http://www.edn.com/archives/1994/051294/10di10.htm
a high-speed, full-wave rectifier-or absolute-value circuit-that uses a high-speed clamping amplifier
http://www.edn.com/archives/1995/033095/07di6.htm
performs accurate full-wave rectification on the input signal
http://www.play-hookey.com/analog/full-wave_rectifier.html
alternative approach to a half-wave rectifier is to use the input-signal polarity to control the signal channel gain
http://www.edn.com/archives/1994/122294/26di5.htm