WTD: Small signal NPN, HFE >= 200 at Ic < 2uA...

BCX70K? Or a fet!

John

Hey, thanks. This very minute I was looking for a lower noise transistor to drive a laser, has to be really low in noise below a few hundred Hertz. The old 3904 doesn't really cut it but it looks like the BCX70 does. And it comes in SOT-23. Nice.

The BCX70 is very well behaved. High beta, not too fast (tends to not oscillate on its own), beta banded, low leakage, cheap.

John

ON BC847C/848C?

BF862 is used in car radios. The IDSS is tighter than most JFETS. I bought 50 on ebay from this vendor:

I just wish there was more data on those kinds of gems. In my case I need the low 1/f at around 10mA, they only give us data for 0.2mA.

Still mulling the BF862, a really nice JFET. en is only given for 100kHz but sure looks nice, 0.8nv/rtHz (typ). Also, Digikey has about 40,000 of these in stock and there has got to be a reason. Unfortunately not pin-compatible with BJTs. Why did they...? Arrgh.

Not to worry, Greg. Many, if not most, small-signal transistors will work very will in the 10 to 2000nA region. Their beta will be somewhat lower, and their f_T will be substantially lower at a given current (by sqrt(1/Ic), but most of the other parameters will work fine. For some BJTs, this holds down to the pA region. Vbe drops by 58mV/decade at room temp, but you already knew that.

If you look at low Iq LDOs, the bandgap references are using current in that neighborhood. Do the math. Figure 10ua for the bandgap, or 5ua each side, the say 8:1 for the transistor ratio. So you have ic=5/8uA, with transistors that are not really exceptional. Note such circuits have replica devices out the wazoo to control (i.e. replicate) leakage due to the low beta. The poster never indicated the function of the transistor, so it is not known if such tricks are required.

Not enough gain at the low end, but thanks for the suggestion.

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Greg Neff VP Engineering

BC847C looks promising. Lots of gain at the low end, and at low temperatures.

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Greg Neff VP Engineering

I am not at liberty to discuss details of the circuit, other than I need low frequency (DC - 100Hz) linear operation, with Vce around 10V.

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Greg Neff VP Engineering

A FET would be great, but I don't have enough headroom to drive the gate. I can barely accommodate Vbe.

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Greg Neff VP Engineering

Yep, the 3904 isn't so great, noise-wise.

Have a look at Zetex (their low Vcesat eline, like FFMT618/718...) They're not noise speced but I did measure them at about 0.5nV/rtHz and

Ohooo, listen Graham... they're much better than the 4400.

Wow, that would be cool. A bit pricey but if they really hold the line under 1nv/rtHz that would be ok. Do you remember at which current you had measured them?

In case others need such a transistor, it's not FFMT but FMMT618. Data sheet:

I measured them with my 3470A, so it must have been between 1mA and 3mA. Will measure them again tomorrow or the day after.

Also, if you're after ultra low noise, I observed that there's a sweet VCE spot at around 2-3V VCE. Not much, but enough to be noticeable.

As I said they're not speced noise-wise but their ultra low Vcesat mandates low Rbb' so... Also the beta wasn't bad, but I don't remember clearly so I prefer not to tell.

I compared them with some 2SB737 I have and there wasn't clear difference between them. Maybe the 737 was a bit better at low frequency, but I didn't have enough time to fully test them.

The down side is that they have pretty high capacitance.

Greg Neff a écrit :

The 5089 is indeed a good one. You might want to check the MPSA18 variant (same die as the 5089) which should be even better. Don't know what the SOT23 number is though. Maybe MMBTA18 but I couldn't find it.

are you running the low current to get the 1/f noise down, or to reduce power consumption?

Please let us all know what you find. I bet there are lots of audio folks here who need low 1/f.

It'll actually be running thereabouts.

Beta isn't a concern in most low noise apps. Has to be driven hard and any resistors around it need to be low anyhow.

Not a problem either in my case. There's a fat laser diode connected to it and I don't have to care about anything beyond a few kHz.

How about a depletion mode FET like the LND150 or the DN3545 or a JFET?