hello > > /rambling preamble > > My reading on transconductance goes back to Ia/Vg Curves, chasing > linearity on valves (as advanced a science as surfing tidal waves) and > such 'prehistoric' notions. > > My current requirement is in switching circuitry, and particularly, > replacing fets in switching circuitry. This stems from the fact that > the local supplier who has the parts today (instead of the day after > tomorrow) fails to discriminate between logic level fets and ordinary > ones, beyond simply specifying the gfs. The gfs cannot be explained to > me by 'tech support', needless to say. > > /end of rambling. > > > > Now, the question: Can I convert the figure specified in Siemens to a > gate turn-on voltage range? This becomes interesting if I am trying to > specify an equivelant to a blown fet and know the gate voltage. If I > match the typical figure is that enough? Does a higher Siemens value > mean a higher gate turn-on voltage? (Presume typical values are > quoted) It is much quicker to match to what _is_ available, than to > wait for what is not. > a: transconductance g_fs (unit Siemens = A/V; g for conductance, f forward, s source is common pole) is typically a figure for linear applications or in a linear bias point. If you multiply gfs by the impedance (Ohms) seen at the drain then you get the voltage gain in this bias point. FETs used as switches are usually overdriven to switch their drain-to- source resistance from very high to very low values. Typically for "logic FETs" this FET can be overdriven with a typical "logic" voltage used as gate voltage (e.g. +5 V typically delivered from TTL/CMOS logic ICs). The on-resistance is given for that bias point. Of course the maximum allowed current (continuous and peak) is another important figure. Also the maximum allowed drain-to-source and gate-to-source voltages must match your application. Especially the newer MOS-FETs do no tolerate GS voltages as high as earlier ones (thinner oxide). Often GS voltage can be limitted by a series resistor and zener-diode in quasi-static (low speed) applications. For high speed switching the driving circuit must be able to deliver high peak currents (even 1 .. 2 A) to the gate of a MOS-FET because of the high input capacitance (Cgs and Cgd). ...
regards Alois B. > > > Further, a Siemens is defined as 1 amp per volt. Does the Siemens > figure take the current of the device into the calculation? If one > device is specified at 4 Siemens, and another at 8, what is that > telling me? > > -- > > With best Regards, > > > Declan Moriarty. > -- > Author: Declan Moriarty > INET: [EMAIL PROTECTED] > > Fat City Hosting, San Diego, California -- http://www.fatcity.com > --------------------------------------------------------------------- > To REMOVE yourself from this mailing list, send an E-Mail message to: > [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the > message BODY, include a line containing: UNSUB CHIPDIR-L (or the name > of mailing list you want to be removed from). You may also send the > HELP command for other information (like subscribing). - - - - - - - - - - - - - - - - - - - - - - - - - - Alois Bauer WORK Microwave GmbH Raiffeisenstr. 12, D-83607 Holzkirchen, Germany Tel. {+49} (0)8024-6408-0 / FAX (0)8024-640840 -- Author: Alois Bauer INET: [EMAIL PROTECTED] Fat City Hosting, San Diego, California -- http://www.fatcity.com --------------------------------------------------------------------- To REMOVE yourself from this mailing list, send an E-Mail message to: [EMAIL PROTECTED] (note EXACT spelling of 'ListGuru') and in the message BODY, include a line containing: UNSUB CHIPDIR-L (or the name of mailing list you want to be removed from). You may also send the HELP command for other information (like subscribing).
