A method of obtaining a negative bias voltage by means of a resistor or zener diode in the centre tap of a full-wave rectifier circuit. The current in the centre tap flows in the same direction for both half-cycles, so the voltage drop is the same for both. This full-wave-rectified negative voltage can be filtered and used as a negative bias supply. The downside is that all the plate current of the output stage flows through the back bias circuit, so it can be impractical for higher-powered amplifiers.