Haven't measured, but they sure make crappy followers.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
The subthread about the circuit begins with a post whose time stamp is
06/05/2014 01:09 PM. On Google it'sor
The actual circuit is slightly different, because the SPICE model abuses the CFA output stage horribly, but this is the gist. It lives and dies by keeping the input capacitance very very low, like wire-bonding the biochip right to the board.
Worked great, and almost made the ridiculous spec.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
A 60 GHz source follower would be interesting.
If you're lucky enough to get actual DC curves (and not just s-params), some phemts have downward-sloping drain curves, namely negative output impedance. Maybe the effect is thermal.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
I built a freebie calibrator circuit into that amplifier, because (a) it needed one, and (b) I wanted to try out those parts in various roles.
I eventually got the circuit working by replacing the ATF38143 follower with a SKY65050. The Skyworks part is generally nicer in circuitry, except that its flatband noise is a decibel or two higher, and its 1/f corner is 50 MHz rather than 10 MHz, at least in my small sample.
I've seen that in SiGe:C datasheets, but not in pHEMTs. Do you remember which ones? That effect could be pretty useful if it's fast.
Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
The circuit is posted somewhere in the "really triangular triangle wave" thread.
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
Here it is in the CLY2, which is a mesfet:
The negative slope corresponds to the high power dissipation region. Maybe these are true non-pulsed DC measurements, so the effect could be thermal. Just like some RF guys to do that.
I'm pretty sure I've seen the same in a phemt, but I can't find it just now.
The DC specs are awful (in the sense of missing) on most RF parts. You are lucky to get anything beyond Idss and drain breakdown voltage, and you shouldn't believe the latter. The app notes literally say "bias it until it works." We run supposedly 6 volt parts at 10, because they actually start to leak at 25. That's RFthink again.
-- John Larkin Highland Technology, Inc picosecond timing laser drivers and controllers jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Hmm, interesting. It does look like a thermal effect, since the slope changes sign as you go to higher dissipation. The top curve drops about
100 mA in 6 volts, which would be a drain impedance of -60 ohms. That would be interesting to stabilize!Cheers
Phil Hobbs
-- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC Optics, Electro-optics, Photonics, Analog Electronics 160 North State Road #203 Briarcliff Manor NY 10510 hobbs at electrooptical dot net http://electrooptical.net
We mostly use these parts switchmode, so those curves don't much matter.
Depletion gaasfets enhance past Idss, as much as 2:1 in some cases, less for lazy mesfets and more for phemts with low pinchoff voltages. The enhancement phemts, like the Avagos, have a weird super-enhancement mode that kicks in with gate current, as much as you dare to force.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Rivets! 1960s technology. Not very reliable, as I recall.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Should be a minority carrier thing, like a UJT. Supposedly, it happens to regular Si JFETs as well (i.e., gate emission reducing channel resistance below nominal Rds(on)), but I've never observed it.
On a related note, you can make a UJT out of a CdS photocell, LED and resistor. Quite a bit slower and larger, but does the job (physically and behaviorally).
Tim
-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: http://seventransistorlabs.com
Seems to be something like that. Phemt gates are fragile (abs max gate currents in the mA range) so you can't go crazy here.
Or a LASCR (light-activated SCR)
-- John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com
Thanks very much for your troubles. That is a very elegant solution to a difficult problem and I had already played with it a great deal. Truly amazing performance!
However, it is a current amplifier. When you mentioned a voltage gain of
50, I thought you meant a voltage amplifier and that's why I could not find a circuit. That was the reason for the question.But the basic principle of driving a phemt in cascode with a bipolar remains sound. I wonder if it ould be possible to adjust your circuit to provide feedback to the source of the ATF38143 and turn it into a wideband voltage amplifier? It might be worth investigating.
In the meantime, I may have found a solution for those who wish to upload a LTspice file but who don't have their own web site or subscribe to Dropbox.
I fixed the line wrap in your circuit and uploaded it, and I wonder if you could take a second and see if it works.
The circuit is at
Just click on 46EAA174.ASC and save it to a convenient folder.
The PLT file is at
Click on 46EAA174.PLT and save it to the same folder, then run LTspice.
The filenames are my local date and time converted to hex. This only occurs once, so there is little risk these files will overwrite anything on your hard disk.
