Operational Amplifier

Op Amp

The operational amplifier (op amp) is a powerful and versatile device with innumerable applications. The op amp takes two voltages as inputs

and supplies an output voltage, . Essentially, , where is the open loop gain of the amplifier – that is, the gain when there is no feedback around the op amp (more on this later).

The op amp is an active element, which means is requires a power supply. Here, it is connected to and .

Ideal Op Amp

For most applications, the inputs draw so little current (order nanoAmps) and the gain is so high (order ) that we can ignore the exact values for both and make these assumptions:

thus describing the ideal op amp model. This model makes op-amp circuits much easier to analyze while making accurate predictions. When we account for the op amp’s power supply (which ensures ) and simplify the op amp’s inputs to a single input (), we can plot vs. as shown below.

Op Amp Open Loop Behavior

Op Amp

The plot on the right shows the three regions of op amp open loop operation. It is the behavior that of the op amp on the left would show if were fixed between and and was swept. This plot shows the three regions characteristic of the ideal op amp:

In the open loop configuration, region 3 is essentially infinitely thin because the open loop gain of the op amp is infinite. To show the power of this model, we can use it to analyze a configuration of the op amp that expands region 3.

Op Amp as a Buffer

Op Amp

The only difference between this circuit and the above one is that and have been connected. The circuit path between the output () and the inverting input () is called negative feedback, and it means that region 3 of the op amp (where ) is expanded.

To understand why, consider how the equality is affected when . Combining, – that is, the one free input to the op-amp, , is equal to the output, . Thus, the vs. plot shows a line with slope 1 (unity) from the negative supply rail to the positive one.