Page 106 - Op Amps Design, Applications, and Troubleshooting

Inverting Summing Amplifier 89

voltage gain of that input. In theory, there is no requirement to have a physical
resistor for R/—the source resistance alone can serve as the input resistor. In prac-
tice, however, the source resistance is usually only an estimate and rarely a con-
stant; therefore, it is generally wise to include a separate resistor as jR z and to make
this resistor large enough to minimize the effects of changes in the source resis-
tance. The application must dictate the degree of stability needed, but in general,
if the input resistor is 10 times as large as the source resistance, then the effects of
changes in the source resistance are reduced by about 90%. If greater protection is
needed, increase Rj accordingly.
For purposes of our sample design, let us choose R n to be 10 times the value
of source resistance. The value of R n then is computed as

Calculate the Required Feedback Resistor (R F). The feedback resistor is
calculated by using a transposed version of the basic voltage gain equation, Equa-
tion (2.6), for an inverting amplifier.

In our particular circuit,

We choose the nearest standard (5% tolerance) value of 36 kilohms.

Compute the Remaining Input Resistors. Values for each of the remaining
input resistors can be calculated by using yet another transposed version of the
basic voltage gain equation, Equation (2.6).