is it possible/practical to build a compressor with mosfets?

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You gotta be more specific.

Do you wanna use the MosFET(s) as:

... a voltage-controlled resistor?

... a variable-gain amplifier?

... as a pre- or post-amplifier block, with some other device handling the actual gain-reduction duties?

MosFETs have two characteristics which make them tricky to apply in, say, a voltage-controlled resistor configuration: the intrinsic drain-source diode, and the fairly high gate capacitance, which limits turn on/turn off speed. For control of AC signals, MosFETs are usually used back-to-back in a "bilateral" arrangement to defeat the rectification effects of the drain-source diodes.

A MosFET would not be my first choice for a gain control element.
 
take an 1176 it uses a jfet as a voltage controlled variable resistor shunt for gain reduction.

so knowing it most likely possible would it be more or less practical to designing something similar using a mosfet...
 
FETs are problematic used as voltage-controlled resistors, whether MOSFET or JFET. If you feed about half of the drain voltage back to the gate it helps a lot, but for relatively low distortion you are limited to a 100mV peak or less. This leads to signal-to-noise ratio issues.

There are also not that many relatively small-signal MOSFETs left anymore with a fourth electrode for the substrate. As Dave points out the typical MOSFET now has the intrinsic body diode and requires the opposed series connnection. And those guys have rather high capacitances even for the smallest ones like the 2N7000.

You can also use them as choppers and do a pulse-width modulation approach. These have their own set of issues, the primary one being filtering out the sideband energy and switching carrier from the output of the chopper. There are nonlinearities arising from the dependence of control charge on the switched voltage. A balanced configuration helps.

The quad bilateral CMOS parts for chopper use look appealing but have some quirks. The simplest structures tend to work the best (CD4016 for example) but beware: because some manufacturers realized they could make spec with a "better" die they started to put them into packages labeled 4016, whereas in fact they were the guts of a 4066. The latter has problems used as a chopper switch due to aspects of the circuitry used to make the on resistance more constant. After a chorus of complaints some of the mfg's started to mention some cautions.
 
There has been quite abit of use of CMOS hex inverters and like for their MOSFETS for voltage controlled resistors in VCF and VCA applications. There was a CMOS abuse binge there awhile back at the synth-diy mailing list. There is some of it here, along with some info that can be applied to plain ol MOSFETs. There are plenty of other sites with alot more of this, but this is the only one i can remember off the top of my head.

http://www5b.biglobe.ne.jp/~houshu/synth/

adam
 
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