This might solve the problem of line wrap wrecking LTspice uploads in a newsgroup post.
Please let me know if it works and if it is convenient to use.
Thanks for all your help.
Thanks, I replied to your next post.
Thanks very much for your excellent explanation. I will have to see if LTapice can duplicate the device curves for the ATF38143 and examine this further.
Very good work! Thanks.
The main difficulty in building your own probes is not the buffer amplifier, but the mechanical construction and impedance management around the probe tip. The tip needs to be strong, but very small to keep the inductance and capacitance down. I documented and published here my project to build an amplified probe using a BF862 (10pF input capacitance), with a 7mm probe tip and nominal 10K impedance. The inductance of that length of probe resonates with the input capacitance, making the probe useless above 350MHz.
Obviously I should have used much smaller (physically) coupling capacitors than 0805, a shorter tip, and a FET with smaller input capacitance. It's possible I could also have used an RC combination to form a voltage divider with the input impedance (at a cost in noise).
However I think you'll get my point that the probe performance isn't mainly about the FET, but about what comes before it. There's a good reason why folk who can make 7GHz probes can charge $x,000 for them.
It was lower than I expected too, but that's what we measured. I welcome your comments on the design and construction. I'm not claiming any special expertise; this was my first project into VHF, let alone higher. The earthy side of the probe is a pogo pin, the signal side just a header pin, soldered both sides on the PCB.
Clifford Heath.
By "let them", you mean that you decide it's cheaper than developing your own.
Right. That's why $$$ probes put the FET just a few mm from the tip.
You won't maximise bandwidth that way though (Phil's amps optimise sensitivity and noise, not bandwidth). Unless you have the source->gate feedback very finely-tuned to get close to the FETs bandwidth (and risk oscillation), the buffering effect may be slower than the signal - so the tip looks reactive at high frequencies anyhow.
To be fair, they do say that this configuration is "For very widely spaced targets" - which is not conducive to high-bandwidth probing.
I look forward to seeing your published designs that do better.
Have to try it and see.
It would be nice if you could do that at 7GHz.
The datasheet says SOT-343, but it looks the same as the SC70.
Where did you get the 0.58 and what is it for? Your calculation may have a typo. I get 0.58*165 + 25 = 120.7 C, not 145 C.
On page 6 of the datasheet they run it at 4V and 120mA. Brave people.
For the follower, I get (0.384 * 165) + 25 = 88.36 C. The maximum channel temperature Tch is 160 C, so if we keep the pins cool there should be plenty of margin.
Perhaps mount the ic on a small 2 sided pcb that is 0.015" thick or so. The bottom is copper, the top has the mounting pads for the ic and a row of ceramic bypass caps for the drain. The other ends of the caps are soldered to the edge of the pcb, which is soldered to the brass ferrule. The pcb bottom is soldered to the ferrule, which is then thermally connected to the coax shield.
This would also provide mechanical support for the probe, which is going to get rough treatment as the user tries to punch through soldermask or a layer of copper corrosion.
Yes, it really looks bad. But we are running as a source follower, not an amplifier. Lots of negative feedback.
Typo. Missing the decimal. If you followed the link it would show the correct price and distributors.
Unfortunately, the price has changed between last night and this morning. Newark now wants $2.54
How well does it work as a follower driving 50 ohms?
It measured dead-flat 100KHz to 250MHz, and after tweaking the trimmer, almost dead flat to 350MHz. Why are you surprised? I thought it might go higher, even though I didn't need it to.
I'd still like to hear your thoughts on where you think the bandwidth limitation is coming from.
My objective was mainly to learn how to design such circuits, not to buy ready-made. And I wanted an impedance higher than 500ohms at least at HF, to probe some very short antennae.
Clifford Heath
I was looking at the 38143 datasheet until I noticed you were mainly talking about the 34143, which has a lower max dissipation. I changed the result but missed the LHS. The calculation is otherwise correct.
I do build 50-ohm stuff, but it's not my main interest.
For FET probes, I use Tek P6249s from eBay (4 GHz, about $250). For slower stuff, i.e. with my 500-MHz scopes, I generally use P6201s (900 MHz).
Cheers
Phil Hobbs
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