<A question occurred to me today : Can you have an embedded analogue <computer, and if so, how many op-amps are needed to have one? Yes, and that's very common. None, a low pass filter(RC) performs a function and can be considered analogue. <I was looking at the service manual for my Micropolis 1203 hard disk, and <I read the circuit description of the servo electronics. It's a fairly <complicated array of op-amps, which combine integral and differential <forms of the position error, positioner current, etc. I would claim that <is an embedded analogue computer. Valid claim, also a good example of a fairly complex function. <On the other hand, I think it would be stretching the definition to call <a simple op-amp wired as a voltage follower an analogue computer. Correct. However often the buffer is between some function or follows one so it's part of the analog system. Other analog systems common to computers: cassette IO (low pass filter on output) and complex filter/differentiator edge/peak detectors for input. Some of the acients used PLLs for clock recovery (KANSAS City is one). Analog to digital conversion (quantification). Digtial to analog conversion (filtering) Disk/tape systems have several layers of analog function for data and control. Allison