In my earlier posts about balanced audio I used an audio transformer for the input to our amplifier. This was easy to show how to identical signals would not be passed. Because there are no differences between two identical signals, no current flows between the two signals and the transformer sees nothing. This is great when noise or distortion are present in equal amounts on both signal wires coming into the transformer. Using what we call common mode rejection, the two common signals are rejected and only differences are amplified. But most inputs on today’s audio electronics no longer us audio transformers. Instead they use something called a differential amplifier.
Like its name implies a differential amplifier performs a similar function to that of the transformer: rejecting that that is in common and amplifies differences on its two inputs. Here is what a simple block diagram of such a circuit looks like:
V1 and V2 are the balanced inputs. V out is the output. Not shown is the power supply connections. Place any signal common to both V1 and V2 and nothing will show at the output. If the signals coming into V1 and V are different, then that that is different will appear on V out. This circuit will not amplify things in common. It will amplify things that are different. This can be handled with a $0.25 op amp or a simple diff pair. The diff pair looks like this:
This drawing shows a basic Diff pair, the heart of any differential amplifier. You see only two transistors (M1 and M2). V in is where the two balanced signals come in. V out is where the two balanced outputs can be found. Again, the same characteristics as in the first example. There are many differences between the two approaches, one is complex with perhaps 50 transistors to make it work, the bottom simple, with two or more transistors. The top one is an op amp, the bottom a discrete circuit.