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Old 23rd April 2010, 07:36 PM   #621
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Here's the picture of my new "Danish Setup" -- it has taken approximately 3 months to undo the damage to my physique caused by eating a tin of these cookies purchased around Christmas:

Click the image to open in full size.

While the big tin will hold a couple of SLA batteries, it's also a magnet for a.c. line fundamental and harmonics.

The Colin low noise preamp is a bit lower noise, but isn't as accurate as the one suggested by WJ. I hope to build the Jim Williams amplifier from AN-124 this weekend.
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Old 23rd April 2010, 08:31 PM   #622
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Hey Jack, I built that one too, with the LT1028, but then moved onto a simple 4 x 2sk170v first stage and 1 x 2sk170bl second stage. Nice gain of 60db, no need for dual supply (works on 24V, 2 x 12V lead acid batteries), and I think has lower noise than the opamp one. I became more fond of the crackers square boxes, bigger, taller, fits my batteries

The problem I have with both is calibrating it for 60db gain with low tolerance.
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Old 24th April 2010, 01:29 PM   #623
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The 2SK170 has pretty high input capacitance -- so when you parallel devices the f3 is reduced. The BF862 is only available in SOT23 but has higher transconductance (45 mS) vs 22mS) and lower Ciss (10pF vs 30pF). Because the BF862's transconductance is so much higher (2x the K170) you can really get out of the weeds when you parallel devices.

In these very high gain amplifiers even a tiny of bit of wire will pick up all kinds of radiation. So the design has to be quite compact and the power supply or battery leads bypassed.
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Old 24th April 2010, 02:32 PM   #624
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Quote:
Originally Posted by jackinnj View Post
The 2SK170 has pretty high input capacitance -- so when you parallel devices the f3 is reduced.
Sorry, what's f3?

Quote:
The BF862 is only available in SOT23 but has higher transconductance (45 mS) vs 22mS) and lower Ciss (10pF vs 30pF). Because the BF862's transconductance is so much higher (2x the K170) you can really get out of the weeds when you parallel devices.
Yes, I'm aware of the lower capacitance bf862, but had a bunch of sk170 on hand so I used them. I might redo it with the bf862, but maybe not, it seems fine,

Quote:
In these very high gain amplifiers even a tiny of bit of wire will pick up all kinds of radiation. So the design has to be quite compact and the power supply or battery leads bypassed.
I agree, I bypassed the battery leads right at the sk170 load. Are you referring to the actual part leads that they can be made shorter too? I wasn't too worried because I also use a lid on the box.
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Old 24th April 2010, 05:31 PM   #625
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Quote:
Originally Posted by ikoflexer View Post
Sorry, what's f3?
Here are 2 amps with identical gain -- you can see that the one with several JFETs rolls off more quickly than one with only one device.

Click the image to open in full size.
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Old 24th April 2010, 06:29 PM   #626
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We filter the high frequency anyway, at least JW does, band pass 10Hz to 100kHz. Beyond a few hundred kHz do we even want to amplify?
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Old 26th April 2010, 09:05 PM   #627
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Jack, how do you calibrate your 60dB gain low noise amplifier?

I have an idea for building a high precision simple AC calibrator in the same amplifier box so that when 1V is measured, with high certainty I can say that the original signal is 1mV. Would you be interested in that?
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Old 27th April 2010, 05:40 PM   #628
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Quote:
Originally Posted by ikoflexer View Post
Jack, how do you calibrate your 60dB gain low noise amplifier?

I have an idea for building a high precision simple AC calibrator in the same amplifier box so that when 1V is measured, with high certainty I can say that the original signal is 1mV. Would you be interested in that?
Since we're measuring noise power, we're really integrating the area under a curve, and need to know the bandwidth at each measurement. WJ used an approximation of "48 root F" as the divisor for a 4th order filter with a Q of 5 to calculate nV/root Hz. We don't know anything with absolute certainty, but we can estimate how accurate with statistics.

I like the idea of using a low noise amplifier's bias current to calculate measurement accuracy:

Analysis and Measurement of Intrinsic Noise in Op Amp Circuits - Part X: Instrumentation Amplifier Noise - audio/videoZONE
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Old 27th April 2010, 08:03 PM   #629
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Thanks for the link. My question was about something else. We build a 1000x (60dB) low noise amplifier. Even using 1% resistors for setting the gain, you will see that JW allows for adjustment in the second stage gain, using a 2k variable resistor. How do you calibrate the amplifier to be as close as possible to 60dB? And then, is the gain stable over time (temperature, battery voltage dropping, etc.).

If the 60dB gain figure is specified with +- 0.xyz dB from A to Z frequency then I can simply measure power of the amplified noise with a spectrum analyzer (which now doesn't have to be so fancy) and adjust the result for the 60dB amplification, and if I trust my 60dB tolerance specs I do have an idea of the error.

Of course the assumption here is that the amplifier self-noise is significantly lower than the noise to be measured.

In order to do this, I want to calibrate and trust the specifications of the 60dB amp. That's what I was talking about, and was asking if you're interested in a discussion about this calibration. I was thinking of a new thread on the subject.
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Old 27th April 2010, 08:32 PM   #630
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... and I was thinking this is a feedback amp with gain set through a feedback network, guaranteeing 60dB.
Hmmm...

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