Op Amp Configurations

Rules:

  • No current can flow in or out of the + or - inputs as Rin = infinite.
  • Current can flow in or out of the output as Rout = 0.
  • There is a virtual connection between the two inputs so they are the same Voltage.

Inverting

\(I_{Ri} = I_{Rf}\)

\(V_{in}/Ri = -V_{out}/Rf\)

\(V_{out}/V_{in} = -Rf/Ri\)

Non-Inverting

\(I_{Ri} = I_{Rf}\)

\(V_{in}/Ri = (V_{out} - V_{in})/Rf\)

\(V_{in}/Ri = (V_{out})(1/Rf) - (V_{in})(1/Rf)\)

\(V_{in}/Ri + (V_{in})(1/Rf) = (V_{out})(1/Rf)\)

\((V_{in})(1/Ri + 1/Rf) = (V_{out})(1/Rf)\)

\((Rf)(1/Ri + 1/Rf) = V_{out}/V_{in}\)

\(V_{out}/V_{in} = Rf/Ri + 1\)