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FETs and Tek 1L5 plug-in


Tim Phillips
 

from Tim P (UK)

How reliable are the J-FETs (usually 151-1004-00) used in Tek plug-ins?
Also, does it really matter which way round they are in the sockets?
The manual shows Source, Gate & Drain, but I thought FETs were symmetrical.
Does it make a difference, particularly on a SpecAnal? On the 1L5, one FET
is just after the input attenuator, the other is the last thing before Pin
1 on the plug-in, with a series inductor of 3 ferrite beads.

Tim


Tom Lee
 

JFETs are indeed symmetrical, but not all pinouts are. If the gate is in the middle (as in this case), feel free to swap them if you want. If the gate isn't in the middle, note that the Miller effect (and other parasitic effects) will be larger if whatever you're calling the drain is next to the gate. So, if it's a high-frequency common-source amp, you'll want to make the choice that maximizes the separation of gate and drain.

And in JFET diff pairs, the small deviations from perfect symmetry produce different offsets. In circuits where that matters, you can experiment with different source/drain assignments to find the configuration with the smallest offset.

I haven't looked at the schematic, but I'm assuming that the first FET is a source follower. That won't care which is the source and which is the drain.

-- Cheers,
Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/4/2021 01:48, Tim Phillips wrote:
from Tim P (UK)

How reliable are the J-FETs (usually 151-1004-00) used in Tek plug-ins?
Also, does it really matter which way round they are in the sockets?
The manual shows Source, Gate & Drain, but I thought FETs were symmetrical.
Does it make a difference, particularly on a SpecAnal? On the 1L5, one FET
is just after the input attenuator, the other is the last thing before Pin
1 on the plug-in, with a series inductor of 3 ferrite beads.

Tim




Jonathan Pyle
 

I have a working 1L5 and I haven't had any problems with Q20 or Q420.


saipan59 (Pete)
 

On Thu, Feb 4, 2021 at 03:30 AM, Tom Lee wrote:


JFETs are indeed symmetrical
In old Motorola databooks (circa late 1960's), some JFETs (such as 2N3823) say "Source and Drain may be interchanged". Many other types do NOT say that.
What's up with that?
I was thinking that a typical JFET was NOT symmetrical, but I could be wrong...

Pete


Tom Lee
 

I've never encountered a JFET that wasn't nominally symmetrical. That's not a proof of non-existence, of course, as I have tested only a tiny fraction of the many JFETs ever made, but I can't think of a good reason for an asymmetrical JFET structure except possibly for making a high-voltage device (which may exist, but which I've never seen).

Many JFET datasheets simply don't bother to advertise that the source and drain may be interchanged, even though they can be. Perhaps that's just one more bit of data among many others that are omitted from some datasheets.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 2/8/2021 20:44, saipan59 (Pete) wrote:
On Thu, Feb 4, 2021 at 03:30 AM, Tom Lee wrote:

JFETs are indeed symmetrical
In old Motorola databooks (circa late 1960's), some JFETs (such as 2N3823) say "Source and Drain may be interchanged". Many other types do NOT say that.
What's up with that?
I was thinking that a typical JFET was NOT symmetrical, but I could be wrong...

Pete