I got burned recently with a BFT25 follower, oscillating at invisibly high frequencies. Base resistance fixed it. It's interesting that phemts make stable followers.
I'll try the loopthru pickoff circuit with a MiniCircuits phemt follower. I don't need voltage gain of 1... something like 0.6 would be fine.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Yes, except that then it doesn't bootstrap Cgs very well.
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
What values do you use for the SKY65050 capacitances, when you're doing back-of-the envelope calcs? I did a bunch of SPICE modeling for this part about 8 years ago, but it makes little sense now. Maybe Ciss = 1.1 to 1.5pF, and Crss must be much smaller than that.
--
Thanks,
- Win
Right. I measured 0.92 pF at the gate with the source and drain grounded, about 1 pF at the drain with others grounded.
0.6 bootstrapping of Cgs would be about 0.6 pF, a decent improvement in my situation.-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Ciss is less than that--my biochip preamp had a total input capacitance of about 1 pF. Crss is probably in the 0.1 pF range.
I built a calibration source into my pHEMT/SiGe:C cascode TIA, using discrete pHEMTs for followers and Schmitt triggers and stuff--it was a very triangular triangle wave oscillator coupled into the summing junction via about 0.01 pF of board capacitance. With the Avago pHEMTs it completely failed to work on account of the low drain impedance, but it worked OK with the Skyworks ones after I fiddled with the biasing a bit.
That was one of those moonshot projects--some biophysics types at Samsung's main research lab were doing DNA sequencing the way a kid eats spaghetti: sucking half strands through a micropore in a Si3N4 membrane electrostatically, and measuring the change in transverse conductance as the different amino acid bases went between the pads on the top side.
The spec was to measure 1 nA at the shot noise limit in a 100 MHz bandwidth. The equivalent measurement time is 5 ns, and 1 nA times 5 ns is 31 electrons. I pointed out to them that their maximum SNR was going to be 15 dB, but they wanted to go ahead anyway.
The only lucky thing was that I insisted that they mount their biochip on my board and wire bond it to the amp input.
I got within a factor of 2 (11 electrons RMS), but you really can't measure anything with a SNR of 9 dB. I suppose that they could have done pattern matching to combine data from many strands, sort of how you do with tree ring dating.
It was great fun, and I think I extended the state of the amplifier art in an unusual direction.
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
Oh, and the ATF38143 datasheet has a super detailed SPICE model for the chip and package, so I used that.
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
cks
'd
UUUGE in fact many small signal monolithic amps introduce serious harmonic
Does jitter worry you? That LEE-19 has 4dB greater NF than any amplifier in the product family.
I often use a directional coupler for this, usually about 10 dB. Mini-Circuits is good for that. I has nice control of impedance at all ports and can be quite wide-band.
IN
| | | | | | | |
OUT
We've just finished some tests, using a few different MMICS with series input resistors, rather than the usual 50 ohm drive.
A LEE-59 with a 510 ohm series input resistor, driving a 50 ohm comparator on its output side, has an overall voltage gain (pickoff point to comparator input) of 1. The thru-path trace won't notice the
560 ohm load, and an 0603 resistor's capacitance will be tiny, something like 0.05 pF.A LEE-59 is rated at 5 GHz. Costs about $2.
This would be super wideband. I'd put a pretty big cap (physically small!) as a DC block in series with the 510 ohm resistor.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
As another alternative, there's a name for the sort of coupler used on these boards, but I can't remember it:
But yeah, a ~500 ohm pickoff resistor should be fine if you don't care about any directional properties.
-- john, KE5FX
I want an invisible, minimally invasive voltage pickoff, from maybe 1 MHz to several GHz. A phemt source follower might work too - haven't tried that yet - but it would probably be worse than the 500 ohms at high frequencies. The s-g capacitance would be bootstrapped, but the follower gain would be closer to 0.5 than to 1.
500 ohms costs about 5% of my signal amplitude and is flat over frequency.MMICS are cool when used off-label.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I did this recently:
Vdd -+- | '--| | .--|
Bootstrap the upper jfet with another fet!
-- John Larkin Highland Technology, Inc lunatic fringe electronics
The actual circuit's source load (is) was this...
R2 | | J1 '-' | |- | |
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