I haven't followed this from the beginning, so perhaps I am missing something, but you have to remember that you won't be able to turn the MOSFET off if the drain is more negative (or positive for PMOSFET) than the body (usually connected to the source). In other words, it is true that it conducts in both directions when fully on, but it will still conduct in one direction when off. This is due to the inherent diode (body diode) created by the PN junction between drain and body (substrate), and the fact that the source is usually connected to the body. If you have a MOSFET where the source is not connected to the body (it has 4 terminals instead of 3), then the asymmetry is gone: as long as the body is kept more negative (or positive for a PMOSFET) than both source and drain, then source and drain are interchangeable. AFAIR, MOSFET analog switches work by keeping the body separate from the source and keeping it more negative than both source and drain (this is easy, tie body to ground and then specify that source and drain must be greater than 0 volts with respect to ground). Sean At 04:27 PM 2/20/02 -0600, you wrote: >On Wed, 20 Feb 2002, Dwayne Reid wrote: > > > Remember that a conducting MOSFET looks like a resistor, NOT a saturated > > bipolar transistor. As such, it conducts BOTH directions when enhanced > > (on). In other words, the Vf of the body diode is swamped by the ON > > resistance of the FET. Because Dave drives both gates at the same time, > > both FETs are turned on. > >See, I didn't know that. I *thought* it might be the case, but couldn't >find any confirmation of it anywhere. I couldn't find any reference in >the data sheets as to whether Rds(on) was the same as Rsd(on)... but I >suppose the R part should have been a give-away, in hindsight. > >Thanks for the correction. Now that I've learned something I guess I can >go home and call it a day! 8-) > >Dale > >-- >http://www.piclist.com hint: To leave the PICList >mailto:piclist-unsubscribe-request@mitvma.mit.edu ---------------------------------------------------- Sign Up for NetZero Platinum Today Only $9.95 per month! http://my.netzero.net/s/signup?r=platinum&refcd=PT97 -- http://www.piclist.com hint: To leave the PICList mailto:piclist-unsubscribe-request@mitvma.mit.edu