R-S flip flop

Cross-coupled transistors.

More generally, positive feedback.

Imagine this: cascade two inverters, then tie the output of the second back to the input of the first. If the input to the first inverter is 0, the output of the first inverter is 1, and the output of the second inverter is

0, so the operating point is stable, and the values are held. Now assume the input to the first inverter is 1. The output of the first inverter is 0, and the output of the second inverter is 1. Again, the operating point is stable, and the values are held. No matter which state the two-inverter system is in, it will hold that state.

The inputs to an R-S flip-flop allow the state at one point in the loop to be overridden, so a 1 can be changed to a 0, or vice-versa, changing the flip-flop state.

-- Mike --

Hysteisis. A flipflop can be looked at as a bistable multi-vibrator. The transfer function looks something like this:

^ Bias point | V | | | +---|------------ Vout | | | | | | | | | ---------+---|---+ | ^ | ^ A B

------------>

Vin

If you push the voltage up past 'B' the output goes high. If it's pushed down below 'A' the output goes low. If it's let alone (at the bias point) the output is stable in the state is was last. An SR flipflop has one input that drives the voltage past 'B', and another that drives it below 'A'. If neither input is asserted the multi- vibrator is stable at the bias point.

--
  Keith