> I've guess the schematic you've point could go at max. 50KHz.
> Can you measure the output modulation with a PIN photodiode
> ( or a standard photodiode if you haven't a PIN one ) and a scope?

Suppose I could when I have a little time

As you say, the 4093 will limit the frequency, although a 74F132
would be OK to 50MHz, if you really need to use a gate at all on
the bypass' base

Is the BD139 too slow ? > 200MHz ? Anyway, there are plenty
of HF power transistors around if you don't like the BD139

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.