# Analyzing AC Op-amp Circuits

Before beginning to discuss AC op-amp circuits, it is assumed that you are already familiar with DC op-amp circuits. Some of what we discuss here is a review of the material covered in the DC op-amp pages. Start by recalling the schematic representation of an operational amplifier:

As long as the op-amp is operating in the linear region, an op-amp AC circuit can be analyzed using the same steps as other AC circuits:

1. Transform the circuit to the phasor/frequency domain.
2. Solve the problem using circuit techniques (nodal/mesh analysis, superposition etc...)
3. Transform the resulting phasor to the time domain if desired.

### Operating within the linear region:

Referring to the above schematic representation of an op-amp, let: $$V_d = V_2 - V_1$$ If we were to increase Vd beyond the linear range, the op-amp becomes saturated and yields: $$V_o = V_{cc^+} \; or \; V_o = V_{cc^-}$$ Most op-amp circuits require operation to be within the linear range for predictable outputs. Therefore the linear range is defined by: $$V_{cc^-} \le V_o \le V_{cc^+}$$

### Rules for ideal op-amps:

The "ideal" op-amp model is used to solve op-amp circuits. We won't go into this any further than simply stating the two rules. Once again refer back to the DC op-amp pages for more information. In reference to the schematic at the top of the page, the two rules for an ideal op-amp are:

$$1) \; V_1 = V_2$$ $$2) \; i_1 = i_2 = 0$$

Let's take a look at an example:

Continue on to AC op-amp circuits example 1