Choosing transistors for radio frequency applications

[rfeecs]

For jFets they don't always seem to specify a transition frequency, how do you figure out what frequency range they are intended for?
Example: http://www.digikey.se/product-detail/en/2N3819/2N3819CS-ND/4806873

That datasheet is pretty sparse, but if you google that part, you will find some much more detailed datasheets.

This one gives a "cut-off frequency" of 700MHz, where they are defining it as where the transconductance starts to roll off:
http://www.micropik.com/PDF/2N3819.pdf
It also has plots of the Y-parameters and S-parameters up to 1GHz.

Here's another one that mentions some RF performance in the description:
http://www.learnabout-electronics.org/Downloads/datasheet2N3819.pdf

[apis]

OK, those are definitely more informative and thanks for the application note on s-parameters, very helpful.

[T3sl4co1l]

Stay away from anything stinking fast (say >3GHz) until you're ready.  Even for anything fT between <your fastest instrument's bandwidth> and there, at least try to get something that can read the presence of extremely high frequencies, like a diode RF probe.  Otherwise, when your circuit does inevitably oscillate, you'll have little idea what's going on, aside from "spooky" behavior like shifting bias or distortion with fingers nearby (proximity effect).

Si JFETs are generally good for the 100s of MHz range.  As transconductance devices, they don't really have a transition frequency as such.  BJTs can be expressed as current mode devices, and therefore have hFE and fT.  But for a true constant current output, you can simply keep increasing the load resistance, and power gain keeps going up (if not maximum power output, because you'll pretty quickly run out of voltage).  You do have the direct tradeoff of band width (which is DC to BW for a wideband circuit, or center frequency +/- BW/2 for a tuned circuit), because drain circuit bandwidth is limited by load resistance versus drain capacitance.  Likewise for gate/source circuit bandwidth tradeoff.  At some point, bandwidth is too narrow to use (plus the real losses in gate and drain terminals) and you can't do much more with it.

Tim

[AF6LJ]

If you can get your hands on this book it would be a great help, complete with circuits to get you started.
http://www.arrl.org/shop/Experimental-Methods-in-RF-Design/

[apis]

Stay away from anything stinking fast (say >3GHz) until you're ready.  Even for anything fT between <your fastest instrument's bandwidth> and there, at least try to get something that can read the presence of extremely high frequencies, like a diode RF probe.  Otherwise, when your circuit does inevitably oscillate, you'll have little idea what's going on, aside from "spooky" behavior like shifting bias or distortion with fingers nearby (proximity effect).

Right, I'm starting to worry I will need better equipment than what's possible for me to get or things might oscillate like crazy without me realizing it. But they did it in the past so it must be possible? A diode RF probe is a great idea though. I guess I'll get some parts and try and work my way up to higher frequencies gradually, hopefully I have a chance to spot where the problems arise so they can be dealt with one by one.

If you can get your hands on this book it would be a great help, complete with circuits to get you started.
http://www.arrl.org/shop/Experimental-Methods-in-RF-Design/

From the description it sounds very tempting, thanks for the tip. I saw another thread that recommended RF books as well, will take a look at those as well.

[AF6LJ]

Not knowing for sure what your goals are, what you plan on building.
One of the first cheap tools I had was a general coverage receiver that had a piece of coax on the antenna terminals and a small 25 or so mm loop soldered across the end of the coax to sniff out my oscillator circuits to see if they were working as intended. The same receiver could be used to test small signal amplifiers and converter circuits. (mixers).

There is a lot you can do when you think like a hacker and use what you have on hand, even at VHF and beyond.

[apis]

I have a couple of rtl-sdr sticks, it might work to connect a coax-loop probe to that! I also have a the VHF radio from the boat. I can't cover all frequencies though but it should help.

One can often get a long way with a bit of ingenuity but ironically it is the experienced and skilled ones who usually can manage without fancy equipment, while the beginners who don't have any are the ones who need it the most.

[AF6LJ]

Those RF SDR sticks will work just fine.

