Page 431 - Op Amps Design, Applications, and Troubleshooting

Instrumentation Amplifiers 407

FIGURE 11.2 Basic instrumentation amplifier used for numerical analysis.

The polarity of the voltage drop depends on the relative polarities and magni-
tudes of the input voltages. For the present example, the lower end of RG is the
more positive, since v 2 is more positive than v^. The current through R G can be
computed with Ohm's Law as

Now, since none of this current can flow in or out of amplifiers A : and A 2
(ignoring the small bias currents), z' G must also flow through the feedback resistors
of those amplifiers. Ohm's Law can be used to determine the resulting voltage
drop across the feedback resistors.

Since electron current is flowing downward through R G, the polarity of R^ and R 2
will be as shown in Figure 11.2.