## PHYS345 Electricity and Electronics

**Answer for S14.1**
a. R_{f}/R_{i} = 100. For example, R_{i} = 1k
and R_{f} = 100k.

b. R_{f}/R_{i} = 99.

c. For the noninverting configuration, the effective input impedance is extremely high
and does not load the transducer. There is no change in the gain.

For the inverting configuration, the effective input impedance is the sum of R_{i}
and R_{TH}. If R_{i} = 10k (and R_{f} = 1.0M)
then the effective gain is 16.7. As R_{i} decreases, the effective gain approaches 20.

To eliminate loading effects or changes in gain when using an inverting configuration,
use a voltage follower between the transducer and the inverting stage.

**Answer for P14.1**

The ratio of R_{f}/R_{i} 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% 1.91k resistor is parallel with
the potentiometer...

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

**Answer for P14.2**

Availability of a -5 V supply makes is easy:
Use R_{f} = R_{i}
and tie v_{2} at -5 V.

To use a different voltage requires a voltage divider. Then we seek a voltage divider of
-15 V to ground via R_{4} and R_{3} in series such that its Thevenin
equivalent is the same as -5 V and R_{f} above.

If R_{f} = 10k above, then R_{4} = 30k and
R_{3} = 15k works.

**Answer for P14.3**

Put a voltage follower between each input and associated transducer.

"http://www.physics.udel.edu/~watson/phys345/protected/exercises/answers/1117.html"

Last updated Dec. 4, 1998.

Copyright George Watson, Univ. of Delaware, 1998.