Discussion about voltage and current in AC circuits?

For example, a solenoid may then become bidirectional: by increasing or reducing the applied voltage in series with the bias voltage, the armature is controllable in two directions instead of one. In the case of a transistor or vacuum tube an analogous improvement takes place, and the device can amplify positive and negative inputs; or the input signal may be superimposed on a better, more linear, part of the transfer characteristic.