- For an amplifier circuit, the electrical angle of the input signal during which the anode or plate current flows in an electron tube. In the case of a transistor, it is the electrical angle during which the collector or drain current flows. For a class A amplifier, the operating angle is 360°. That is, in a class A amplifier there is a current output at all times, regardless of the stage of the input signal. For a class AB amplifier the operating angle is greater than 180°, but less than 360°, for a class B amplifier it is 180°, and for a class C amplifier it is less than 180°.