3n138 equivalent

[Steve]

Anyone know of an equivalent for a 3N138? I've found them online, but
~$40.00 for a single fet seem a bit steep.

Thanks,
Steve

 

[legg]

Anyone know of an equivalent for a 3N138? I've found them online, but
~$40.00 for a single fet seem a bit steep.

3N128 is 20V and a wider Gm spread.
3N139 has slightly higher capacitances.
3SK33 is 25V and lower cap.
40467/40468 are 20V were considerably less rare a few years ago..
Pro-Electron subs would have different pin-out.,
but BFS28 at 20V would ~fit.

What are you trying to do? If this is for a repair of useful
equipment, it's cost is probably less than the labour involved or
(likely) the cost of replacing the equipment.

If it's for a new project or an app note, then perhaps another n-mos
depletion-mode device or analog switch would work just as well,
particularly at low frequency.

These parts are expensive probably because they are no longer mfred,
and are static sensitive - diifficult to store (for 20 years), ship
or install. If it arrives in a simple plastic or paper container, send
it back.

RL

 

[Michael Black]

Anyone know of an equivalent for a 3N138? I've found them online, but
~$40.00 for a single fet seem a bit steep.

Thanks,
Steve

A dual gate mosfet? Or is it so old it only has one gate? I can't
remember.

To a large extent, it seems like small-signal RF mosfets have faded
out. They were big at one time, but then people learned more about
transistors and realized some of the problem with bipolars were they
weren't being used properly. Once they got over the mentality that
the key to transistors were low power, it seems like bipolars got
better at handling strong signals, and the need for mosfets (that
were once touted as being so great for strong signal handling) faded.

So basically you are looking for an RF dual gate mosfet, and one
that won't cost a lot. It's like small signal germanium diodes
at this point, the key being germanium rather than part number.

One place that has a small selection of mosfets, and is set
up for dealing with hobbyists, is "Dan's Small Parts", and
you might look over his webpage at
http://www.danssmallpartsandkits.net
at the very least, it might give you an idea of what might
be available to search for those numbers at some other parts
place.

Michael

 

[Steve]

3N128 is 20V and a wider Gm spread.
3N139 has slightly higher capacitances.
3SK33 is 25V and lower cap.
40467/40468 are 20V were considerably less rare a few years ago..
Pro-Electron subs would have different pin-out.,
but BFS28 at 20V would ~fit.

What are you trying to do? If this is for a repair of useful
equipment, it's cost is probably less than the labour involved or
(likely) the cost of replacing the equipment.

If it's for a new project or an app note, then perhaps another n-mos
depletion-mode device or analog switch would work just as well,
particularly at low frequency.

These parts are expensive probably because they are no longer mfred,
and are static sensitive - diifficult to store (for 20 years), ship
or install. If it arrives in a simple plastic or paper container, send
it back.

RL

It is for a repair, and you are correct, the labor and replacement
cost far outweigh the cost of the FET. However, I don't know if it's
bad, I don't have any experience checking these types. Pinout isn't
important, the board has room for bent pins.

Thanks,
Steve

 

[Steve]

A dual gate mosfet? Or is it so old it only has one gate? I can't
remember.

To a large extent, it seems like small-signal RF mosfets have faded
out. They were big at one time, but then people learned more about
transistors and realized some of the problem with bipolars were they
weren't being used properly. Once they got over the mentality that
the key to transistors were low power, it seems like bipolars got
better at handling strong signals, and the need for mosfets (that
were once touted as being so great for strong signal handling) faded.

So basically you are looking for an RF dual gate mosfet, and one
that won't cost a lot. It's like small signal germanium diodes
at this point, the key being germanium rather than part number.

One place that has a small selection of mosfets, and is set
up for dealing with hobbyists, is "Dan's Small Parts", and
you might look over his webpage at
http://www.danssmallpartsandkits.net
at the very least, it might give you an idea of what might
be available to search for those numbers at some other parts
place.

Michael

This is for an audio oscillator, single gate. I'll give the site a
shot.

Thanks,
Steve

 

[Chris Jones]

This is for an audio oscillator, single gate. I'll give the site a
shot.

Thanks,
Steve

You can turn a dual-gate mosfet into a single-gate mosfet by connecting the
two gates together. Of course all of the other parameters may not be the
same. If you post a schematic of your application circuit then people may
be able to suggest a substitute that will work there.

Chris

 

[Steve]

You can turn a dual-gate mosfet into a single-gate mosfet by connecting the
two gates together. Of course all of the other parameters may not be the
same. If you post a schematic of your application circuit then people may
be able to suggest a substitute that will work there.

Chris

Actually, I did some testing this afternoon, and the original fet is
fine. The problem lies in one of the op amps, as I posted in the CIC
replacement parts post. It would be nice to have a backup, but since
it's still working, I think I'll just keep using it.

I appreciate the help though,

Thanks,
Steve