Re: fast discrete PHEMT one-shot

On Thu, 16 May 2024 00:00:56 -0000 (UTC), Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2024-05-15 12:47, John Larkin wrote:

 
https://www.dropbox.com/scl/fi/agatzclr8pvr5470g6mc4/Phemt_One_Shot_1.jpg?rlkey=cwnx0qd7ajgnh8otf627x5lku&raw=1
 
Regular monostables are terribly slow. This one has low prop delay and
high rep-rate, if the sim is to be believed.
 
SAV541 is mostly specified as an RF part, but it's a dynamite switch.
 
I can post a link to the files if anybody wants to play with this. All
my values are first guesses, no math involved, and it works!
 
My SAV541 Spice model is a revision of Phil Hobbs' original.
Mini-Circuits is adamant that they will never provide Spice models, a
typical RF-bigot attitude.
 

 
With a low-resistance drain load like that, you don't care too much
about the low drain impedance of the pHEMT.  The old Avago ones were
around 160 ohms iirc.  The transconductance is high enough (400

>
millimho or

Rds-on is about 2 ohms with a half a volt or so on the gate.

Interestingly, they seem to keep turning on harder with insane gate
voltages, like +1 or so. Ig is about 16 mA, way past abs max, at 1
volt, but I'll just pulse it so it will be fine. Right?

something ridiculous like that) that you get gobs of voltage gain even so.
 
Really really crappy for followers though!

Tell me about that. My triggered 50 MHz colpitts oscillator squegged
at around 4 GHz. Tons of jitter.

I designed a new osc using a BUF602 and it's great.