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How to Solve the Summing Amplifier

An operational amplifier, or op amp, is a versatile type of integrated circuit that performs many basic electronic functions on analog electrical signals. Its simple design has two inputs: One inverts, one does not. The op has a single output. By connecting several resistors to the inputs, it becomes a summing amplifier, adding the input voltages together and producing a total voltage at the output. You need to know the values for each input resistor and the feedback resistor connecting the output to the inverting input.

Things You'll Need

Calculator
Pencil and paper

Instructions

- 1
  Divide the value of the feedback resistor in ohms by the ohm value for every resistor connected to the op amp's non-inverting input. Divide the feedback resistor value by the input resistor. Write the results as negative values. For example, the inverting input has 2 resistors, a 100K ohm and a 20K ohm. Your feedback resistor's value is 100K ohms. 100K / 100K = 1, and 100K / 20K = 5. Write these as -1 and -5.
- 2
  Write an equation using your calculated numbers as multipliers and the number of each input as a variable, such as i1 and i2. Draw parentheses around each group of multipliers and variables, and add each group together. For example, the numbers for the inverting input are 1 and 5. Write these as "(-1 x i1) + (-5 x i2) = output."
- 3
  Calculate the summing amplifier's output by substituting voltage values for each input. For example, the the first inverting input to 1 volt and the second to 2 volts. The output becomes ( -1 x 1) + (-5 x 2) = -11 volts.

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