Click here to purchase the entire book in PDF format. Differential Amplifier The mixing circuit above adds a number of different signals and outputs the result (albeit inverted in polarity and possibly with a little gain added for flavour...). It may also be useful (for a number of reasons that we'll talk about later) to subtract signals instead. This, of course, could be done by inverting a signal before adding them (adding a negative is the same as subtracting...) but you could be a little more efficient by using the fact that an op amp is a differential amplifier all by itself. Figure 2.70: An op amp in a simple differential amplifier configuration You will notice in the above diagram that there are two inputs to a single op amp. The result of the circuit is the difference between the two inputs as can be seen in the following equation. Usually the gain of the two inputs of the amplifier are set to be equal because the usual use of this circuit is for balanced inputs which we'll discuss in a later chapter.