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Multiplication by a Constant

Consider the circuit shown in Figure 5.

Figure 5:  Scaling the Magnitude of a Signal

Because no current flows into the op-amp and because all current at a node must sum to zero, the current entering the input leg must be equal to that flowing throught the feedback leg from the input to the output ( tex2html_wrap_inline1288 ). We also know that the voltage at the inverting input must be zero (that is the voltage indicated by the ground shown connected to the non-inverting input). From our properties of resistors, then, we know that




which means that we're multiplying the magnitude of the input voltage by the constant tex2html_wrap_inline1290 .

Notice that the sign of the voltage has changed. This is a universal trait of the type of op-amp circuits we'll be using in analog computers. As we'll see in the example, it doesn't cause any significant problems in using the computer.

Brian L. Stuart
Fri Mar 14 11:57:03 CST 1997