In one aspect, a bias circuit includes a rectifier, a negative bias level setter, and a negative bias extractor. The rectifier has a rectifier input and a rectifier output. The rectifier is configured to produce at the rectifier output a negative rectified voltage signal from an alternating input signal applied at the rectifier input. The negative bias level setter couples to the rectifier output and provides a path for current establishing the negative rectified voltage signal produced at the rectifier output. The negative bias extractor has an extractor output and an extractor input coupled to the rectifier output. The negative bias extractor is configured to produce at the extractor output a substantially constant negative bias signal from the negative rectified voltage signal produced at the rectifier output. In another aspect, a bias circuit includes a biasing output coupled between a positive voltage source and a negative voltage source, and a switching circuit coupled between the positive voltage source and the biasing output. The switching circuit is configured to define two or more different current paths through the switching circuit and thereby produce two or more respective biasing states at the biasing output.