PHYS345 Electricity and Electronics

Answer for P14.1

The ratio of Rf/Ri needs to vary between 1 and 20 for each input. This may be accomplished, for example, by using a feedback resistor of 100k and an input network of a 5k resistor in series with a variable resistance (potentiometer) that can vary from 0-95k.

You may be satisfied with using a 100k trimmer potentiometer which is typically available; full-range variation will yield a variation of c from 20 to 0.95 (only a 5% error on the low side).

The low-end stop of the range may be improved by putting 1% 1.91M resistor is parallel with the potentiometer...

(You may find many tables of "standard resistor values" on the web. Another one!)

Last updated Dec. 7, 1998.
Copyright George Watson, Univ. of Delaware, 1998.