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.
More tomorrow.
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