# JFETS in a strange land...

#### Hypatia's Protege

Joined Mar 1, 2015
3,226
Attached is an excerpt from Linear Technology's application note #118

I draw your attention to "D5" and "D6" --- Here a 'brace' of JFETs are employed in a so-called 'clamp' arrangement (I tend to favor the term 'clipper' inasmuch as level translation is not the point -- but to continue...) --- I must confess that I fail to see the advantage of such a scheme over a pair of 'fast' diodes?

Granted, the pictured arrangement would feature 'near infinitesimal' leakage current, still I can't 'shake' the feeling that I'm afflicted with a 'mental scotoma' as regards some essential property of JFETs and am, hence, missing 'the big picture'???

Any insight, ideas or idle musing will be greatly appreciated!

HP

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#### OBW0549

Joined Mar 2, 2015
3,566
Granted, the pictured arrangement would feature 'near infinitesimal' leakage current, still I can't 'shake' the feeling that I'm afflicted with a 'mental scotoma' as regards some essential property of JFETs and am, hence, missing 'the big picture'???
Diode-connected JFETs are used for D5 and D6 simply because the 2N4393 has extremely low gate leakage; switching speed isn't a factor.

#### KL7AJ

Joined Nov 4, 2008
2,229
Attached is an excerpt from Linear Technology's application note #118

I draw your attention to "D5" and "D6" --- Here a 'brace' of JFETs are employed in a so-called 'clamp' arrangement (I tend to favor the term 'clipper' inasmuch as level translation is not the point -- but to continue...) --- I must confess that I fail to see the advantage of such a scheme over a pair of 'fast' diodes?

Granted, the pictured arrangement would feature 'near infinitesimal' leakage current, still I can't 'shake' the feeling that I'm afflicted with a 'mental scotoma' as regards some essential property of JFETs and am, hence, missing 'the big picture'???

Any insight, ideas or idle musing will be greatly appreciated!

HP
A Jfet in this configuration simply has a more abrupt transition than a diode. Think of it as an amplified diode...sort of.

#### KL7AJ

Joined Nov 4, 2008
2,229
A Jfet in this configuration simply has a more abrupt transition than a diode. Think of it as an amplified diode...sort of.
Clarification.....by "abrupt" I mean with respect to voltage, not to time.

#### crutschow

Joined Mar 14, 2008
29,825
A Jfet in this configuration simply has a more abrupt transition than a diode. Think of it as an amplified diode...sort of.
I don't see why it would exhibit such a characteristic. What is your justification for that?

#### Hypatia's Protege

Joined Mar 1, 2015
3,226
Well hey! Many thanks for the informative replies! Being, as I am, 'less than au fait' with 'off-label' applications, I've learned something today!
Diode-connected JFETs are used for D5 and D6 simply because the 2N4393 has extremely low gate leakage; switching speed isn't a factor.

#### RichardO

Joined May 4, 2013
2,271
Now, those diodes cost \$11 each in 500 quantity!
I think the parts we were using may have been FDH11oo but I can't find an info on the web... I will have to check my old Fairchild data book to be sure.
We never paid anywhere never that much for the ones we were using. They cost more than 1N4148's but not _that_ much. I know that has to be the case because the were used 64 at a time.

#### #12

Joined Nov 30, 2010
18,223
I think the parts we were using may have been FDH11oo but I can't find an info on the web...
My 2 best sources say there ain't no such animal.

#### RichardO

Joined May 4, 2013
2,271
My 2 best sources say there ain't no such animal.
I am beginning to suspect that my brain cells storing the part number have failed or been re-assigned to other valuable information.
I am petty sure that the part -- maybe by a different part number -- is in my 1980's vintage Fairchild diode data book. I am not near my data books right now so the answer will have to wait.