phemt follower

It looks simple enough that it's possible to do the matrix math with the S or Y parameters and pencil and paper to find out if the structure is likely to oscillate at the new operating point at least, if the manufacturer is kind enough to provide that sort of stuff with some data points that're appropriate for the OP and bandwidth in question. I understand that's a big "if", though.

I normally don't run them that hot, so I don't know what the stability properties are up there, but it's really hard to get them to oscillate at 15 mA or so. (*) The output resistor is probably a bit on the big side, because 1/g_M is probably at least 10 ohms even up at 40 mA.

The only other thing is the pHEMT's poor linearity as a follower, which will distort the output some. For following pulses, it's maybe not a big deal.

Cheers

Phil Hobbs

(*) I've used them to run a bootstrap over a 50-mm FFC cable, believe it or not. Trashed the bandwidth, but it worked like the bomb for the customer. It's been in production for a couple of years now.

I don't do s-parameter math... I'm a time domain guy. And there are resistors and caps and traces and stuff involved.

I think Phil knows about using the SAV parts as followers.

Looks like I can't find an L1 value that helps, so that's gone.

Here's an xray of the SAV541. Those wirebonds are scary.

The data sheet says 392 mS typ at 60 mA, which would be 2.5 ohms, so I'd expect 4ish at maybe 45 mA, R4=100. I could reduce R3 a bit.

I was surprised when Jonathan reported 140 ps edges with R2 = 499. The source-bootstrapped gate capacitance must be nil.

torsdag den 2. december 2021 kl. 02.01.45 UTC+1 skrev John Larkin:

do you need the gain?

No, the monitor could be maybe 0.1 of the signal we're snooping.

Passive resistive scope probes, like the HP54006A, can be crazy fast and very clean. But I don't want to load my signal much, and the customer has radical requirements for reverse isolation. I need an active device in there.

Deadline moved to Monday.

The SAV-541 really wants to oscillate as a source follower. We think it's too much risk. We're considering using a MMIC as our pickoff amplifier. They are remarkably stable.

I don't recall ever running one that hot, but they're remarkably stable at lower current--15 mA or so.

Cheers

Phil Hobbs