Low noise transistors

The *2SB737* used to be one of the best devices you could get at a sensible price for mic preamps. Rohm no longer make it though.

Graham

Ah, 2SB, right. Used to be able to read things that small, but not any more. Thanks. So what do you use instead?

Cheers,

Phil Hobbs

Ah, 2SB, right. Used to be able to read things that small, but not any more. Thanks. So what do you use instead?

Cheers,

Phil Hobbs

The 2SA1084 ( widely used in this application ) suffices. I think most of the really nice devices designed for audio ( like moving coil preamps ) are now out of production. In the distant past we used 2N4403s btw ( larger than usual die AIUI makes for lower intrinsic resistance and hence lower noise ). It can't match the more modern parts though. 2SA970 was another we used too. Soundcraft has used the 2SA1316 and that can still be found on Toshiba's website - and the 970 as well. Woohoo !

There's a thread in rec.audio.pro where Jim Williams mentions a Renesas ( fromerly Hitachi ) part. 2SC2545. Ah ! PNP - it's the complement of the aforementioned 2SA1084.

Graham

2sd786 and 2sb737 BJTs are close to the available limit, and folded cascode circuits are a good choice. But if the source impedance is near 200 ohms (real), that corresponds to 1.8nV Johnson noise, much higher than the 0.55nV that 2sd786 are capable of. What about the base-current noise, is that a problem with your source impedance? Have you run a network analyzer on your source impedance? You may want to run your transistors at lower currents to reduce the i_n base-current shot noise and suffer the higher voltage noise.

BTW, my hp 4470A Transistor Noise Analyzer is a convenient tool to evaluate low-noise transistors, it's worth looking for one on eBay.

--
 Thanks,
    - Win

Thanks, Win. Glad that I seem to be on the right track. I'll have a look for a 4470A--though I'll want IBM to buy it, so eBay is usually out, more's the pity.

The emitters of the diff pair are driven by a low-noise adjustable current source, and the diff pair collector loads are current sources as well, driven from the same adjustment pot via massively degenerated current mirrors. (+-15 V supplies are such a luxury for low-noise circuits.) The tracking isn't perfect, but the error mainly shows up as a common-mode shift in the dc level at the output of the inverted cascodes, so it isn't a worry. The idea is to be able to change the bias with one pot to optimize the noise for the impedance of the device.

The devices of interest range from about 10 ohms to 500 ohms. Theoretically they should exhibit about twice full shot noise power, which will give them a noise temperature of about 300K (PN junctions used as resistors have a noise temperature of 150K at room temperature, as I mentioned in another post.) The quieter I can get the amplifier, the more confident I am in the measurements--my goal is to be able to calibrate it with a 100-ohm resistor at room temperature and 77K, the way you do with an RF LNA.

I'm very interested in their popcorn and 1/f performance too, since that will tell me a fair amount about device reliability and the contributions of individual atomic asperities and trap states. I don't anticipate there'll be much. The tunnel barriers in Ni-NiO-Ni are so low that the tunnel current isn't dominated by the thin spots in the barrier the way it is in the Al/Al2O3/Al system, for example.

My optical coupling efficiency into the silicon waveguides still stinks, and I broke my last lensed tapered fibre coupler, so for the next few days I'm going to be doing noise measurements. This is all to the good, because I need to know the noise-equivalent power (NEP) as well as the responsivity before I can claim specific SNR numbers.

I ordered some very low Vce(sat) devices from Zetex, which have f_T of around 100 MHz at 10 mA and betas of 300-800. Their saturation V_BE and V_CE suggest that the extrinsic resistances are less than an ohm each, so they may be decent input transistors. I'll have a whack with them and see.

Cheers,

Phil Hobbs

Thanks, Win. Glad that I seem to be on the right track. I'll have a look for a 4470A--though I'll want IBM to buy it, so eBay is usually out, more's the pity.

The emitters of the diff pair are driven by a low-noise adjustable current source, and the diff pair collector loads are current sources as well, driven from the same adjustment pot via massively degenerated current mirrors. (+-15 V supplies are such a luxury for low-noise circuits.) The tracking isn't perfect, but the error mainly shows up as a common-mode shift in the dc level at the output of the inverted cascodes, so it isn't a worry. The idea is to be able to change the bias with one pot to optimize the noise for the impedance of the device.

The devices of interest range from about 10 ohms to 500 ohms. Theoretically they should exhibit about twice full shot noise power, which will give them a noise temperature of about 300K (PN junctions used as resistors have a noise temperature of 150K at room temperature, as I mentioned in another post.) The quieter I can get the amplifier, the more confident I am in the measurements--my goal is to be able to calibrate it with a 100-ohm resistor at room temperature and 77K, the way you do with an RF LNA.

