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Fri Jan 7 20:08:39 EST 2011

RC-decay envelope, FET controlled?


RST o---------------------o------o
                        __|      |
         R1           ||         |
PWM o--/\/\/\--o------||       =====
               |      ||__       | C2
               |          |      |
             =====        |      |
               | C1       |      |
               |          |      |
               o----------o------O


Principle: resistance modulation.  RST is mostly for resetting and
possibly voltage feedback, i.e. ADC or 1/2 time CMOS trigger.

In general a FET resistor like this is nonlinear but it is possible to
linearize using a simple voltage divider.  This sounds nice, but hase
some problems:

  - Very sensitive to V_T for low condictivity (high resistance)

  - Linear, while we need exponential range.

  - V_DS needs to be less than the V-V_T distance, which means we
    can't have a large resistance combined with a large signal.

    This looks like it's a show stopper.




[1] http://graffiti.virgin.net/ljmayes.mal/comp/vcr.htm





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