Fri Dec 17 02:40:37 EST 2010

The variable gain/resistor problem

Looks like I'm out of luck.  It's either the transistor/diode ladder
or expensive or cumbersome chips (LM13700N dual ota @ $1.5 futurlec).

Let's dig deeper.  Why is variable gain so difficult?  Because it's a
multiplication, and we don't really have many multiplying components,
so we use other tricks based on non-linear resistances and small
signals: transistors and diodes and the exponential voltage/current
transfer function.

What about FETs?  When the gate voltage is larger than both source and
drain, it behaves as a variable resistor.  How useful is this to build
variable gain, variable time constant circuits?