I'm very interested in their popcorn and 1/f performance too, since that will tell me a fair amount about device reliability and the contributions of individual atomic asperities and trap states. I don't anticipate there'll be much. The tunnel barriers in Ni-NiO-Ni are so low that the tunnel current isn't dominated by the thin spots in the barrier the way it is in the Al/Al2O3/Al system, for example.

My optical coupling efficiency into the silicon waveguides still stinks, and I broke my last lensed tapered fibre coupler, so for the next few days I'm going to be doing noise measurements. This is all to the good, because I need to know the noise-equivalent power (NEP) as well as the responsivity before I can claim specific SNR numbers.

I ordered some very low Vce(sat) devices from Zetex, which have f_T of around 100 MHz at 10 mA and betas of 300-800. Their saturation V_BE and V_CE suggest that the extrinsic resistances are less than an ohm each, so they may be decent input transistors. I'll have a whack with them and see.

Cheers,

Phil Hobbs

Phil, I drew a basic preamp, which is basically independent of temperature and allows also very low frequencies to be measured. The gain is also 60dB, but a following differential amp can make up more. The gain depends only on passive components, (1+2*4k99/10) and the current can be tailored to meet the source impedance. But I used PNP transistors because they have lower noise. Maybe 1nV/sqrtHz overall in this case.

+-----+--------------+--o+15V | | | /| | | | OP177 /+|-GND | | +------)-----+-< | .-. .-. | .-. | \\-|-+-+-+ | |3k3| | .-. | | | ||\\| | | | 2k| | | | | | | |2k +-||---+ | | '-' '-' 6k8| | '-' 0u47|| .-.-. | | '-' | | | | >| | | |< 1M| | | |---+-------+----| MAT03 '-'-' /| ___ |\\ | | GND +--------|___|-------)--------------)-+ | | 4k99 | ___ | | .-. +---+ +---+---|___|--+ | 1k2| | |22p| ___ |22p| 4k99 | | | | --- +-----+--|___|--+----+ --- | | '-' --- | | 10R | | --- | | | | |< |< >| >| | | | o--+---+-------| THAT120A |------+ | | |\\ |\\ /| /| | OP227 | | | | | | | |\\ | | +-----+ +----+---)-----|-\\ | | | | | | >-+-)---o o--+-------|--------------------)---+ +-|+/ | | | | | |/ | .-. | | | |\\ | 1k2| | +--------------------)-------)-|-\\ | | | | | | | >---+---o '-' .-. .-. +-|+/ | | | -15V | | | |/ === | |2k4 o 2k4| | | GND '-' | '-' o | _V_ | -10 +--------|___|-------+ 200R (created by AACircuit v1.28 beta 10/06/04

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ciao Ban
Apricale, Italy

Tucker has one you can order with a P.O., but of course they charge a lot more than a typical eBay seller will get from we cheap bidders. However 4470A don't come up very often on ebay, for example none are available now, or in the last 30 days either. A Google search will show a few other used-instrument sources that might have one.

--
 Thanks,
    - Win

Thanks, Ban. My circuit has more transistors and fewer op amps than yours, but operates not so differently.

Interim report:

It's running about 1.1 nV/sqrt(Hz) in 1-11 kHz, with some line harmonics lower down that I have to fix--probably with an on-card regulator, since my cap multipliers don't have enough rejection down there. Adding a

50-ohm source resistance (300K) brings it back up to 1.5 nV, which is about right.

There's something broken about the inverted cascodes, because ripping them out dropped the noise by about 3 dB, which it shouldn't. The clue was that bypassing their bases either to ground or to the supply

*raised* the noise by about 10 dB! Their emitters are supposed to stand still, so this is quite odd. There's probably an optimal V_CE for the 2SD786s, too--interelectrode capacitance and all that.

I'll try using 2N4250s instead of 2N3906s for the cascodes--I probably damaged one of them during torture testing. The feedback network has a

10k : 10 ohm divider and two 1000 uF, 16V aluminums back to back, giving unity gain at dc, with a low-frequency gain 3 dB point of about 32 Hz, which is fine for my purposes, but it would be nice to get rid of the capacitors. I may re-do the bias loop to eliminate them, but that will take a bit of work.

Cheers,

Phil Hobbs

Hello Win,

If you buy through dealers on occasion what is your favorite used equipment dealer? Tucker? I bought a lot of my stuff at auctions when companies closed down. But lately that has dried up, mostly because companies often just don't have anything other than computer stuff anymore. IOW they didn't have any staff that could operate an analyzer.

