Ladyada has an interesting rant on her new blog. In linear circuit analysis, we tend to ignore the difference between the positive and negative inputs. However, in practice positive feedback results in a bistable circuit. (Also read up on negative feedback.)

Usually everyone ends up remembering this detail the hard the way. While designing a tape head preamplifier she was utilizing this circuit from an application note:

After a few hours of staring at the circuit and debugging and wondering “man why the hell is this railing?” I finally look back at the datasheet and realize: oh its in positive feedback, of course its railing.

Just a reminder that the textbook (or pdf) isn’t always right!

DIY Live has an interesting article up describing voltage dividers as power sources. This is an excellent way to create plus/minus voltages from a single positive voltage sources. Unfortunately, the downfall is continuous current flow (and power drain) though the divider. The following ±4.5V supply is detailed in the article. (UPDATE: Read the comments for a discussion on the possible issues with this circuit)

The mathematics behind a voltage divider are quite simple. After dividing the 9V supply into 9V, 4.5V, and 0V, the ground reference is changed to 4.5V. This leaves us with -4.5V, 0V (new ground reference), and +4.5V.