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From: Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net>
Newsgroups: sci.electronics.design
Subject: Re: Drat. NTE is gone, and they took the last 5-GHz PNP with them.
Date: Fri, 6 Sep 2024 00:07:45 -0000 (UTC)
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john larkin <jl@650pot.com> wrote:
> On Thu, 5 Sep 2024 22:44:36 -0000 (UTC), Phil Hobbs
> <pcdhSpamMeSenseless@electrooptical.net> wrote:
> 
>> john larkin <jl@650pot.com> wrote:
>>> On Thu, 5 Sep 2024 10:49:42 -0400, Phil Hobbs
>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>> 
>>>> On 2024-09-05 10:14, john larkin wrote:
>>>>> On Thu, 5 Sep 2024 07:30:50 -0000 (UTC), Phil Hobbs
>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote:
>>>>> 
>>>>>> Jan Panteltje <alien@comet.invalid> wrote:
>>>>>>> On a sunny day (Wed, 4 Sep 2024 17:54:19 -0400) it happened Phil Hobbs
>>>>>>> <pcdhSpamMeSenseless@electrooptical.net> wrote in
>>>>>>> <6b1217f0-f55b-95b4-6516-6914d18d0e91@electrooptical.net>:
>>>>>>> 
>>>>>>>> The NTE2403, similar to the BFT92, but a bit better overall.  Dunno who
>>>>>>>> actually made them. (I saw the news on s.e.repair today.)
>>>>>>>> 
>>>>>>>> Rochester claims to have 1,500,000 of the 2SA1462 (1.8 GHz) for 20
>>>>>>>> cents, but that's all she wrote.
>>>>>>>> 
>>>>>>>> I have a couple of reels of BFT92s and one of BFG31s, so I'm good for
>>>>>>>> protos and small production, but I can't use PNP wraparound bootstraps
>>>>>>>> for customer designs anymore. :(
>>>>>>>> 
>>>>>>>> Barstids.
>>>>>>>> 
>>>>>>>> Cheers
>>>>>>>> 
>>>>>>>> Phil Hobbs
>>>>>>> 
>>>>>>> Lots if RF snall signal low noise stoff in LNBs:
>>>>>>> https://www.infineon.com/dgdl/Infineon-Design_guide_for_RF-transistors_and_diode_in_Low-Noise-Block-ApplicationNotes-v01_00-EN.pdf?fileId=8ac78c8c7e7124d1017f01f071aa5b8f
>>>>>>> Does that help?
>>>>>>> 
>>>>>>> 
>>>>>>> Complete LNBs inclusive transistors and peeseebees are 5 dollies on ebay.
>>>>>>> 
>>>>>>> 
>>>>>> 
>>>>>> Thanks.  It?s not a 50-ohm system, so using those would be hard. I have
>>>>>> thousands  for personal use, but can?t put them in licensed designs, which
>>>>>> is what I?m moaning about.
>>>>>> 
>>>>>> Cheers
>>>>>> 
>>>>>> Phil Hobbs
>>>>> 
>>>>> Can you use mmics? There are some really cool, fast, cheap, low-noise
>>>>> things around. Just because the RF boys test everything at 50 ohms
>>>>> doesn't mean we have to use them at 50 ohms.
>>>>> 
>>>> 
>>>> For the wraparound topology, which is the second-best follower I know of,
>>>> 
>>>> VDD 0------*---------*
>>>> |         |
>>>> R         |
>>>> R         |
>>>> R         /
>>>> |       |V
>>>> *-------|  BFT92
>>>> |       |\
>>>> |--*         \
>>>> In  0-->|  CPH3910   |
>>>> |--*         |
>>>> |         |
>>>> |         |
>>>> *---------*----0   Follower output
>>>> |
>>>> V (tail current source)
>>>> 
>>>> the BJT needs to be a PNP.
>>>> 
>>>> It's a nice circuit, because the PNP reduces the output impedance a lot 
>>>> without adding much noise at all--way better than an NPN follower after 
>>>> the FET.
>>>> 
>>>> Because of the local feedback, the transistors need to be fairly 
>>>> different in speed to maintain stability.  The FET is about a 750-MHz 
>>>> device, so a 5-GHz PNP is great.  The alternative would be a 100-MHz 
>>>> PNP, which would be too depressing to contemplate. :(
>>>> 
>>>> The very best follower topology I know about is a fancy bootstrapped 
>>>> version of the White cathode follower, where the feedback is applied via 
>>>> the tail source.  That's much harder to stabilize, because there are 
>>>> three transistors in the local feedback loop, but on the other hand its 
>>>> gain is 0.9997 at baseband and above 0.995 at 10 MHz.  (You can't 
>>>> readily measure those sorts of numbers directly, so I inferred them from 
>>>> its performance as a bootstrap.)
>>>> 
>>>> The reason I care about getting such accurate bootstraps is a bit 
>>>> subtle--probably I'd have enough bandwidth improvement with a gain of 
>>>> 0.9, but that extra 10% shows up as a gnarly settling transient at late 
>>>> times, which screws up measurements.  0.9997 is dramatically better.
>>>> 
>>>> Cheers
>>>> 
>>>> Phil Hobbs
>>> 
>>> The CPH3910 is a jfet. Might an un-assisted PHEMT be better?
>>> 
>>> 
>> 
>> I’ve used them a fair amount in front ends, generally with a SiGe NPN
>> cascode-slash-drain bootstrap.  Their transconductance is a few times
>> higher than a CPH3910’s, but not as good as the local-feedback circuit’s. 
>> 
>> pHEMTs have very low drain impedance—the late lamented ATF38143 had a
>> voltage gain of ~0.7 as a follower, even with a current sink in the tail.  
>> 
>> Their amazing noise floor (~0.3 nV in 1 Hz) makes them well worth patching
>> up, but they do take some patching. 
>> 
>> Cheers 
>> 
>> Phil Hobbs 
> 
> What is the Vdd in your circuit above?
> 
> I learned about jfet impact ionization from AoE. Keep the drain
> voltage low!
> 
> 

Yup. I run them between 2.5 and 3 V_DS. 

That gives full transconductance, low noise, low leakage, and low
dissipation. 

Cheers 

Phil Hobbs 

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