Simple lab amps

The amp sounds like a cool product (send me one!), but why do you need a low noise amp to make noise?

Real parts always seem to add in their own 1/f noise component which cryptographers might not like. The cryptographic noise source presumably doesn't, and the low noise amplifier presumably doesn't add in enough 1/f noise to worry them. The FET noise corner is at a higher frequency than the bipolar one.

The BF862 puts it at about 100Hz, and if the customer doesn't care about the frequency content below 100Hz, that wouldn't worry them

On a sunny day (Mon, 10 Oct 2022 20:49:22 -0400) it happened Phil Hobbs snipped-for-privacy@electrooptical.net wrote in snipped-for-privacy@electrooptical.net:

Note that if you go for some noise at low levels an adversary gets the advantage to make their own not-so-random noise by sending all sorts of RF signals to your location. Apart from your 60 Hz wall-warts with their harmonics etc. and local radio stations with the latest hits and jive.

Am 11.10.22 um 02:49 schrieb Phil Hobbs:

Don't forget to simulate / measure the real part of the input impedance. I found it impossible to combine 250pV/rtHz, > 1MHz, feedback through opamp to input FETs and stability. Yes, it won' necessarilly oscillate, but -100 real Ohm input impedance is sth. I don't want in am amplifier to work with. And a 150 Ohm gate stopper destroys the low noise properties. Cascode helps somewhat.

That can be measured: <

Where the green line leaves the Smith diagram's outer circle, there comes more energy back from the port than we put in. add an inductor, and we have an oscillator.

Oh, and Win's amplifier in AOE3 had that also. <

Yellow line between 100 KHz and 20 MHz.

Talking about Win, I hope he's well? Last time he disappeared was during preparation of AOE3. Do we get a new book?

The only opamp that kinda worked was one from TI with 3 or 4 GHz BW. Its 1/f noise was so bad that it dwarfed the noise of the FET stage even after the first stage gain.

I ended up without FB around the FETs. 16 CPH3910 at 45 mA total seem to be a sweet spot where the pos and the neg TK of the FETs cancel, at least in simulation. Then gain is constant over temp, at least. For this new, overpriced TI FET that seems to happen near Idss, no way.

The third of the board to the right is switchable gain to 86dB, BW limiting and such. 340 pV/rt Hz.

If someone wants to play with it, some empty boards are left.

Cheers, Gerhard

Hi, Gerhard,

Thanks. This one is okay even with an open-circuited cable on the input, thanks to a BLM18BB100SN1D ferrite bead in series with the gate of the input FET and 100-ohm base stoppers on their cascodes. (The other one has a 990/10 ohm feedback divider.)

It's pretty simple, just a cascoded BF862 pair driving a THS4631 differentially, plus an RLC in the feedback network to flatten out the frequency response.

Something slower and quieter is probably next--those SiGe BJTs have super low Rbb' and Ree', at least according to the Spice model.

It'll also be fun trying out a TMUX1511 as a chopper amp--2 ohms, 5 pF.

Cheers

Phil Hobbs

I had cause to email Horowitz a few months ago and asked about Win. He's reported to be well but won't do another edition.

We ordered 50 boards, stuffed with most of the components excluding the op amps and some of the transistors. Two hundred bucks total from JLCPCB, parts included.

Not too shabby.

Cheers

Phil Hobbs