There is a paper published by National Semiconductor that gives some insight into FET gain control: AN129.
I have just realized that, not only is it missing on their website, it isn't in paper-copies from the mid-1980s. In fact the only copy I could track down is in my 1977 FET Databook, which is old and yellow-brown and falling apart.
I'm going to risk posting it. NatSemi distributes similar papers free, and I don't see anything in AN129 that they may have regretted publishing.
The paper is mildly "flawed" in that it discusses Gain control up to a gain of around 1,000, absurdly high. But if you look at it backward, it is an attenuator with loss of 1 to 1,000.
We would probably never want that much loss, nor that high of a series resistor (300K); but the general trends apply with the more typical 22K-100K resistors historically favored for good audio FET attenuators.
It also concludes with a Volume Expander application, which had already fallen out of fashion in the 1970s.
Right/long-Click and "Save As...": My copy of National Semiconductor's AN129 from 1977, 5.5 MegaByte PDF
Apologies for the size: this was printed on cheap paper over 25 years ago, the ink has faded and the paper has "ripened with age" like a banana gone brown. And ink is seeping through the page. It does not scan as black/white, and I could not force high contrast without clobbering detail, so the file includes all the shades of grey left on/in the paper.
I have just realized that, not only is it missing on their website, it isn't in paper-copies from the mid-1980s. In fact the only copy I could track down is in my 1977 FET Databook, which is old and yellow-brown and falling apart.
I'm going to risk posting it. NatSemi distributes similar papers free, and I don't see anything in AN129 that they may have regretted publishing.
The paper is mildly "flawed" in that it discusses Gain control up to a gain of around 1,000, absurdly high. But if you look at it backward, it is an attenuator with loss of 1 to 1,000.
We would probably never want that much loss, nor that high of a series resistor (300K); but the general trends apply with the more typical 22K-100K resistors historically favored for good audio FET attenuators.
It also concludes with a Volume Expander application, which had already fallen out of fashion in the 1970s.
Right/long-Click and "Save As...": My copy of National Semiconductor's AN129 from 1977, 5.5 MegaByte PDF
Apologies for the size: this was printed on cheap paper over 25 years ago, the ink has faded and the paper has "ripened with age" like a banana gone brown. And ink is seeping through the page. It does not scan as black/white, and I could not force high contrast without clobbering detail, so the file includes all the shades of grey left on/in the paper.