Regards, Joerg

I've bought plenty of stuff through Tucker (mostly their eBay stuff), but I prefer some of the other Internet used-equipment vendors. Most of them also have lots of eBay activity. The best let you return the equipment, no questions asked, and use Purchase-Order purchases, just like all of the other true industrial-instrument sellers.

--
 Thanks,
    - Win

Phil, me think what you desribe is a design flaw as a differential amp has quite a different and non balanced impedance for common mode signals. So it might be the gain goes up and down and not the noise. It will also induce a lot of distortion. Maybe you should also put a ferrite bead on the base of those

3906 to get rid of the negative input impedance there, seem to oscillate with the lowish cap impedance. Can you post a schematic, because I do not quite understand where your feedback goes. Another thing I do not understand is this gain of 250 what you mentioned before. Do you really have 2.5k resistors in front of the differential amp or what? When you want to try the circuit I posted, do not forget to put 2 diodes 4148 from emitter to base, as the opamp can pull the leg quite high when overdriven and you do not want to damage those expensive matched Quads. I also have some MAT04 here which could be taken as well. If you want to reduce gain at DC, put just a 3300uF or bigger cap in series with the 10R, since the differential voltage swing will be very small there, no need for bipolar Cs. The input impedance is somehow higher, so a pair of 100u is enough there. I also omitted some small(15p) caps from output to inv.IN of those opamps.

--
ciao Ban
Apricale, Italy

Okay, it's done, and all measured up. Replacing the inverted cascodes with PN4250s fixed the excess noise problem. The measurement problem was that my HP dynamic signal analyzer lies through its teeth about its measurement bandwidth per point! When measuring power spectral density--the real bandwidth is almost twice what they quote (Grr.).

I plotted the curve, and whadda ya know, it was nearly perfect but ~3 dB higher than I calculated, everywhere. I made up a stupid little RC lowpass/highpass box and ran it into my trusty HP 3400 true-RMS AC voltmeter, and whadda ya know, the numbers came out 2.84 dB lower, which is right on the money.

With two 2SD786s running at 3 mA each, beta = 750 or so on the input device (4 uA base current), I get an astonishingly good fit to the parameters:

eN=0.635 nV/sqrt(Hz), iN = 1.7 pA/sqrt(Hz).

(That's R_N = 373 ohms, T_N = 19.5 K. Pretty amazing for a BJT--those Rohm guys knew what they were doing.)

The measured noise is well within 0.2 dB of that calculated from these values everywhere from 0 to 10k source resistance. These correspond to the shot noise of the two collector currents divided by the transconductances, plus the Johnson noise of 7.3 ohms, or 3.6 ohms per transistor, which is about right.

The only discrepancy from the prediction of fundamental physics is that the noise current should be 1.6 pA/sqrt(Hz) instead of 1.7, so that's probably caused by the base current cancellation circuit.

Amazing, this theory stuff--it really works. Off to measure some junctions!

Cheers,

Phil Hobbs

Hello Win,

Thanks for sharing your opinion. Buying used equipment is sometimes like buying used cars. I have done both but one has to be careful and mind the experience of others.

Regards, Joerg

I have measured ~ 0.3 nV/rtHz white noise for a discrete medium-power SiGe bipolar transistor at room temperature. Wrote about it into Superconductor Science and Technology. Even if they are intended for GHz frequencies, you can operate them at low frequencies, too.

1/f corner is pretty high (~100 kHz) at standard bias powers but I just found out that the corner goes down when the bias power is reduced. Exactly how low I don't know yet (intend to measure it), in particular at room temperature. There are some noise spectra measured at 4.2K in a paper I just submitted to the WOLTE-7 workshop.

Regards, Mikko

What are some part numbers you took measurements on? BTW, at how low a temperature can you use these transistors?

Mikko, what's the easiest way to read your papers or view your presentations?

--
 Thanks,
    - Win

The Infineon BFP650 appears to be the best performer for my purposes. The measured noise is consistent with 1.5 ohm base-spreading resistance, whereas the Infineon Spice parameters specify 1.03 ohm. I suppose such a small Rbb is a combination of the geometry for a medium-power transistor and the high doping level in the base region typical for SiGe. At least those were my clues when I started looking for a low u_N.

I use them in liquid He ( 4.2 K ), there is no point in going lower because of the heat load to the refrigerator.

You can find preprints in the XEUS web page

which exists (non-updated) for the moment in my personal web space due to some ridiculous web publishing policies my institution brought in effect recently.

Regards, Mikko