Low noise amplifier.

[Marco]

Even a humble BSS84 can be relatively low noise (limit case 1 nV/rtHz) ... it just has a 50 kHz noise corner.

[T3sl4co1l]

MOSFETs are noisier than JFETs, especially at LF.

I forget what the lowest noise CMOS op-amp is, but most are upwards of 20nV/rtHz, while the better bipolar amps are in the single digits.

Though, MOSFETs have a good history of RF amplifier use.  Many of the "high tech" types do, too: PHEMTs, GaAs FETs and HBTs.  I forget which, but some of them have a 1/f knee in the MHz; all of them are distinguished by very high Gm, very low capacitance, and very low noise (in their intended frequency range), with fT into the 10s of GHz.  A daring few have dared harness them for "DC" use (Phil Hobbs is particularly fond of using bootstrapped combinations for wideband laser diode and high impedance probe amps).

Tim

[sdouble]

MOSFETs are noisier than JFETs, especially at LF.

I forget what the lowest noise CMOS op-amp is, but most are upwards of 20nV/rtHz, while the better bipolar amps are in the single digits.

Though, MOSFETs have a good history of RF amplifier use.  Many of the "high tech" types do, too: PHEMTs, GaAs FETs and HBTs.  I forget which, but some of them have a 1/f knee in the MHz; all of them are distinguished by very high Gm, very low capacitance, and very low noise (in their intended frequency range), with fT into the 10s of GHz.  A daring few have dared harness them for "DC" use (Phil Hobbs is particularly fond of using bootstrapped combinations for wideband laser diode and high impedance probe amps).

Tim

as far as AsGa FET are concerned, we have to face their rather high leakage current. Due to that unfortunate feature, I use them at 50+MHz only.

[ogoun]

Hi Glen from a fellow aussie
I have been toying with a low noise front end design for audio band use for several months now, with no success in using an integrated solution.
The big challenge I am trying to overcome is the input voltage swing.
The design must accommodate up to +/- 60 volts input, while maintaining extremely low noise and distortion performance. Additionally, I am looking to minimise quiescent current draw (this will be battery operated).

The input is from a single ended source, with impedances that range from tens of ohms up to around 7k. This is due to the varying specs of the microphone preamp output signal, for different model mics. The mics in question are professional capacitor mics, such as those made by gras or bksv.

Since I plan to attenuate the signal after the front end, what i am looking to design is a unity (or less) gain imledance converting buffer stage.

Others have solved this problem with a bootstrapped op-amp power supply (shifts the +/- op amp supply rails in sync with the input signal), but this adds significantly to Iq, as the bootstrapper is linear. I tried a switchmode equivalent, but couldnt get it stable, probably because the smps chip's control loop was too slow, or maybe just my crap design

Anyway, I am biting tbe bullet and looking at doing a discrete front end, and was going to try a matched jfet pair in one of the configs from art of electronics, however your design may be adaptible to my needs...

What are your thoughts on this, as your discrete design skills are more polished than mine.

l8r,
pete

[T3sl4co1l]

I don't understand how you desire a "low noise" amplifier, when the source impedance is low (kohms or less) and the desired gain is less than 1...

For gain < 1, the only thing you can do, that's better than a resistor divider, is a resistor in a feedback loop so that the thermal noise is reduced.  (Assuming the amp is quieter, which might not be the case.)  Or the classic: a transformer.

Unless you meant that the two situations are exclusive, i.e., you want low noise only when gain is required.  In which case, the input signal would be much smaller, under 1V, right?

Tim

[ogoun]

hi tim,
problem is, i cant predict what the input impedance will be.. just using a divider would involve the divider's total resistance being greater than 10x worst case source impedance...  thats a noise price i am not prepared to pay.
the design needs to be accurate from tens of microvolts to tens of volts input, with an unpredictable source impedance between zero and around 7k.

The ultimate objective is to get the signal down to a low level, maybe +/- 2.5v, and use a fully differential signal path all the way to a high quality 24 bit adc.

l8r,
pete

[eeFearless]

I used my home brew noise measurement test set to measure the amplifiers noise:

@GK

That home-brew noise measurement test set looks quite interesting.  Where could I learn more about your project?

[Marco]

Since I plan to attenuate the signal after the front end, what i am looking to design is a unity (or less) gain impedance converting buffer stage.

Others have solved this problem with a bootstrapped op-amp power supply (shifts the +/- op amp supply rails in sync with the input signal), but this adds significantly to Iq, as the bootstrapper is linear.

It's not like the mic pre-amp has no quiescent current, the opamp would only add something in the same region. I'd just use something like one of these circuits with a LTC6090 (the differential pair is less sensitive to temperature). It's expensive, but it saves you some headaches.

