RF PNP transistor - Page 1
Forum section: electronics / projects

author: sdouble
author_rank: Frequent Contributor
timestamp: March 15, 2019, 08:53:52 pm
content: Hi everybody,I'm currently designing a cascode amplifier. Actually, I intend to build a folded cascode amplifier. It will be based on a j-FET and a RF transistor. To allow for more headroom, I do want to use a low noise RF PNP transistor.It seems that there is not a single RF PNP on the market anymore. How is that possible ?!?

author: Benta
author_rank: Super Contributor
timestamp: March 15, 2019, 08:57:22 pm
content: My crystal ball just brokeVoltage?Power level?Frequency?...

author: MasterT
author_rank: Frequent Contributor
timestamp: March 15, 2019, 09:15:43 pm
content: https://www.digikey.ca/products/en/discrete-semiconductor-products/transistors-bipolar-bjt-rf/283?k=&pkeyword=&sv=0&pv74=2&sf=0&FV=ffe0011b&quantity=&ColumnSort=0&page=1&pageSize=25

author: sdouble
author_rank: Frequent Contributor
timestamp: March 15, 2019, 09:35:08 pm
content: Quote from: Benta on March 15, 2019, 08:57:22 pmWhat happened to your crystal ball ?I personnally prefer coffee grounds readings. more reliable and less prone to falling from a height.Quote from: Benta on March 15, 2019, 08:57:22 pmvery low power/low voltage/current (3 V, 15 mA)600 MHz min

author: Benta
author_rank: Super Contributor
timestamp: March 15, 2019, 09:47:33 pm
content: Quote from: sdouble on March 15, 2019, 09:35:08 pmFTor operating frequency? Your question does not get better...

author: sdouble
author_rank: Frequent Contributor
timestamp: March 15, 2019, 09:50:18 pm
content: 3db cutoff frequency

author: Zero999
author_rank: Super Contributor
timestamp: March 15, 2019, 10:41:14 pm
content: The BFT92, only because I've got some in my junk box.

author: Gerhard_dk4xp
author_rank: Frequent Contributor
timestamp: March 16, 2019, 06:18:20 am
content: Intersil / Renesas HFA31... transistor arrays have the last fast PNPs. ft = 5 GHz or so.I still have a reel of BFG31Regards, Gerhard

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2019, 10:35:04 am
content: MMBTH81, get 'em before they're gone forever. Only fT is up there, mind.BFT92 may still be around in places. (I have some in my bin too, actually; good luck..)Your alternatives are, do it with NPNs anyway (consider SiGe HBTs, or PHEMTs, or GaAs or GaN FETs, with fT in the 10s of GHz -- may not need cascode), or do it monolithic (custom IC, or find a commercial all-in-one solution that you can adapt to your application).Tim

author: sdouble
author_rank: Frequent Contributor
timestamp: March 16, 2019, 10:03:55 pm
content: I have some spare BFT92 around too. But I do need to design a new equipment. I need to source "active" components only.I had to give up with the modern HBT, PHEMTs, GaAs FET due to their leakage current. This prevent from using them at relatively low frequencies (below say 100 MHz).My current plane is to cascode a good ol' "high-gm" JFET with a very low noise SiGe HBT from Infineon. Those could be prone to oscillate in the GHz range. We'll see.Quote from: T3sl4co1l on March 16, 2019, 10:35:04 am

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 16, 2019, 10:46:11 pm
content: Phil Hobbs is fond of cascoding two of them, afraid I forget which though. One of them has awful Early effect (or the FET equivalent), so that goes on the bottom; the other doesn't, but has awful low frequency noise, so goes on the top. Yes, the layout must be tight and usually demands a small ferrite bead on the base/gate, which must be carefully chosen since you still want some high frequencies, not just lower ones.Tim