ferrick@acsu.buffalo.edu (patrick k ferrick) (11/13/90)
OK, all you EE types out there! Maybe this is kind of a dumb question, but is there a simple device out there that, if given a certain voltage will produce as output a signal proportional to the log of the input? In other words, input output (1-1000 VAC) -----> (1-3 VAC) I'd appreciate email responses...Thanks, Pat (ferrick@acsu.buffalo.edu)
cjp@megatek.UUCP (Christopher J. Pikus) (11/13/90)
From article <45753@eerie.acsu.Buffalo.EDU>, ferrick@acsu.buffalo.edu (patrick k ferrick): > OK, all you EE types out there! Maybe this is kind of a dumb question, > but is there a simple device out there that, if given a certain voltage will > produce as output a signal proportional to the log of the input? > > In other words, > > input output > (1-1000 VAC) -----> (1-3 VAC) > > Well here is a log function for DC that should get you started. This is from memory so the details are pretty sketchy. Get a basic college electronics textbook to get the details (and calculations) right. Note that This only does DC but maybe you can live with that. This is a classic transform discussed in Circuits I class concerning OpAmps. The key element is that a Diode has a logarithmic transfer curve between non-conduction and conduction (0.5 - 0.65V). thus you have to bias a diode pretty close to that point and use it as a feedback element in a simple amplifier. Diode transfer curve: i (current) | | | | | | | | | | | | | / |------' +----------------------------------- V ^ - 0.6V Op amp with logarithmic feedback. (the actual values are left as an excercise to the student :-) |\ | \ Linear In -/\/\/\--|+ \ U1 R1 | \ | \____________ Log out | / | | / | +-----|- / | | | / | | |/ | | | |_____|/|__________| | |\| / D1 \ / \ R2 / \ | --- - . -- Be Seeing You ... Christopher J. Pikus, Megatek Corp. INTERNET: cjp@megatek.uucp San Diego, CA UUCP: ...!uunet!megatek!cjp