PS. isn't the mic pre-amp also just an impedance converting buffer?

[Audioguru]

Why does your preamp have the Jfets? The TL072 opamp already has extremely high input impedance Jfets on its inputs.

[Marco]

Why does your preamp have the Jfets? The TL072 opamp already has extremely high input impedance Jfets on its inputs.

The LTC6090 is CMOS, so if he just wanted the input impedance it alone would suffice, but presumably he wants lower noise. The JFET stages have amplification, so the opamps there and the one from the main topic of the thread don't influence noise.

[3roomlab]

would using 2SK879 suggest better SNR at near DC range?
http://www.mouser.com/ProductDetail/Toshiba/2SK879-GRTE85LF/?qs=sGAEpiMZZMvplms98TlKY3rIOtqo%252bKm62v0tHK6313Y5kqoZKMKXnw%3d%3d

[Kleinstein]

The 2SK879 is still a rather small JFET - good for really high impedance sources (e.g. MOhms range, maybe input stage of a capacitive microphone), not a really low noise type for lower impedance sources. You need something line 30 of them in parallel to get something comparable to a BF862.  Good low noise audio range JFETs are 2SK170 / 2SK117 / SK389.

[sdouble]

gm is too small

[sdouble]

do you know the 2SK932 ?
Very close to BF862 in term of overall performances.
3 ranges of IDSS

[phenol]

or the old russian KP903 jfet.

[3roomlab]

The 2SK879 is still a rather small JFET - good for really high impedance sources (e.g. MOhms range, maybe input stage of a capacitive microphone), not a really low noise type for lower impedance sources. You need something line 30 of them in parallel to get something comparable to a BF862.  Good low noise audio range JFETs are 2SK170 / 2SK117 / SK389.

how about this 1?
http://www.mouser.sg/ProductDetail/ON-Semiconductor/2SK3557-7-TB-E/?qs=sGAEpiMZZMvplms98TlKY72UPjEzp3ixViUnpXrpp6o%3d
how do you see if it is relatively "big" or "small"

[GK]

Hi Glen from a fellow aussie
I have been toying with a low noise front end design for audio band use for several months now, with no success in using an integrated solution.
The big challenge I am trying to overcome is the input voltage swing.
The design must accommodate up to +/- 60 volts input, while maintaining extremely low noise and distortion performance. Additionally, I am looking to minimise quiescent current draw (this will be battery operated).

The input is from a single ended source, with impedances that range from tens of ohms up to around 7k. This is due to the varying specs of the microphone preamp output signal, for different model mics. The mics in question are professional capacitor mics, such as those made by gras or bksv.

Since I plan to attenuate the signal after the front end, what i am looking to design is a unity (or less) gain imledance converting buffer stage.

Others have solved this problem with a bootstrapped op-amp power supply (shifts the +/- op amp supply rails in sync with the input signal), but this adds significantly to Iq, as the bootstrapper is linear. I tried a switchmode equivalent, but couldnt get it stable, probably because the smps chip's control loop was too slow, or maybe just my crap design

Anyway, I am biting tbe bullet and looking at doing a discrete front end, and was going to try a matched jfet pair in one of the configs from art of electronics, however your design may be adaptible to my needs...

What are your thoughts on this, as your discrete design skills are more polished than mine.

l8r,
pete

Unfortunately this is a rather high gain (x1000) measurement amplifier for very small signals that isn't readily adapted for unity gain / high-level signals. The current consumption is not low either. For a discrete circuit that might do what you want (buffer those mic preamps that run on +120V or +/-60V supply rails and deliver comparable-level signal output) google "diamond buffer".     

[Kleinstein]

The 2sk3557 is somewhat comparable to the BF862. The good thing is it seems to have specs down to the audio range too - the DS I know are missing specs on LF noise.
Voltage noise of JFETs depends a lot on the trans-conductance. So a high trans-conductance is needed to get a really low noise.

[T3sl4co1l]

All JFETs are pretty much the same, with small tweaks in voltage rating (high voltage JFETs long ago disappeared, so what's left is in the 20-30V range and not much outside of that), and capacitance (junction layout, bond pads?).

The only truly free variable is junction width: bigger means less Rds(on), more Id, more gm, and more C.

Best performance is had when you choose the part with Rds(on) or X_C or 1/gm closest to system impedance (more or less).

Noise mechanisms are poorly documented.  Probably something like trapped impurities in surface or junction layers, or oxide/passivation.  It's not clear if a quiet JFET can be "designed", or if they are always selected.  AoE3 shows that some RF JFETs are quiet and some are not, at LF.  Simply put: you get what you pay for.  If you didn't buy a low noise (at LF) part, you aren't likely to get one.

