Amplifier (general symbol) An amplifier circuit with one input. Really this is a block diagram symbol because it represents a circuit rather than just one component.
Electricity can flow only if there is a complete circuit from the battery through wires to components and back to the battery again.
A NOT gate can only have one input. The o on the output means not. The output of a NOT gate is the inverse (opposite) of its input, so the output is true when the input is false. A NOT gate is also called an inverter.