1.1 Operational amplifiers

Operational amplifiers and other analog components

The op amp is perhaps the most versatile building block for analog circuits. With an op amp and a few passive components, you can make amplifiers of all sorts with inverting and noninverting gains. You can make integrators, differentiators, filters and voltage-level shifters. You can perform signal rectification, convert voltages to currents and vice versa. Applications information is widely available on Texas Instruments' website and across the Internet by searching with a few keywords.

Op Amps for Everyone is an excellent general reference. This section of the book will also briefly cover special amplifiers and related component types such as instrumentation amps, comparators and difference amps.

Let's start with some basics. If you recall the ideal op amp assumptions, the most important are infinite gain and infinite input impedance. The infinite gain assumption can be troubling. Think of it this way: When negative feedback is connected from output to input, the output seeks a voltage that creates 0 V between the two input terminals.

In Figure 1, the noninverting input voltage is clearly defined; it’s 0 V connected directly to ground. The voltage at the inverting input is determined by the voltage divider formed by R1 and R2. The op amp (with its feedback network) performs a balancing act, driving the output to a voltage that will make V _X = 0 V. Any small deviation of V _X away from zero will nudge the output in the direction to regain balance at 0 V. Some simple nodal equations involving V _IN , V _O and V _X = 0 will yield the transfer function of this circuit.

The noninverting amplifier shown in Figure 2 is essentially the same circuit, with the input signal applied at a different node. The “input” side of R1 is now connected to ground and the noninverting input of the op amp is now the signal input terminal. Through feedback, the output seeks a voltage that causes the inverting input terminal voltage to be equal to V _IN . Again, simple nodal analysis will yield the transfer function and gain. Note that the gain is different in this case with a “+1” term added.

Although many complexities exist involving the nonideal behavior of op amps, understanding an application circuit always starts with a firm understanding of how the ideal circuit is meant to operate.

Finding your op amp

You’ve probably used a prescribed op amp in your labs, and your professors have designed assignments to suit the characteristics of the op amp you are using. When you need to select an op amp that meets the needs of your project, you will find that there are hundreds to choose from. Where do you start?