# Transistor Selection

T

#### Tim Williams

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

Tim

D

#### DJ Delorie

Jan 1, 1970
0
Tim Williams said:
It just occurred to me, how the hell do you select transistors?

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).

F

#### Fred Bloggs

Jan 1, 1970
0
Tim said:
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.

J

#### John Popelish

Jan 1, 1970
0
Tim said:
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

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.
http://www.zetex.com/3.0/pdf/ZTX325.pdf
http://www.fairchildsemi.com/ds/MP/MPSH10.pdf
http://rocky.digikey.com/WebLib/On-Semi/Web Data/MPS918, MPS3563.pdf
http://rocky.digikey.com/WebLib/On-Semi/Web Data/MPSH17.pdf
http://rocky.digikey.com/WebLib/On-Semi/Web Data/MPS5179.pdf
http://www.fairchildsemi.com/ds/BF/BF240.pdf

R

#### RST Engineering $$jw$$

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

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.
In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz
fT
range, with V, I and package similar to a 3904 and such.

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

B

#### [email protected]

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

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.
Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

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.
In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

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
2N3904?

T

#### Tim Williams

Jan 1, 1970
0
A little bit, thanks.

For example, I hobbled together a 20~50MHz differential cascode stage using
ancient 2N3646's -- the mushroom shaped things. They happen to have a date
code of 71xx or so. Evidently they are somewhat fast, 350MHz compared to
the 100-300MHz of 3904, etc. So that's about 1/10th fT, which I would
certainly consider fast (especially for a mere breadboarded circuit).

A shame I have to make estimations of bandwidth based on rise time using a
square wave that's not particularly fast, so my estimate is 20-50...

I can push 4401/4403 into the 30ns range, but that's no good if I'm to
explore something like the FM band. It'd also be fun, probably more out of
bragging rights than anything remotely practical, to say I can make discrete
logic in the single digit nanosecond range, too.

I've pulled some UHF transistors from a bunch of high-def monitors. These
things used a cascode/emitter follower video amp stage: PH2369 / Philips
2N2369 in TO-92 for the bottom, BFQ225 (I think), a TO-202, 100V transistor,
and a complementary (two diode biased emitter follower of BFQ221/241 to
drive the grid or cathode or whatever it was. I've tried making a similar
circuit out of these things, wiring as carefully as I can, point-to-point,
but they're a bitch to stabilize, and it's hard to even see how they're
oscillating -- I suspect they are fond of 500MHz or so, which is a shame as
my scope is only 200MHz!

So yeah, I could look up other transistors as these, but what if I want
other ratings, or packages, or pricing? That's why I want a good
selection...

Tim

T

#### Tim Williams

Jan 1, 1970
0
John Popelish said:
I would be surprised if you had a very big list to search
with that package and fT.

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.
TO-92 isn't much use at 1000 MHz, is it?

No, probably not -- though see my reply to RST.
most of the fast transistors have low voltage and current
capability.

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...
I doubt you will find a 40V 100 mA rated TO-92
with an fT>300MHz.

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

Tim

E

#### Eeyore

Jan 1, 1970
0
Tim said:
It just occurred to me, how the hell do you select transistors?

NPN / PNP (assuming bipolar)
Darlington ?
Voltage rating
Current rating
Current gain @ low / medium current
Current gain at higher currents
Power dissipation / thermal resistance / max operating temp
Saturation voltage where an issue
fT
Secondary breakdown characteristics where an issue
Noise figure where an issue

Graham

E

#### Eeyore

Jan 1, 1970
0
Tim said:
an IRF540 should still be good out to, erm, at least 20MHz I would guess

I hate to think what the gate drive power would be !

Graham

J

#### John Popelish

Jan 1, 1970
0
Tim Williams wrote:
(snip)
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?
(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.

D

#### D from BC

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

Tim

I get realy lazy and just order the transistors (models) found in
D from BC

S

#### Spehro Pefhany

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

Tim

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

R

#### RST Engineering $$jw$$

Jan 1, 1970
0
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

W

#### whit3rd

Jan 1, 1970
0
Why is that anyway, hFE vs. collector capacitance, or perhaps more to the
point, hFE per collector area or somesuch?

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.

L

#### linnix

Jan 1, 1970
0
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

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

Fairchild SS9018
TO-92
1100 MHz Typ
30V Vce
50 mA Ic
\$0.04 each

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

J

#### John Popelish

Jan 1, 1970
0
Tim Williams wrote:
(snip)
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?
(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.

J

#### joseph2k

Jan 1, 1970
0
RST said:
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
gain * bandwidth product does seem to be remarkably consistent at the device
level and in simple circuits. It even works for opamps.

L

#### LVMarc

Jan 1, 1970
0
Tim said:
It just occurred to me, how the hell do you select transistors?

Digikey has a couple brazillion in stock and no way to select more than the
most fundamental characteristics of their stock! Mouser is no more helpful.
Where do you go?

In particular, I thought I'd look for high speed BJTs in the 300-1000 MHz fT
range, with V, I and package similar to a 3904 and such.

Tim
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

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