low noise amplifier for high impedance source

Or is it more like a voltage source with small series capacitance ?

Makes a big difference.

Assuming the latter is the case, the dominant noise source in your circuit is the bias resistors. A 1 megohm resistor has 130 nV / rt Hz of noise while a good J-FET will have only 100th of that.

The trick is to make the bias resistors as high in value as possible and so allow the capacitance of your source attenuate their noise contribution. For each doubling the resistor value, the attenuation factor increases by 6dB while noise increases by only 3 dB.

To maximise available signal voltage, make the input capacitance of the pre-amp as low as possible, ie install the pre-amp right next to the sensor.

........ Phil

Thanks for your reply Phil.

Reckon the laws of nature have changed since then ???

....... Phil

No I remember - YOU posted the SAME query back on July 25. Yes i did have another post but the problem was DIFFERENT at that time. I don't have to think about cryogenic operation at the moment. The thing i want to understand is THIS: HOW the input transistor capacitance and my bias resistor effect the signal to noise ratio?

No I remember - YOU posted the SAME query back on July 25. Yes i did have another post but the problem was DIFFERENT at that time. I don't have to think about detecting a single charge and there is no cryogenic operation. The thing i am trying to understand is: How the input transistor capacitance and my bias resistor effect thesignal to noise ratio?

No I remember - YOU posted the SAME query back on July 25. Yes i did have another post but the problem was DIFFERENT at that time. I don't have to think about detecting a single charge and there is no cryogenic operation. The thing i am trying to understand is: How the input transistor capacitance and my bias resistor effect the signal to noise ratio? And i would appreciate helpful description.

Bugger off.

...... Phil

But this isn't really a current source. Charce delivered into a known capacity means a voltage, isn't it? You also can make a Current measurement but not by enabling the current flowing through resisitve path. Then you should use transimpedance amplifiers. TI have niche appnotes discussing noise in transimpedande amps.

Marte

Hi Marte, Thanks for replying. Here i have the fundamental paper talking about the electrical model of my detector by Comisarow, which is a penning trap. I

archiees wrote

The schematic you give in your .jpg drawing is

. |-- . ,---------+-------------->|-- JFET . | +--/\\/\\-- gnd . | ICT +---||--- gnd . | cs |-- . '-(->)----+-------------->|-- . +--/\\/\\-- gnd . +---||--- gnd

where the resistors are 1M, and the capacitors 20pF. The signal voltage developed across a low-value resistor like 1M will likely be much less than you like, as Phil implied. Resistor noise density is i_n = (4kT/R)^1/2, so you'll want a high R, like 100M or higher. The load capacitance will start reducing the signal (including the resistor noise) voltage above a frequency fc = 1 / 2pi R C, which is only 8kHz for 1M, and 80Hz for 100M. So clearly you want to lower value of the load C if you can.

One point, a portion of the JFET's capacitance, Crss, will be added to C, unless you employ tricks to prevent this.

Once you know the signal voltage vs. current, as a function of frequency, you can compare it to e_n, the JFET's voltage noise at that frequency. You will find low e_n JFETs have high capacitance. Just a little gotcha from Mother Nature.

For example, a classic 2n4392 has en = 3nV and Crss = 3pF. Compare this to a bf862 with en = 0.9nV and Crss = 1.9pF. Ooops!! Actually, that's a bad example, contrary to Mother Nature, as well as Murphy's Law, because the Philips bf862 JFET is far, far better than it should be! :-)

Now *WE* have to F****NG pay money ( US$23) to read this arrogant wanker's reference !!

........ Phil

Thanks Win, I appreciate the explaination. It makes complete sense to me now. Earlier, i was misled about matching the capacitance stuff. I have been working a design using JFETs from Interfet having en =

-Arch

You don't have free access to a decent library that has it, or can get it for you? It's an old, basic reference in JCP, which is an longtime basic journal available in millions of libraries, and is readily available online through tens of thousands, if not hundreds of thousands, of libraries, etc. Specialized journals, like the Journal of Mass Spectroscopy, or Ultramicroscopy, etc., may be another matter, even though for many these are now among today's basic journals.

"Winfield Hill"

He apologised for the blunder and made it available freely two hours ago.

Go shove your f****it opinions where the sun don't shine.

........ Phil

As Comisarow discusses in that paper, above the fc you calculate the RC looks like a low pass filter and so attenuates the signal voltage as the cyclotron frequency rises. However, the current is proportional to cyclotron frequency and so these offset, giving constant signal voltage as the frequency (and thus the mass/charge ratio) varies. Not saying rising signal with decreasing m/z wouldn't be nice, it's just how it falls out. It is convenient that the proportionality between number of ions of a given m/z and the signal voltage is constant, and not a function of m/z (assuming constant cyclotron radius).

What kind of Penning trap system are you working on, Arch?

-- Regards, Carl Ijames carl.ijames at verizon.net

On 16 Oct 2006 15:53:38 -0700, Winfield Hill wrote in Msg.

I don't have the 4329 datasheet, but note that the BF862's noise is spec'd at 100kHz. But the BF862 does seem to have a rather large gm for its low capacitance. When you say "contrary to Mother nature" it sounds as if you meant to imply "contrary to the laws of physics" which would imply "faked datasheet". Did you mean to say that? Personally, I wouldn't put anything past Philips and especially NXP.

robert

Its noise isn't appreciable higher at 1 to 10kHz. I'll look for my data and post it.

No, I was just being funny. I suppose it's really the older part that's at fault in the comparison. The bf862 does what Philips claims, and better. IIRC, the bf862 has a higher gm at a given current than the '4392. JFET noise goes as 1/gm, so that must give the bf862 a leg up. NXP or not, it's a nice JFET, easy to get, and cheap. What's not to like?

Philips states an e_n = 0.8nV spec at Idss, which could be as high as 25mA, and even tho this is the industry-standard way to publish noise specs, it's not the best to my way of thinking. But the bf862 JFETs I measured are no more than 1.1nV or so at 3mA, which makes them very useful parts.

On 17 Oct 2006 04:55:28 -0700, Winfield Hill wrote in Msg.

-- if you parallel N FETs you get N times the transconductance, but you also get N times Ciss and Crss and 1/sqrt(N) the noise.

I'm probably at least half wrong here, so please correct me.

Nothing. I like it fine.

robert

Carl, We have a open cylindrical ICR cell.