PAD5

What are you going to design with that diode then?

Happens to pretty much anything these days when no longer needed by key accounts. Key accounts as in manufacturers of cell phones, game consoles or DVD players.

It's the amplifier that's going to operate in vacuum. The signal is nanovolts at very high impedance, with occasional massive RF blasts. I was thinking of using them as back-to-back clamp diodes to protect the front end. Vf is 1.5 volts at 10 mA, so they're not going to protect a lot.

Their zero-bias resistance should be pretty low, but looks like I'll have to measure it. That's a nisance at pA levels. Maybe I can get one of those tins full of Danish cookies...

John

If I remember rightly, the PAD series of low leakage diodes are just low leakage FETs, specified and tested as low leakage diodes.

Winfield Hill, in "The Art of Electronics" call the PAD series "FET diodes", and the book's discussion of FET gate current (section 3.09) credits the 2N4868A N-chsnnel FET with a 1pA gate leakage current - at room temperature and less than 10V of reverse voltage.

-- Bill Sloman, Nijmegen

Did you consider using smt LEDs as low-leakage diodes? (painted, of course)

Electrometer-grade JFETs are another source of low-leakage diodes. I seem to remember that's what some of the PADs series are--JFETs.

Cheers, James Arthur

This Vishay part sure looks like a selected jfet. The channel resistance is in series with the gate junction, which is why Vf is 1.5 volts at 10 mA.

I need low capacitance, so LEDs are out. The little blue Infineon I like is 150 pF!

I think I'll try some transistor c-b junctions just for fun.

I wish I could post the circuit I seem to be evolving. It is getting very, very weird.

John

Pad V? See:

Can't you clamp against two rails and make sure the amp is able to tolerate a few volts? Even if the rails have to be set up just for that purpose. In low current apps simple back-to-back diodes can cause distortion.

Some of the zippy dual-gate low-C FETs such as the BF998 can stomach

Only if your cholesterol levels are ok :-)

I have a huge copper box for such jobs and for EMI hunting. About 2-3ft long/wide/high. Depending on the weather, when moving its lid you can often see a bright blue flash and hear a pop.

I'm trying to find suitable diodes; I need to keep total input node leakage below, say, 10 pA, and capacitance ballpark 10 pF.

There are several issues: input current, input current noise, input damage from overloads, and transmit/receive recovery time. They all fight one another, as any good design problem does.

Skyworks makes some spiffy little RF limiter diodes, 0.8 pF, but of course they don't specify leakage. The RF people usually don't acknowledge that DC parameters even exist.

I'm about halfway through building my diode tester box. I made it through all the box machining without bleeding even once.

John

Maybe you could shave an LED to a tiny nub, cutting the pF? Might not be too hard to do with a Dremel and a diamond wheel, since the die's already fixed and mounted in clear epoxy.

And/or bootstrap it.

Something tiny, like a 3V Vce(br) GHz ft transistor maybe?

Very intriguing. I'm spending my time profiting off Obama's chaos. Lucrative, but boring.

James

isn't because they normally use PIN diodes which of course don't work at DC as they do at R.F. ?

Couldn't the non-signal side of those diodes be biased to the same DC level as the input signal; that would tend to decrease leakage some..

You'd have to measure that but I guess it will be disappointing.

I'd rather do that as well but the honey-do list said that I had to perform my most favorite task. The yearly pellet stove cleaning. Yuck. Well, at least it won't happen again until spring 2010.