(BJTs, on the other hand, often have provocative noise performance, even for rather more distantly spec'd types, like switching transistors.  There are few if any "oddball" noise sources, so that the biggest contributors are base resistance (Rbb') and base current noise.  The downside, isn't really a downside, just a matter of different needs for different applications: the impedance range is much lower, kohms to single ohms.  AoE3 plots all of this.)

Tim

[GK]

@GK

That home-brew noise measurement test set looks quite interesting.  Where could I learn more about your project?

I once had an incomplete write up on a webpage, but needs to be re-written into my new site format. I've got the revised PCB for my low noise measurement amplifier etched, drilled and lacquered and once that is done/tested I'll be giving this project a webpage of its own which will include the Gerber files. I'll be using my noise test set in the measurements so this will be as good a reason as any to re-write and web-publish the test set again. I'm currently getting a couple of other incomplete projects out of the way and off the bench, so will probably take a few weeks.
 

[eeFearless]

@GK

That home-brew noise measurement test set looks quite interesting.  Where could I learn more about your project?

I once had an incomplete write up on a webpage, but needs to be re-written into my new site format. I've got the revised PCB for my low noise measurement amplifier etched, drilled and lacquered and once that is done/tested I'll be giving this project a webpage of its own which will include the Gerber files. I'll be using my noise test set in the measurements so this will be as good a reason as any to re-write and web-publish the test set again. I'm currently getting a couple of other incomplete projects out of the way and off the bench, so will probably take a few weeks.

I'm defined looking forward to learning more.  Please keep us posted when you are ready ...

[vindoline]

I too am interested in how you measure and characterize the performance of a low noise amplifier. I'm looking forward to more!

[GK]

The 2SK879 is still a rather small JFET - good for really high impedance sources (e.g. MOhms range, maybe input stage of a capacitive microphone), not a really low noise type for lower impedance sources. You need something line 30 of them in parallel to get something comparable to a BF862.  Good low noise audio range JFETs are 2SK170 / 2SK117 / SK389.

how about this 1?
http://www.mouser.sg/ProductDetail/ON-Semiconductor/2SK3557-7-TB-E/?qs=sGAEpiMZZMvplms98TlKY72UPjEzp3ixViUnpXrpp6o%3d

If the Noise Figure charts in that parts datasheet are to be believed the 1/f noise performance looks very good indeed, at <100 Hz. However the NF versus Rg plot showing essentially a flat ~1dB NF regardless of the value of Rg makes no freaking sense whatsoever.

The 1dB NF plotted/specified (for Rg=1k) computes to an e_n of 2.1nV/rthz. This is at Id=1mA. For comparison the NXP datasheet for the BF862 specifies 0.8nV but the value of Id is neglected. My guess (from measurements) is that the 0.8nV is for Id @ typical Idss (~16mA). 16mA test versus 1mA test = 16/1^0.25 = 2:1 noise test disadvantage for the 2SK3557 (JFET voltage noise is directly related to gm and gm goes by Id^0.25). So in reality the 2SK3557 is likely only marginally more noisy than the BF862, as one should predict from the comparative gm of 35mS (typ) of the 2SK to 45mS (typ) of the BF. Simplified JFET voltage noise can be computed from the equivalent channel resistance which = 0.67/gm.

Dunno how the 2SK compares in terms of input current noise though as (unlike in the NXP datasheet for the BF862) gate leakage current isn't comprehensively charted. The BF682 is a very good performer in this regard. At the Vds and Id I am running them at gate leakage according to the chart in the datasheet is in the order of ~1pA per gate.



 

[ap]

@GK
have you ever measured the amplifier's noise density allone (input shorted)? Connecting a high ohms resistor at the input, as you did, does not say much about the expected extraordinary noise performance. So would be interesting, should be somewhere arround 0.3nV/sqrtHz.

[GK]

@GK
have you ever measured the amplifier's noise density allone (input shorted)? Connecting a high ohms resistor at the input, as you did, does not say much about the expected extraordinary noise performance. So would be interesting, should be somewhere arround 0.3nV/sqrtHz.

Yes. As detailed in reply#30 I measured 0.361nV with the input shorted in an audio A-weighted bandwidth and that was with a lot of rectified mains hum-pickup (the amplifier was unshielded at the time), as shown on the attached scope screen photo.

The test with the 100k resistor (that I posted in a later reply) that returned the theoretical thermal noise of 100k demonstrates the amplifiers negligible current input noise.