I need it to be really linear, and act as a variable resistor based on gate to source voltage. I need it to compress audio.
What would be the best device ? I need an Rds that goes down to maybe 2 ohms, but the main thing is that the Rds stays constant with varying Vds at any given Vgs.
Any recommendations ?
In the low ohms range,
In the low-ohms range, a 2N7002 with 100% to 200% AC gate feedback works pretty well. The linear range is limited to about 2 to 10 ohms, so you'd need several stages to get a wide range.
The usual choice is a JFET with 50% AC gate feedback, but you'd need to parallel a bunch of them to get to 2 ohms.
Why is the impedance level so low?
Cheers
Phil Hobbs
What sort of Vds range do you have in mind? Fets can be pretty linear to +- a tenth of a volt or so, but not volts.
A linear multiplier IC might be better.
A pair of fets used antiphase might be interesting.
Well, something out of the box like a piece of thin nichrome with a DC current to heat it while picking off your AC signal with a cap might be interesting. Variation of the low distortion filament oscillator.
Interesting suggestion! A small incandescent filament lamp would have a cold/hot resistance ratio of about ten times. So might need to cascade several stages and attack time might not be the fastest but distortion would be really low.
piglet
Unless you have a small DC range in mind, (or a very small AC range), a JFET in voltage-controlled-resistance is going to be disappointing.
Try an OTA solution instead, that's what they really do well. Figure 26 here may be useful:
nichrome has a low thermal coefficient, but stainless, or tungsten, etc would work,
a 5W 12V lamp has a cold resistance of about 2.4 ohms (measured) which rasies to about 29 ohms (computed) at full power
presumably a 200mW 2.4V lamp would follow the same trajectory.
'Wein bridge oscillator'?
No, if you use the FET as the shunt element and keep the p_p audio small across the FET And use the 50% feedback trick mentioned by another poster, You can get very good results. Distortion well under 1%. The worst case distortion occurs at about 6 dB attenuation.
e:
gate to source voltage. I need it to compress audio.
2 ohms, but the main thing is that the Rds stays constant with varying Vds at any given Vgs.current to heat it while picking off your AC signal with a cap might be int eresting.
Low current filament gives the fastest attack. Driving it symmetrically eli minates ac components of the control voltage appearing on the audio line, w hile not needing to slow down control to minimise this. Filaments can handl e sizable pulses if you need attack to be faster. But limit those pulses, y ou don't want to get filament spots too hot. Using the whole 10x R range me ans running them white hot, which would give short life.
NT
The OP wanted 2 ohms for some reason, which would take a lot of (matched) J FETs, and the 50% feedback trick doesn't work with MOSFETs. MOSFETs in rhe low ohms region actually want 100% to 200%.
I did some measurements of this a few years back with NXP 2N7002s and found that 150% worked well. That is, I put the two FETs in series, drove their gates in parallel through a resistor, and put a big cap from the gates to t he drain of the top device. That's ~200% FB for the bottom one and ~100% fo r the top one. In the low ohms region THD was around -60dB.
Cheers
Phil Hobbs
How about a platinum RTD, e.g. Pt-100?
The resistance range is disappointingly small. Of course if you use two of them in a bridge, you don't need much of a range--the audio will cancel out when delta-T is zero.
Cheers
Phil Hobbs
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