Probably the main reason to use a differential input is to cancel the dc offset drift of the base-emitter voltage with temperature. The two base emitter voltages drift in the same direction and thus cancel each other (quite well if they are close together on the same chip), therefore the dc output bias level remains constant. This is important for DC coupled circuits, and to keep the internal bias levels constant in AC input coupled circuits. Most ICs, even if they are AC coupled on the input, have several interal dc coupled stages.But explain to me this then. Why do they use these on the input of allot of chips? What do they do to get the extra power gain? They don't feed both base's do they?
An ideal differential stage's output differential voltage gain is essentially the same as a single transistor stage. Thus there is no gain advantage to a differential stage.