Squarerooting transistors.

[Erik The ViKing]

>
> What I'm wondering is this: You can use two transistors to create an
> exponential current source from a linear input voltage. Is there a way to
> wire  them in reverse so that you have a square-root (reverse log?) voltage
> source  from a linear input current?
> Just a point in the proper direction would be helpful.
>

Uhm.... square-root isn't the same as reverse log. (Exponential isn't the
same as square!!!!) But it could be something like this:

                + xV
                |
          +--------------->
          |     |
          |c    |c
          /     \
       b|/       \|b
>----+--|         |---+
     |  |\       /|   |
     /    \e    /e    |
     \    |     |    ===
     /    +--+--+
     \       |
     /       /
     \       \
     |       /
     |       \
    ===      /
             \
             |
             |
             - xV

Both transistors NPN. You could start trying with some 10k resistors.

(Not sure if this is going to work!!!)

Erik