little photo diodes and an opamp?

There are still lots of fast (5 GHz and up to 100G) NPNs around, and lots of BFT25 equivalents. Try a Digikey search.

I'm currently designing an oscillator with two BFT25s! Spice is showing 1.2 volts Vbe at a reasonable collector current. It's a small chip, but that's ridiculous. When Da City allows me to legally go back to work, I might test one.

Fast PNPs are pretty much history.

We use a fiber-coupled, very fast photodiode that's 0.7 pF.

Also check out the new OPA818 (haven't used it yet).

How much bandwidth and gain do you need? The 1/f noise contribution is often drowned out by the feedback impedance Johnson noise.

? David

You can get about 15 dB improvement at high frequency by using a SAV-551+ bootstrap, if you keep the stray capacitance low enough.

So I was looking at opamps with small C-in.

No. The eN*C noise power density goes as f**2, whereas 1/f noise power goes as (drum roll) 1/f.

That's why pHEMTs make very good bootstraps despite having 1/f corners of 10 MHz or so--the f**2 from the admittance of the capacitor knocks out the 1/f from flicker noise.

For a common-base stage it's C_CE, but you generally don't care because it appears across r_E.

For a FET bootstrap it's basically C_DG + (1-Av) C_GS. For a pHEMT that can be way below 1 pF, so you're limited by the strays.

Cheers

Phil Hobbs

Your entire company is shutdown?

Have you found any with the super low C-B leakage?

Cheers

Phil Hobbs

C_BE, but you generally don't care because

Yes. I go in part-time to check on things, which is technically legal for fundamental company functions and to enable other people to work from home. Most engineers are working at home, but of course production people aren't. It's absurd.

We are currently still paying everyone.

Imagine Apple, Google, Intel, Salesforce, Twitter, all of SF and Silicon Valley and Napa Valley and Tahoe ski areas shut down. Glen Canyon is full of unemployed people walking kids and dogs. The crush at Safeway is over: how much rice and toilet paper will people buy until they run out of space? Still can't find flour for some reason, so the St Patrick's Day Guinness Cake is on hold.

Musk doesn't want to shut down the plant in Fremont. I'm sure Tesla fans approve.

Haven't tried, but that's a good idea. I'll get some and fire up the old Boonton. I wonder what SiGe leakage is like.

Well, I'll get some when UPS can deliver them.

BFT25 seems to leak single digits of fA as a diode. 0.25 pF too.

I'm just finishing up the layout of my Colpitts test oscillator. It's a 4-layer prototype. I punched a hole in the L2 ground plane and inserted a copper patch below the critical oscillator nodes. I'll compare tempco with the patch grounded, or connected to the emitter as a bootstrap/guard.

PADS is being obtuse. I can't flood the copper guard over connection point TP5 without it throwing a clearance error. Too bad.

Thanks.. I think I searched DK for low power and the BFT25 wins. I'm ordering some. I need the cascode transistor to work at ~100nA I_c... all the curves I see peak in mA and then head down.

Is there some DC spec I can look at? Take these BFP405 from Infineon

Do these DC cut-off currents tell me anything. (well, do they tell you something. Mostly clueless.

George H.

Huh.. Right I was thinking of a situation where eN is all 1/f noise. (say I pick one of these jfet or cmos opamps with a 1/f corner at

I'm thinking I'll be groveling for signal at ~100 kHz...

Hmm OK. At some point I just have to build it and measure.

Thanks as always.

George h.

You can use rice flour. If you can only get whole rice, use a coffee grinder until you're sure it's fine enough then grind for the same amount of time again.

NT

... but the result won't keep unless frozen. Store with care, don't leave it sat about untrefrigerated, and don't keep it for days in the fridge.

NT

There's lots of flour at Safeway now. People have moved on to panic buy other things. I'll finally get my Guinness Cake.

I think we'll do chicken and dumplings tonight.

I use a burr grinder for my coffee. Putting the grind through a second time would do nothing more. What sort of crappy grinder would? Do you use a blender-style one? That's a very bad way to make coffee.

I have thought of baking bread by grinding wheat in the coffee grinder. The main problem is the small size of the top and bottom receptacles... and the current unavailability of the grain.

CH

but an effective way to grind rice

the short duty cycle of blender style coffee grinders is more limiting. But it's perfectly doable.

details details :)

NT