spice test

The circuit doesn't care what you call it.

But you weren't going to look at it!

I invented that circuit a long time ago, using a bipolar NPN, as a cheap replacement for a comparator. Of course a saturated NPN would be slow. But the phemt g-s works about the same as a bipolar b-e and is super fast. It does with a few parts what could be slower and a lot more complex.

Rds-on of that little SAV part is about 2 ohms.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

No I said I hadn't looked at it, not I wouldn't look at it. Something similar and slower was done with thyratrons 100 years ago.

Presenting circuits on drop box isn't all that helpful. LTSpice .asc files work a whole lot better, but makes it more difficult to gloss over the inconvenient details.

Looking at John Larkin's circuits is persistently annoying - why can't he learn how to design stuff properly, or learn how to create intelligible schematics?

--
Bill Sloman, Sydney

People who are prepared to admire it without reservation.

Why would anybody bother?

--
Bill Sloman, Sydney

They are pretty nice parts, for sure. My Mini Circuits rep and I take full credit for persuading MCL to sell the transistors barefoot. ;)

I use them a lot in bootstraps and samplers. Magic.

Last fall Simon and I did a 24-channel SAV-581+ time-stretcher board that was new and different (to us at least). It's a POC for a system intended to replace a big-iron digitizer system in airborne bathymetric lidar.

It uses eight EL5167 CFAs, each driving the source of a SAV-581+ whose drain runs an 8-pF capacitor and the input of a simultaneous-sampling

14-bit ADC (LT2351). The gates are controlled by FIN1011 line receivers, which hold the gates in forward bias in track mode and reverse-bias them in hold mode. (The circuit is the usual diode+JFET+RC track/hold, except with a pHEMT.)

One input of each FIN is controlled by a DAC, and the other is a ~10 ns ramp generated by an RC on the output of another FIN.

We were quite happy with it--the spherical cows of LTspice predicted that the ramp signal would pick up a lot of artifacts as it went along, but in real life it didn't.(*) That was good, because we needed to interleave the samples in such a way that each one of the op amps had the maximum time to recover after each T->H transition.

Cheers

Phil Hobbs

(*) Verified using a TDS 694C 3-GHz scope with an ac-coupled P6249 4-GHz FET probe (

You get my intense gratitude for that. I used your Spice model too. They are great for discharging caps and such, fast. GaNs need too much gate swing.

Yes, I've used the ATF parts that way, and the SAVs seem to have lower

1/f noise. (I haven't measured that yet, but should.)

My most radical TIA to date was a pHEMT/SiGe cascode from 2011, iirc. In a 100-MHz bandwidth, it got within 6 dB of the shot noise of a 1-nA current. (It was actually for biochip use.) In the one-sided frequency picture, 100 MHz corresponds to 5 ns, and 1 nA in 5 ns is 31 electrons. So the TIA noise was around 11 electrons RMS in 5 ns.

With capacitive transducers such as photodiodes, pHEMTs make good medium- to high-speed TIAs and bootstraps because they're very fast, their flatband noise is very low, and the linear frequency dependence of the transducer's admittance wipes out the 1/f noise pretty well.

Might need to be packaged like those flip-chip GaN FETs you love/hate, though. The g_M/C_dg ratio is key.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I discovered that FOM for myself, when I was a teenager, Gm over plate capacitance. The frame-grid tubes, like 6DJ8 as I recall, were breakthroughs.

Audiophools seem to like 6SN7's, about as bad as it gets.

--

John Larkin      Highland Technology, Inc 

The best designs are necessarily accidental.

sources of three samplers, each made

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com

I last built a tube circuit in about 1990--it was for an air ion drift chamber, where I needed it to dork the collector grid voltage by ~1 kV in a microsecond and then go entirely away so I could collect all the charge.

At the time there no solid state thing available that would touch the performance of an 811A in that circuit.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
ElectroOptical Innovations LLC / Hobbs ElectroOptics 
Optics, Electro-optics, Photonics, Analog Electronics 
Briarcliff Manor NY 10510 

http://electrooptical.net 
http://hobbs-eo.com