[Laser] TX Circuit for high speed data.

[James Whitfield]

From: "Paul B. Webster VK2BZC"
>
>   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.................

It is true that the voltage drop is not a problem.  The shunt circuit, which
looks like a regulator but is not used as such for this application, need
only "steal" enough current from the laser diode to quench the laser action.
It may still have sufficient current flowing through the laser diode for it
to continue to emit light, just not with the intensity of the laser.  If you
are testing the circuit with an LED, its linearity will continue to emit
significant light untill the LED trigger voltage is lost.  As such an LED
test may show more degradation ( rounding ) of the signal than the laser
diode will.

>................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).
>
I do not see how the diode would prevent gate to drain breakdown.

>
> 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.
>
I looked at the data sheet which shows the "equivalent circuit".  The output
circuit shown has the output pin connected to the drain and the source
grounded.  Neither line has shows resistance to limit current in the "ON"
state, but of course the "real" devices will have some small resistance in
in the trace conductors and measureable voltage drop across each output FET
which will vary with load current.  I made the suggestion to add balancing
resistors based on my own, limited and ancient, experience with paralleling
logic devices for power to drive atypical devices.   I presume that Paul has
more experience than I with modern logic devices.  That seems to allow him
to assert that all six buffers, being on the same die, are matched and will
track each other sufficiently to be used in this circuit effectively "wire
OR'ed".  To do so will simplify the modification to the circuit and I see no
risk to any of the other components.


73

James
 n5gui