Another app note from Jim Williams and Linear Technology [PDF!], describing various bridge circuits. Bridges described here are mostly made of passive components, and should not be confused with bridge motor drivers.
Bridge circuits are among the most elemental and powerful electrical tools. They are found in measurement, switching, oscillator and transducer circuits. Additionally, bridge
techniques are broadband, serving from DC to bandwidths well into the GHz range. The electrical analog of the mechanical beam balance, they are also the progenitor of all electrical differential techniques.