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Why this transistor?

  • Thread starter Anthony Fremont
  • Start date
A

Anthony Fremont

Jan 1, 1970
0
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.
http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can think
of?
 
A

Andrew Holme

Jan 1, 1970
0
Anthony Fremont said:
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.
http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can
think of?

Lots of NPN transistors, including the ones you mention, would work equally
well there. He probably had a spare 2N2369 in his junk box.

The op-amp needs to work from a single 5V supply, with (ideally)
rail-to-rail input and output range - or as close as you can get to it.
 
M

martin griffith

Jan 1, 1970
0
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.
http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can think
of?
I think he just has a big bag of 2N2369s
http://homepage.eircom.net/~ei9gq/2m.html


martin
 
E

Eeyore

Jan 1, 1970
0
Anthony said:
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904?

Because he had one already probably !

Why did you expect to see him use a 2222 or 3904 ? Other devices do exist you
know ! Most are quite considerably superior.

Graham
 
A

Anthony Fremont

Jan 1, 1970
0
Andrew said:
Lots of NPN transistors, including the ones you mention, would work
equally well there. He probably had a spare 2N2369 in his junk box.

Ok then, I was just wondering if there was something special about it. It
does specify a much faster Toff, but that is with a much lighter collector
current (if that matters?).
The op-amp needs to work from a single 5V supply, with (ideally)
rail-to-rail input and output range - or as close as you can get to
it.

OK, well that's not a big problem either. I notice he only wants a range of
1V - 4V out of it. Thanks. :)
 
J

Jim Thompson

Jan 1, 1970
0
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.
http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can think
of?

2N2369 is GOLD-DOPED, thus fast recovery from saturation (if
over-driven).

...Jim Thompson
 
G

Graham Holloway

Jan 1, 1970
0
Anthony Fremont said:
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.
http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can
think of?
Try running a 2N2222 at 30MHz square wave/saturated and you see why he uses
a 2N2369. Storage time! The alternative is a Baker Clamp.

Don't know about the op amp. I'm sure something cheap and nasty would do the
job.

Graham H
 
A

Anthony Fremont

Jan 1, 1970
0
Eeyore said:
Because he had one already probably !

That seems to be the consensus.
Why did you expect to see him use a 2222 or 3904 ?

Generally, when it doesn't really matter, these parts tend to be the ones
actually specified or listed as alternates. Why do you think I'm asking?
I'd like to know about some decent/better alternatives myself. I was hoping
that someone that had used the 2N2369 would speak up and say "ya, ist gut"
or something like that. ;-)
Other devices do
exist you know ! Most are quite considerably superior.

Ok, good to know. I'll be sure to stock up on the quality part numbers you
provided. Maybe this is why 3904's and 2222's rule?
 
A

Anthony Fremont

Jan 1, 1970
0
Jim said:
2N2369 is GOLD-DOPED, thus fast recovery from saturation (if
over-driven).

Like this Eeyore.....this is the kind of thing I was hoping to see......an
actual reason to use that part over a more common jelly-bean. I'm guessing
this is where the Toff thingy comes in? Of course it's now a moot point
since it's an obsolete part, but maybe the designer liked the metal cans.
 
J

Jim Thompson

Jan 1, 1970
0
Like this Eeyore.....this is the kind of thing I was hoping to see......an
actual reason to use that part over a more common jelly-bean. I'm guessing
this is where the Toff thingy comes in? Of course it's now a moot point
since it's an obsolete part, but maybe the designer liked the metal cans.

I still have a bunch of them myself ;-)

...Jim Thompson
 
A

Anthony Fremont

Jan 1, 1970
0
Graham said:
Try running a 2N2222 at 30MHz square wave/saturated and you see why
he uses a 2N2369. Storage time! The alternative is a Baker Clamp.

Thanks Graham. :) The fT's aren't that far seperated, so I'd ass-u-med
similar characteristics in that area, but I'm learning differently now. ;-)
Don't know about the op amp. I'm sure something cheap and nasty would
do the job.

Ok, he sure makes a deal over it, but doesn't explain why.
 
A

Anthony Fremont

Jan 1, 1970
0
Jim said:
I still have a bunch of them myself ;-)

Is Toff the thing I should have been looking at to determine this? Gone are
the days of loads of charts relating everything, now I mostly see a few
paramaters specified that (at least AFAIC) leave no way to extrapolate
anything useful. :-?
 
J

Jim Thompson

Jan 1, 1970
0
Is Toff the thing I should have been looking at to determine this? Gone are
the days of loads of charts relating everything, now I mostly see a few
paramaters specified that (at least AFAIC) leave no way to extrapolate
anything useful. :-?

