Transistor Selection

I'm usually not picky, so... narrow it down to the Ice you want and basic functionality (NPN? dual? built-in bias resistors?). Then I usually pick the smallest SMT package of what's left, then pick whatever Digikey stocks the most of (i.e. most popular).

Tim Williams wrote: > It just occurred to me, how the hell do you select transistors? >

You use something called a *selector* *guide*, the OEMs like ON, Zetex, and Fairchild have them.

guide: n. Something that serves to direct or indicate.

select: v.tr. To take as a choice from among several; pick out.

So you want to use this "something that serves to direct or indicate" to select as in "to take as a choice from among several; pick out" the one you deem best suited for the job. You may even SPICE it in the prospective circuit.

I would be surprised if you had a very big list to search with that package and fT. TO-92 isn't much use at 1000 MHz, is it? I think you also have few choices that include both that high fT range and the voltage capability of a 2N3904. most of the fast transistors have low voltage and current capability. I doubt you will find a 40V 100 mA rated TO-92 with an fT>300MHz.

i.e.

You get on here and ask a few thousand of your closest friends what they use. Most of us use the least expensive device that will do the job.

My low power 50-500 MHz. RF device of choice is the gold-doped 2N5770. You will be pushing it very hard at a gig with the TO-92 package but if you know what you are doing with matching devices it isn't too difficult ... and the usual PCB layout with splayed 1/2" leads isn't going to cut it much above quarter of a gig. The base lead is the bitch of the bunch as you want minimum L for maximum bandwidth -- component side short and fat traces; the collector isn't quite as bad because you will probably be having some L in a tank circuit there anyway. The emitter is even easier, as you simply choose an emitter bypass to either give you gain less than maximum, or for maximum gain, simply series resonate the lead inductance with the package inductance with the capacitor lead inductance at the frequency of choice.

The venerable 2N918 isn't too bad either, and the 2N5179 comes in a rather distant third. Them's my choices.

Did that help?

Jim

Work out what you want the transistor to do, read a few selection guides, and see if Farnell stocks anything that looks as if it might do the job.

Manufacturer's data sheets, though they can be a bit sparse for individual transistors. National Semiconductor's transistor data book had an interesting section at the back, which gave more detailed information about various transistor processes - you could find out the process used to make a specific transistor and use the process data sheet to fill out the information that wasn't specified on the device data sheet.

What do you want to use it for? Why do you want to buy a high frequency part in the antediluvian low frequency package used for the

-- Bill Sloman, Nijmegen

V, I and package, no problem. fT, who said anything about fT? Like I said, those vendors don't have any speed specs to pick from.

No, probably not -- though see my reply to RST.

Why is that anyway, hFE vs. collector capacitance, or perhaps more to the point, hFE per collector area or somesuch? For sure, most of the GHz range ones are those icky black lumps with flat leads coming off and are rated in mA up to 12 or 25V.

And what makes an RF power amplifier special, anyway? A lot of the transistors I see for that kind of duty are rated for a specific frequency range. I might guess that's due to peculiarities in their performance curves, but still, a transistor is a transistor, why couldn't I use a 10W VHF amp to make a fast-assed DC power switch?

And speaking of long lists of questions, what keeps a MOSFET from doing VHF work? Lead inductance and gate capacitance, sure, but an IRF540 should still be good out to, erm, at least 20MHz I would guess. And lead L and gate C are just circuit constants, those could be varied, if not by the circuit then by the manufacturer, to get a certain passband of interest...

Any leaded package, really. TO-18 and TO-39 are common too...

Tim

-- Deep Fryer: A very philosophical monk. Website @

Tim Williams wrote: (snip)

(snip)

In a nut shell, you get a fast transistor by making it very small, for the current and power rating (to minimize capacitances), and use the thinnest possible junctions to minimize the carrier transport time (but you also get low break down voltage). Those things go against high power ratings. Fast transistors are fragile, compared to slower devices, where bigger chunks of silicon can be used to carry the same current and drop larger voltages.

I get realy lazy and just order the transistors (models) found in spice programs. ex: switchercad III D from BC

You could try the TO-92 C3355. Vcbo 20V Vceo 12V Vebo 3V Ic 100mA hFE 50-300

Cob 0.65pF typ ft @ 20mA 6.5GHz typ

Best regards, Spehro Pefhany

I'll admit to being gabberflasted. 6 gig in a TO-92? I'll bet that was an extrapolated number from the gain at a gig or so.

Jim

RF performance is limited by 'base spreading resistance' . Both current gain and breakdown voltage get lower when you increase base doping to decrease the spreading resistance.

Just out of curiosity, I picked the first RF NPN from mouser.

Fairchild SS9018 TO-92

Sound good enough for me. You can gamble on it for 4 pennies.

Tim Williams wrote: (snip)

(snip)

There is no reason, except that your design has to contain all that high speed gain without accidentally producing any instability all the way up into the gHz region. So you have to be a UHF designer, or be very lucky to get a DC amplifier to function without also being a UHF transmitter, because an inch of trace is also an inductor and delay line at a frequency where the transistor still has lots of gain.

gain * bandwidth product does seem to be remarkably consistent at the device level and in simple circuits. It even works for opamps.

Tim,

good question! I start with a search on the VCE and gain bandwidth product. then from this set the lowest cos most common for prodcution runs.

If there is are any other circuit requirments than they enter the selection and filtering process. IN summary the circit rquiremtn take the ga zilion choice down to a sensible few..

Best regards,

marc Popek