Modern SMT BJTs to replace classic TO-39 BJTs in Amateur HF amplifiers - Page 1
Forum section: electronics / rf-microwave

author: MisterHeadache
author_rank: Regular Contributor
timestamp: April 03, 2025, 03:32:45 pm
content: I’m starting on a new project for my YouTube channel which will attempt to answer this question: “Are there modern, economically priced, BJT’s that can reasonably be made to perform satisfactorily in RF amplifier circuits originally designed for TO-39 transistors, specifically for the Amateur HF frequency range (3-30MHz)?”. If anyone else has thoroughly answered that question, I have yet to find evidence online. Maybe it’s because the answer isTHERE ARE NONE, but I want to make the effort to understand the answer whichever way it goes.More scope elements:The 2N2218/2219 are of prime interest to replace because they aren’t ‘true RF’ BJTs but work satisfactorily nevertheless in the HF band. Secondarily are ‘true RF’ TO-39’s like the 2N5109 and 2N3866 and 2N3553. All of those are no longer made in high volume/economically priced series production.Candidates must be listed as of Apr25 as active – no last time buys or announced end of life.Candidates need to be appx 1-2 watt devices (like the TO-39). Intended applications are for medium power circuits. Otherwise for lower power I’d just use the MMBT3904…SMT is greatlypreferred over through hole.I am hesitant to use modern ‘True RF’ SMT devices because they have Ft’s in the multiple GHz range (risk of oscillations), plus cost (not interested in BJT’s that cost $20 apiece, I want ~$1 or less).My investigation will begin with data sheet properties to select candidates similar enough to the older TO-39’s (in particular voltage ratings, transition frequencies, and Cob). Next will be building simple feedback amplifiers and measuring/comparing performance between them and the older TO-39’s in the same circuit. Criteria like gain compression, distortion, IMD, and noise.Early candidates include SOT-89’s like the 2SC5994 from OnSemi, the 2DD1664P/Q/R from Didoes Inc, and the 2SCR5XX series from ROHM.Hopefully with a little bit of effort, I’ll find a few that can be pressed into HF service and extend the life of many of the amplifier designs that worked with the older TO-39’s.Questions:What other data sheet parameters should I be looking at?Besides the early candidates listed above, are there any others you are aware of that would be of interest to evaluate?What other test data should I be looking to collect and compare?Lastly, I’m not interested in change-of-scope suggestions like “Why BJT’s, just dump those designs and use MOSFETs”, or “Just use an XXYYZZ amplifier integrated circuit chip”. Call me stubborn, I just want to work on answering the question as stated please.

author: Kleinstein
author_rank: Super Contributor
timestamp: April 03, 2025, 04:08:18 pm
content: Essentially the same silicon as 2N2222 , 2n2219 is still available in a SOT89, e.g. as DXT2222 or PXT2222.

author: szoftveres
author_rank: Regular Contributor
timestamp: April 03, 2025, 04:40:00 pm
content: SMD:BCP55Thru-hole:The BD135 - BD137 - BD139 are TO-126 devices with ft >190MHz, which very few datasheets call out (the original Philips did). The same die in TO-92 package is called the BC635 - BC637 - BC639 series.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: April 03, 2025, 06:16:57 pm
content: Quote from: Kleinstein on April 03, 2025, 04:08:18 pmInteresting. I dismissed the DXT2222 earlier at a glance because I saw it as ‘just’ a repackaged 2N2222. Looking at specs, it does look like it fits between the TO-92 2N2222 and the 2N2219 in terms of Icmax and Pmax. It’s an active production part, I’ll add it to the list.Quote from: szoftveres on April 03, 2025, 04:40:00 pmBCP55 shows up on OnSemi as active. Ft and Cob look acceptable, comes in SOT-223 package. I’ll add it to the list. The BD135G/137G/139G show up as discontinued on OnSemi’s site. BC635 shows as replaced by BC337 which has 100MHz Ft. Tentatively on the list. BC637 and 639 show as obsolete with no replacement.

author: G0HZU
author_rank: Super Contributor
timestamp: April 03, 2025, 07:09:15 pm
content: The PZT3904T1G from Onsemi is worth considering, especially if you use more than one in parallel.

author: Zoli
author_rank: Frequent Contributor
timestamp: April 04, 2025, 06:01:12 am
content: More SMT: Rohm's 2SAR574&2SCR574.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: April 04, 2025, 04:45:49 pm
content: Quote from: G0HZU on April 03, 2025, 07:09:15 pmWould have to use many in parallel to get same current capacity, they're 200mA max collector current. The TO-39's are higher.Quote from: Zoli on April 04, 2025, 06:01:12 amYep, those are in the 2SCR5XX family I have on my list already.

author: MisterHeadache
author_rank: Regular Contributor
timestamp: April 19, 2025, 08:23:46 pm
content: Guys, thanks again for the tips. I've just finished my first episode on evaluating five choices, which includes LTSpice simulations. I'll be building a simple amplifer next to evaluate them.Link to my YouTube channel is in my profile.

author: coppercone2
author_rank: Super Contributor
timestamp: April 19, 2025, 08:27:21 pm
content: I made through hole replacements with adapter PCB's. I like to use gold clad wire for the leads but it makes people on eevblog short out, so you can use brass or bronze (gold clad inserts nicest into bread board, its thermo-mechanically clad wire that is around 5% gold by composition if its in spec). Also useful for dual - diodes if you want to try to bread board them.If it worked TH on the old part, it should work TH on the new part. Well mostly, its a bit bigger. But I had it with trying to prototype with SMT.I spent some time thinking about a low inductance adapter to make SMT into TH (not so simple) to try to mimic those parts that are pushed down flush against the PCB with minimum leads, but it requires more thinking, probably something to do with PCB edge metalization and spot welding or maybe high temp solder etc. Its probobly possible to shield it too, but its a little tricky. Or being clever with vias. I really think you can get something very similar... also the solution makes me think of how jewels are seated in jewelry, there is something there too.Actually, if you lap joint solder a tiny PCB, instead of using holes, you can get it really tiny. The middle pin of the package needs to be on a via so you can put a bare trace to solder to on the back. Sn10/Pb88/Ag2 solder should make any problems from soldering go away if the leads are bonded with it