Yes. Though I've only "looked" at a 2N2369 in the past 20 years ;-)

...Jim Thompson
 
W

Wimpie

Jan 1, 1970
0
I'm looking at this schematic and am wondering why the designer chose a
2N2369 over a 2N2222 or 2N3904? I looked at the datasheet for the 2N2369,
and I didn't see anythng amazing about jumping off the page. Did I miss
something. The maximum frequency expected would be under 30MHz.http://homepage.eircom.net/~ei9gq/stab.html

He also makes a big deal out of the particular op-amp chosen as (what
appears to be) a simple voltage follower. Any reason that you all can think
of?

Hello Anthony,

If I had to choose between the 2N2369 and the 2N2222 (for your
application), I would choose the first one.

The 2N2222 is designed for higher current operation (larger die size,
higher capacitance). Because of higher Ft and lower reverse
capacitance (2n2369 device), the gain of your circuit will be higher,
so you need to pick up less signal from your local oscillator. Check
the datasheet from Fairchild MMBT2369A (same chip in different
package).

At low current (for example 3 mA), the Ft of the 2N2222 device drops
to below 200 MHz. It remains high for the 2N2369 device (they specify
at 500MHz at 10 mA). When you overdrive (as Jim mentioned), the 2n2369
device remains fast, so you get good pulses out of it. The 2N2222
device will perform less (at your relative low currents).

The 2n3904 is in-between the two.

Regarding the opamp, I do not have the datasheet, maybe somebody knows
whether it is equal to the OP-07.

Best Regards,

Wim
PA3DJS
 
J

Joerg

Jan 1, 1970
0
Anthony said:
Is Toff the thing I should have been looking at to determine this? Gone are
the days of loads of charts relating everything, now I mostly see a few
paramaters specified that (at least AFAIC) leave no way to extrapolate
anything useful. :-?

When using the 2369 or similar gold-doped transistors with fast
switching times just keep in mind that their hfe is much lower than what
you may be used to from the 3904. That's the price to pay for speed. IOW
there is no free lunch:

http://www.nxp.com/acrobat_download/datasheets/PMBT2369_4.pdf

BTW the TO-92 version of this one seems to be on the way to memory lane,
could become hardcore unobtanium soon. Unless Jim can spare some from
his bag.

The author might have used it in the circuit you posted because he
needed the speed but not much in terms of current gain. Just one comment
here: It isn't a good practice to drive a logic gate straight off the
collector of a transistor. There should be a fast Schmitt first. If the
input signal is smallish this counter might otherwise display next
week's lottery numbers instead of the frequency.
 
L

Lionel

Jan 1, 1970
0
Lots of NPN transistors, including the ones you mention, would work equally
well there. He probably had a spare 2N2369 in his junk box.

That'd be my guess too. Most of us design with whatever we're
comfortable with, & have in the parts drawers. In my case, I use a lot
of BC5xx transistors because they're what I grew up up with, & I can
buy them in bulk locally at dirt cheap prices.

When you're working from a circuit like that one, as long as you've
checked the spec's of the original part, there's no harm in
substituting something equivalent that you already have in stock.
 
L

Lionel

Jan 1, 1970
0
That seems to be the consensus.


Generally, when it doesn't really matter, these parts tend to be the ones
actually specified or listed as alternates. Why do you think I'm asking?
I'd like to know about some decent/better alternatives myself. I was hoping
that someone that had used the 2N2369 would speak up and say "ya, ist gut"
or something like that. ;-)


Ok, good to know. I'll be sure to stock up on the quality part numbers you
provided. Maybe this is why 3904's and 2222's rule?

Yep. They're ubiquitous in American designs. Yurrupeen designs tend to
have lots of BC548s & BC549s for the same reasons.
 
E

Eeyore

Jan 1, 1970
0
Anthony said:
That seems to be the consensus.


Generally, when it doesn't really matter, these parts tend to be the ones
actually specified or listed as alternates.

Not in my world. They're the very last parts I'd suggest.

2N2222s are needlessly expensive simply because they have a metal can.

Why do you think I'm asking?
I'd like to know about some decent/better alternatives myself. I was hoping
that someone that had used the 2N2369 would speak up and say "ya, ist gut"
or something like that. ;-)


Ok, good to know. I'll be sure to stock up on the quality part numbers you
provided. Maybe this is why 3904's and 2222's rule?

They don't rule in any way shape or form.

For general purpose medium voltage small-signal npn I'd use BC214 in the past
and BC549 today.

There are literally hundreds of transistors that would substitute perfectly for
these parts.

Graham
 
E

Eeyore

Jan 1, 1970
0
Anthony said:
Like this Eeyore.....this is the kind of thing I was hoping to see......an
actual reason to use that part over a more common jelly-bean.

Yet quite irrelevant to this application most likely. The signal source isn't
shown.

Graham
 
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