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My version of the G = 1000 low noise measurement amp (for Ikoflexer).
My version of the G = 1000 low noise measurement amp (for Ikoflexer).
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Old 19th July 2018, 10:03 AM   #471
Waly is offline Waly
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Originally Posted by gerhard View Post
Controlling the current source looks elegant, but I cannot (and would not like to) run an IF3601 at/near Idss, let alone 4 of them. That could easily be 1A, for one.

Large source resistors are impossible because of their thermal noise, so controlling the gate voltage is the only solution.
BTW, to lower the drain current, there’s no reason why you could not bias the gate(s) at a fixed negative bias, like you are doing in your beloved jfet based RF amps, and then keeping the drain CCS bias control loop.
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Old 19th July 2018, 01:38 PM   #472
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That could also make some unlucky JFET pairs work together.

One might use left & right outliers for the CS and cascode with the same Vgs when I look at the picture below. Extra bonus if you publish that on diy without a word about the selection. Just the circuit and "It sounds absolutely eargasmic!"

What 1/f corner did you see for these ON FET/BJT cascodes?
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Last edited by gerhard; 19th July 2018 at 01:56 PM.
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Old 19th July 2018, 02:46 PM   #473
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100 something Hz for 8 x CPH05 each close to 15mA Idss (attached). Gain is 60dB.
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Old 19th July 2018, 03:26 PM   #474
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Was a tif file and the forum software silently dropped it, trying again as jpg.
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Old 19th July 2018, 04:27 PM   #475
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BTW, not my measurement, posted the noise data with author's permission.
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Old 19th July 2018, 05:15 PM   #476
jackinnj is offline jackinnj  United States
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My version of the G = 1000 low noise measurement amp (for Ikoflexer).
Quote:
Originally Posted by Waly View Post
BTW, to lower the drain current, there’s no reason why you could not bias the gate(s) at a fixed negative bias, like you are doing in your beloved jfet based RF amps, and then keeping the drain CCS bias control loop.
See Post #1 on this thread.
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Old 19th July 2018, 05:30 PM   #477
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I've only built myself a version with BF861 cascoded with individual bipolars. Aluminum milled case 12mm thick on all sides renders about 1.5nV of 50Hz harmonic noise (under 1nV @150Hz) in a VERY noise polluted lab.

My conclusion was that BF861's have lower flat noise compared to CPH05 (8 x BF861 @12mA Idss was consistently 0.28nV/rtHz) but have higher corner noise (about 400Hz).
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Old 19th July 2018, 05:38 PM   #478
Waly is offline Waly
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Originally Posted by jackinnj View Post
See Post #1 on this thread.

Not sure what you mean, I see only a DC bias feedback loop encompassing a photovoltaic SSR. My suggestion is to bias the gate(s) with a fixed negative voltage and use a DC bias feedback loop on the CCS load (schematic above). This arrangement would have a high enough PSRR to allow a simple 317/337 power supply (which is what I did). Using a simple resistive load (and no DC bias feedback loop) requires a very low noise power supply, or batteries as Gerhard is using.
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Old 2nd August 2018, 08:01 AM   #479
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Quote:
Originally Posted by 1audio View Post
1A7 manual can be had here: http://bama.edebris.com/manuals/tek/1a7
The 1A7A here: TEKTRONIX 1A7A PLUGIN SM Service Manual download, schematics, eeprom, repair info for electronics experts

The 1A7 does have resistors in parallel with inductors and paralleled tubes. The 1A7A has only resistors.

Right, exactly like I said in the first place. Thanks for the link.

My version of the G = 1000 low noise measurement amp (for Ikoflexer).-gridstop2-png

My version of the G = 1000 low noise measurement amp (for Ikoflexer).-gridstop-png
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Last edited by poldaaudio; 2nd August 2018 at 08:15 AM.
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