The Inverting Op Amp

The noninverting input of the inverting op amp circuit is grounded. One assumption made is that the input error voltage is zero, so the feedback keeps inverting the input of the op amp at a virtual ground (not actual ground but acting like ground). The current flow in the input leads is assumed to be zero, hence the current flowing through RG equals the current flowing through RF. Using Kirchoff’s law, we write Equation 3–4; and the minus sign is inserted because this is the inverting input. Algebraic manipulation gives Equation 3–5.

Figure 3–3. The Inverting Op Amp

Notice that the gain is only a function of the feedback and gain resistors, so the feedback has accomplished its function of making the gain independent of the op amp parameters. The actual resistor values are determined by the impedance levels that the designer wants to establish. If RF = 10 k and RG = 10 k the gain is minus one as shown in Equation 3–5, and if RF = 100 k and RG = 100 k the gain is still minus one. The impedance levels of 10 k or 100 k determine the current drain, the effect of stray capacitance, and a few other points. The impedance level does not set the gain; the ratio of RF/RG does.

One final note; the output signal is the input signal amplified and inverted. The circuit input impedance is set by RG because the inverting input is held at a virtual ground.