BJT base current 1/f noise

How about a Sziklai pair sort of thing, with a BF862 and a PNP? Or a Pfet-PNP darlington? Disappear the base current.

I need it to be a PNP-type when it's done, because the lasers are usually grounded-cathode. (Keeping the high frequency output impedance high is the name of the squeezing game--with an emitter follower you get twice the shot noise.) If I could get good PFETs, that would be great, but they're scarce these days and their noise tends to be horrible.

The Darlington-bandaid approach could be done with an NPN and a couple of auxiliary current sources, but it would be a bit of a mess. The advantage would be that (done right) it would have the noise of the BFP640, which is very low.

Tomorrow I'll try reducing the base impedance by 10x and see if that fixes it. Because of the 39-ohm emitter resistor, it doesn't make sense to go very much lower than that.

One nice thing about 1/f noise is that a 10x amplitude improvement gets you 100x lower corner frequency. At that point I can put in a low-frequency bandaid if I need to, such as sensing the voltage across the base resistor and dumping the equivalent into the emitter so that the op amp tracks out the LF noise. The trick will be to avoid a noise peak at the crossover frequency of the bandaid.

Cheers

Phil Hobbs

No idea, did you try more than one? A tangential question; I use a pfet in 'the same'* circuit, is the pnp better.. lower noise?

George H.

Never mind, just read your response to Tim W. GH

Nice work!

For the unloaded batteries plotted on P 11, below 100 Hz the dependence seems to be much more like 1/f**2 or even 1/f**3, which looks to me like temperature drift rather than actual 1/f noise. I don't know of a mechanism for making 1/f noise at zero current, but you're clearly seeing it above about 10-100 Hz. Interesting.

Cheers

Phil Hobbs

Here's an idea that I've never actually built:

I don't remember what those two caps are for.

MOSFETs are generally much noisier than BJTs, although obviously they don't have the base current problem. If I can get a factor of 10 amplitude improvement I'll probably just declare victory for this iteration and start hacking more complicated topologies for the next try.

Cheers

Phil Hobbs

No, the coupling capacitor to the preamp is too small. It should be smaller than needed for -3dB.

f-3dB is 0.1 Hz for the preamp, but that's still not enough to to short the 10K bias resistor efficiently via the small resistance of the DUT.

I have bought some wet slug tantals, but with the 1/f of the Agilent

Cheap experiment.

Chears, Gerhard.

At least, one can see from the paper that NiCd are best, and size matters. I have some Samsung 18650 LiIon batteries to test, but that has to wait.

Interesting. It has the current noise of the FET at low frequency, though, and looks like it has full shot noise at high frequency on account of the caps.

I'd be looking at something vaguely of that kind for the bandaid approach--next time I have a Saturday afternoon free I'll have a whack at it.

Cheers

Phil Hobbs

Have you considered a bootstrap current source? That solves a lot of problems. Adding an inductor somewhere makes it better at high frequencies, so you can concentrate on the low end. Any copper TC error is easily compensated.

The bootstrap sources I know about use op amps to control things at all frequencies, so they're at least 10-15 dB noisier at medium-to-high frequency than a transistor with Rbb' of an ohm. It's really just the

Cheers

Phil Hobbs

Chopamp?

The simple op amp + BJT + local feedback approach has really stellar noise performance at high frequency. The issue at low frequency is that the base current 1/f noise doesn't get tracked out by the op amp loop because it appears in the collector circuit but not in the emitter circuit, which is where the FB is applied.

ISTM there are four ways of fixing this wart without degrading the HF noise. From easiest to hardest:

1. Reduce the 1/f noise by reducing the base circuit impedance.

1. Do some Darlingtonish thing to return the base current to the collector circuit without adding significant voltage noise.

2. Use some more complicated local feedback scheme to synthesize a copy of the base current and stick it into the emitter circuit so that the main loop tracks it out.

1. Come up with some completely new scheme.

Because the laser current-tunes, its linewidth is degraded more by strong low-frequency components than by the same noise power spread out over the full bandwidth. (This is just like an FM transmitter.) Thus it would be worth trading off a bit of HF noise to fix this issue, but I'm greedy and want to have it all. ;)

Chopamps tend to have low frequency noise around 20-30 nV in 1 Hz all the way down to DC, which isn't bad at all--it would flatten out my noise below about 20 Hz. My faves are the OPA2188, which is a 32V dual, and the OPA378, which is a bit noisier in the flatband but whose noise doesn't rise at all at low frequency, at least down to 10 uHz (~1 cycle/day) which is as far as I've measured it.

I'd be very interested to hear about yours and others' fave chopamps--there's a depressing amount of specsmanship going on in their datasheets.

Cheers

Phil Hobbs

We've use a bunch of them, but they tend to have high wideband noise and sometimes weird charge injection problems. The well-behaved LTC2057 is "only" 11 nv/rthz, but the ADI parts are around twice that, except the ADA4638, which is 66!

The 2057 has a noise spike around 50 KHz, so for extreme performance I'd think about compounding it with another amp. Let the chopamp take over below the 1/f corner of another amp.

Wait, then where does the SiGe go? I don't see any in PNP...

Tim

Would paralleling a few help? Decorrelated noise and all that?

piglet

Good idea. A bunch of BCX71K's or some similar high-beta PNP might be interesting.

h is

eat,

Needs a couple of quiet auxiliary current sources and some level shifting s o that the noisy base current can be returned to the collector circuit. Tha t'll all hurt the flatband noise.

Cheers

Phil Hobbs

Phil, the above bothered me. Isn't the emitter current the sum of base and collector? But I think I understand... base current 1/f noise appears in emitter, and gets taken care of by FB... but that screws up the collector current which has no 1/f noise. Or is my head on backwards this morning?

You could feed back with a sense resistor on the same side as the load. I guess that means either floating the load (laser diode.. not good) or floating the sense R. (or I'm confused)

George H.