Tech Forum

You ask how to get "a positive and a negative part of a sine wave from a circuit that runs on a nine volt battery." It is not clear how you are going to use that output, so I'll presume this is to satisfy your curiosity (or perhaps to help on your homework). The circuit suggested is not efficient, and would not make a good power supply.

First, a 9-volt battery produces direct current, DC, with a steady amplitude of 9 volts (gradually decreasing as it is drained). One simple circuit that produces a fair approximation of a sine wave from DC is a phase shift oscillator, shown below on the left side, (adapted from [url=http://www.learnabout-electronics.org]http://www.learnabout-electronics.org[/url]). The sine wave can be observed between the Out and the 0V test-points.

Second, the sine wave must be separated into positive- and negative-going signals, which is done by the half-wave bridge rectifier. The two half-sine waves can be observed between the + or - test-points and the 0V test-point. The waveforms are shown below, (adapted from [url=http://macao.communications.museum]http://macao.communications.museum[/url]). The negative half-wave should look like the second graph flipped upside-down.
 

If this did not answer your question, you might provide some more information as to how the signals would be used.