[vk3yedotcom]

Not knowing for sure what your goals are, what you plan on building.
One of the first cheap tools I had was a general coverage receiver that had a piece of coax on the antenna terminals and a small 25 or so mm loop soldered across the end of the coax to sniff out my oscillator circuits to see if they were working as intended. The same receiver could be used to test small signal amplifiers and converter circuits. (mixers).

Definitely highly desirable.  Two receivers is probably desirable.  (1) a transistor radio with shortwave ranges that allow you to tune across the whole spectrum very quickly (but with poor stability and frequency readout) and (2) A communications receiver, receiver part of an HF transceiver or SDR that allows selective reception, accurate frequency readout and easier tuning once you know the approximate frequency.   

But before all that you need to have an indication that RF is being emitted at all.  A crude but extremely useful tool is just an RF choke, diode, potentiometer and clip lead on the back of a meter movement (250uA).  That gives a rough idea of level and even amplification.  An RF probe going to your multimeter can also work and allow measurement, but I admit to the  satisfaction of pinning the needle when an RF circuit is working! 

[AF6LJ]

Good suggestion.

[apis]

Definitely highly desirable.  Two receivers is probably desirable.  (1) a transistor radio with shortwave ranges that allow you to tune across the whole spectrum very quickly (but with poor stability and frequency readout) and (2) A communications receiver, receiver part of an HF transceiver or SDR that allows selective reception, accurate frequency readout and easier tuning once you know the approximate frequency.

A short-wave radio is a good idea, a transceiver will be more difficult. I think I need a license to own one, not entirely sure though (and they are expensive I think).

But before all that you need to have an indication that RF is being emitted at all.  A crude but extremely useful tool is just an RF choke, diode, potentiometer and clip lead on the back of a meter movement (250uA).  That gives a rough idea of level and even amplification.  An RF probe going to your multimeter can also work and allow measurement, but I admit to the  satisfaction of pinning the needle when an RF circuit is working!

Do you mean connect it like a crystal radio or some other way? Sounds like a good suggestion as well, thanks.

[vk3yedotcom]

1. Rules vary with country re owning transmitting gear. A used ham transceiver wirh general coverage receiver can be better value than a used receiver alone.

2. Yes crystal set with meter in place of headphones is a good indicator - known as an absorption wave meter. It can give an idea of frequency if you calibrate the scale of the tuning capacitor. Or replace the tuned circuit with a my ref choke to operate as a broadband rf indicator.

[dom0]

Ohh, this thread comes up at the right time, since I'm looking for a replacement RF transistor.

PNP, fT ~ 800-1200 MHz, Uces about -15 V. Preferably something with leads. Anyone?

[edavid]

Ohh, this thread comes up at the right time, since I'm looking for a replacement RF transistor.

PNP, fT ~ 800-1200 MHz, Uces about -15 V. Preferably something with leads. Anyone?

MPSH69 (TO92) or 2N5583 (TO39)?

[T3sl4co1l]

MPSH81 also comes to mind (give or take).

[edavid]

MPSH81 also comes to mind (give or take).

Well, only 600MHz f_T on the datasheet.  Is that what you meant by "give or take"?

[dom0]

RF PNP is really... hard to source.

I really really wonder where the hell HP sourced them ... in the 60s! And why they used so many of them in the 3400...

[AF6LJ]

RF PNP is really... hard to source.

I really really wonder where the hell HP sourced them ... in the 60s! And why they used so many of them in the 3400...

Motorola of all people used them, and even had some that were power transistors, 50W VHF parts.

[zmetzing]

ON Semi SS9018HBU
RF TRANS NPN 15V 1.1GHZ TO92-3

Bag of 100 for just over $10 on Digikey.

[edit: A great article on this sort of thing at https://wiki.analog.com/university/courses/electronics/text/choosing-transistors]

[Jack Cornwall]

the s-parameter article cited here is available here https://m.eet.com/media/1072747/C0473edited.pdf
well, today it is