturation and charge recovery
delays.
View in a fixed-width font such as Courier.


.
.
.
. V
. dd
. |
. +-------+
. | |
. | c
. |\ |/ Voh-Vbe,on-Vled(I)
. ----| >----| I~ ------------------
. |/ |\ Re
. CMOS e
. buffer |
. / saturation not possible
. Re
. /
. \
. |
. ---
. \ /
. ---
. |
. gnd
.
.
.
.
.
.
.
. V+
. |
. ---
. \ /
. V ---
. dd |
. | c
. |\ |/ Voh-Vbe,on
. ----| >----| I~ ----------
. |/ |\ Re
. CMOS e
. buffer |
. / V+> Vo,h + Vled(I) + 0.7V
. Re
. /
. \
. | saturation not possible
. gnd
.

Thank you to all for all the advice. In the schem directly above, if the
Vdd is the same as V+, which is usually the case with only the single
supply, the conditions couldn't be met. So using the circuit in the
first schem above would be the wiser choice.

Here's how a guy in Prague, Czech Republic does it. He uses massively
paralleled 74HC04s to drive the HP spider LED.

http://ronja.twibright.com/transmitter/building.php (scroll down to near
bottom for the schem)And for the main project page:
http://ronja.twibright.com/main.php

 

[James Arthur]

On 12/4/04 The dark-vanquishing "Watson A.Name" wrote:

<fast LED drivers snipped>

Thank you to all for all the advice. In the schem directly above, if the
Vdd is the same as V+, which is usually the case with only the single
supply, the conditions couldn't be met. So using the circuit in the
first schem above would be the wiser choice.

Here's how a guy in Prague, Czech Republic does it. He uses massively
paralleled 74HC04s to drive the HP spider LED.

http://ronja.twibright.com/transmitter/building.php (scroll down to near
bottom for the schem)And for the main project page:
http://ronja.twibright.com/main.php

Second the 2n2369a idea, which can be darn quick
when driven by a modern CMOS gate.

If you're set on push-pull drive as in the cool 'HC04 driver
you cited above, other CMOS families, such as 74AC, would
be much faster & stronger with fewer driver gates needed.

Need any free parts? -- they're yours for the asking.

Cheers,
James Arthur

 

[Nicholas O. Lindan]

"James Arthur" <arthurj@aol.comet.net> wrote

Need any free parts? -- they're yours for the asking.

You _always_ pay, how is sometimes not apparent, but you
always pay.

Naked ye came, naked ye go. The book of accounts opens
with $0.00 and it closes with $0.00.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer: Electronics; Informatics; Photonics.
Remove spaces etc. to reply: n o lindan at net com dot com
psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

 

[John Larkin]

On 05 Dec 2004 08:23:07 GMT, arthurj@aol.comet.net (James Arthur)
wrote:

Not bought: a 5 1/2 digit Fluke DMM, offered for $25. I
hemmed over this one a while, but, already having three of like
capability, couldn't justify yet another.

The real hazard is ebay: it's a 24/7 flea market. I have bought
*eight* 1180x sampling scopes so far. Somebody stop me!

Electronics is fun.

Yup.

John