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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: Sun, 8 Sep 2024 00:02:34 -0000 (UTC)
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bitrex <user@example.net> wrote:
> On 9/5/2024 6:06 PM, john larkin 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?
>> 
> 
> What about a PFET/JEFT cascode buffer kinda like:
> 
> <https://imgur.com/a/5fBR8Gb>
> 

There aren’t any decent discrete pfets left, and there never was a 5 GHz
one. 

Cheers 

Phil Hobbs 

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