An operational amplifier and more specifically a differential amplifier have two inputs.
When we see a single input amplifier, like this:
- The input and output voltage connections are shown as single conductors, because ==it is assumed that all signal voltages are referenced to a common connection in the circuit (ground)==.
This is the same circuit as a two-inputs amplifier
- The amplifier in the figure can be any two-inputs amplifier, for example an operational amplifier.