[Laser] TX Circuit for high speed data.

[Paul B. Webster VK2BZC]

On Sat, 2007-06-30 at 21:15 -0400, Chuck Hast wrote:

> I am assuming that the circuit was really designed for the low speed testing

  No doubt.

> Is there any problem with removing the 1K resistors and driving the gate
> directly with the TTL buffered data output?

  If you read John's (K3PGP) original article, you will find that these
resistors are provided purely and simply to prevent your driving circuit
feeding excess current to the laser in the event of a gate-to-drain
breakdown of the FET.  In fact, the only likely cause of such an event
in the first place, is over-voltage from an external modulating source,
such as an audio amplifier, signal generator, or so on.

  If on the other hand, your "modulator" is integral to the whole
assembly and operating from the same power supply (or a separate 5V
supply which also has a 5V Zener "crowbar" protection), then this
destructive breakdown is most unlikely.

  Now, note John's circuit for modulating multiple diodes using 1N4007s!
This demonstrates that in this shunt regulation circuit, the voltage
drop of such a diode is no problem, and since that diode *also* prevents
the adverse effect of a gate-to-drain breakdown of the FET, it renders
the series resistors quite unnecessary (except for a very brief current
"spike" due to the discharge time of the slow 1N4007 at the moment of
the breakdown - perhaps a single 10 ohm resistor might still be in
order).

  For non-linear work (and with the diode in the drain), the 74HC07 hex
buffer sounds quite OK - I am inclined to say you cannot put a
linearising resistor in the source lead but - I suppose you still can if
you are using all six stages in parallel.  Is there an actual advantage
to using the (second) 74HC07 instead of the logic FET - presumably it is
cheaper?

The shunt resistor was only ever a pulldown - has no connection with
response time.

74HC07 gates incidentally do not need balancing in parallel - they are
matched - on the same die - and self-balance due to their own source
resistance.

http://www.k3pgp.org/laserbias.htm
-- 
  Cheers,
    Paul B.