From Newsgroup: sci.electronics.design
Joerg <
news@analogconsultants.com> wrote:
On 7/21/26 11:23 AM, Phil Hobbs wrote:
On 2026-07-21 12:04, john larkin wrote:
On Tue, 21 Jul 2026 10:46:55 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
[...]
I'm thinking a high-side series GaN FET driven by a transformer, like >>>> this:
IbiasDC-a (very very quiet)
0------*--||--*
-a-a-a-a-a-a-a |-a-a-a-a-a |
-a-a-a-a-a-a-a |-a-a-a-a GND
-a-a-a-a-a-a-a *--|
-a-a-a-a-a-a-a-a-a-a |-----*-a-a-a *--||-- Gate drive
-a-a-a-a-a-a-a *--|-a-a-a-a |-a-a-a |
-a-a-a-a-a-a-a |-a-a-a-a-a-a-a 3 || 3
-a-a-a-a-a-a-a |-a-a-a-a-a-a-a 3 || 3
-a-a-a-a-a-a-a |-a-a-a-a-a-a-a |-a-a-a |
-a-a *----*--------*-a-a GND
-a-a |-a-a-a |
-a-a |-a-a ---
-a ----a \ / --
-a ----a-a V-a-a /
-a-a |-a-a ----a -->
-a-a |-a-a-a |
-a GND-a GND
with the laser bypass chosen so that the current doesn't drop too far
below threshold in the OFF periods, and the Ibias bypass big enough to >>>> keep the bias supply running normally without destroying its
sub-Poissonian character below 1 MHz.
This approach saves dissipation, which is important for satellite use. >>>> (There are terrestrial applications too, but its space that's paying the >>>> bills at the moment.)
It may be better to use a push-pull architecture and have the base of
that at an elevated voltage where the laser diode just barely does not
yet turn on. That way you don't have to muscle through its capacitance
every time.
Cheers
Phil Hobbs
You might consider an RF technique, like a series RC or tuned circuit
or something, matched into the laser diode. From a GaN fet running at
low voltage maybe.
That could be low power, which probably matters in space.
MMICS waste a lot of power, as do diffamps.
My hope is to make a little tile with an EPC8004 (52 pF Cin) resonated
with a few nanohenries of inductance, and drive it at a Q of around 3.
The voltage gain of the FET will be less than 1, so Mr. Miller shouldn't
be a big problem.
I just don't have a lot of experience doing 1-ns stuff with GaN FETs.
It'll be a lot of experimenting. GaN aren't the only game in town. LDMOS sometimes come in pairs but it can be hard to find them for such low
power. Assuming you need pulses here and there you'd drive them through
a small RF transformer so it becomes push-pull despite the common source architecture. Like an RF power amp. Instead of GND, the base of the secondary of the transformer which is connected to the laser diode could
be at an elevated DC level so the laser diode remains somewhat pre-biased.
Right, the notion would be to choose the capacitor values and operating frequency such that the bias current is stable at low frequency to control
the noise down there, but drops below threshold long enough that the oscillation has to build up from noise on each cycle.
Because of the hideously wide chirp, that last might not even matterrCoitrCOs an analogy to old oscillator-keyed radars, but the radar oscillator has
stable circuit constants, which the laser really doesnrCOt.
Cheers
Phil Hobbs
--
Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics
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