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Electronics Forum / Basics / December 2005MOSFET transconductance
I'm trying to analyze a trivial common-source amplifier based on an IRFP9240 power P-channel MOSFET (datasheet at http://www.irf.com/product-info/datasheets/data/irfp9240.pdf). The circuit is simply this: -24V | | .-. | | Load | | 100R '-' | | ||-+ ||-> IRFP9240 Vin ---||-+ | | === GND Now, I know that voltage gain = gm * Rd. But how do I find gm? I'm interested in the condition where the MOSFET will be operating in its linear region, with Vgs close to Vt; Vds around 1V, Id around 200mA. The datasheet specifies forward transconductance of 4.2S, but that's at 7.2A and 50V, in the saturation region. The transfer characteristic curves only go down to 400mA, and anyway they're at 50V also. Similarly, the output characteristic curves don't show the region I'm interested in. Given the available data, how can I determine the transconductance at the operating point of interest? Thanks for any help! > I'm trying to analyze a trivial common-source amplifier based on an > IRFP9240 power P-channel MOSFET (datasheet at [quoted text clipped - 32 lines] > > Thanks for any help! The simplest way, is to run spice. The work has already been done for you. Don't reinvent the wheel. If you want the equations, again, check in the documentation of one of many spices out there. I had a quick check on the irf site. They have the spice model in a .subckt. The main model in the subckt is: .MODEL MM PMOS(LEVEL=1 IS=1e-32 +VTO=-3.73073 LAMBDA=0.0109168 KP=7.97276 +CGSO=1.08608e-05 CGDO=1e-11) This is enough information for you to either, put the device in a spice circuit and let spice compute the gm from this data, or secondly, enable *you* to *look* up the equations, with this data and manually calculate the gm. If we tell you everything, you wont learn anything. I know of one spice that will trivially plot this gm as a function of Id:-) Kevin Aylward 431infoEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. >I'm trying to analyze a trivial common-source amplifier based on an IRFP9240 >power P-channel MOSFET (datasheet at [quoted text clipped - 32 lines] > >Thanks for any help! Why not measure it? John >Why not measure it? Presumably parameter variation. It wouldn't have much relevance to the next one in the batch.  Signature"What is now proved was once only imagin'd" - William Blake >>Why not measure it? > >Presumably parameter variation. It wouldn't have much relevance to the >next one in the batch. Well, that's a problem, but how would a Spice model be any better? In fact, I'd not trust any Spice model of such a fet operating at such low current and drain voltage. If Id were forced somehow, I'd imagine Gm would be pretty consistant across devices. Gate threshold voltages will be all over the place, of course... been there, done that, got scars. John >>>Why not measure it? >> >>Presumably parameter variation. It wouldn't have much relevance to the >>next one in the batch. > >Well, that's a problem, but how would a Spice model be any better? It probably wouldn't be, in the unlikely event that you're using KevSpice. I'll wager he still hasn't got that parameter-spread algorithm sorted out properly. ;-) Signature"What is now proved was once only imagin'd" - William Blake >>>> Why not measure it? >>> [quoted text clipped - 6 lines] > KevSpice. I'll wager he still hasn't got that parameter-spread > algorithm sorted out properly. ;-) Ahmmmm...the default variations are actually pretty reasonable. The issue with typical powerfet vendor models are that they are usually just a simple level 1 model. This misses a lot of detail, especially subthreshold. It can be better to fake a Bsim3. Kevin Aylward 431infoEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. Paul Burridge wrote... >> Why not measure it? > > Presumably parameter variation. It wouldn't have much > relevance to the next one in the batch. Actually, parameters like g_m vs Id often don't change much from part-to-part and batch-to-batch, for a given manufacturer's MOSFET type, in my experience. It's well worth the time to take measurements and analyze them in a spreadsheet. Going from one manufacturer to another, that's another matter. But you can explore that as well. SignatureThanks, - Win > [...] > Why not measure it? Indeed I could. Or I could use a simulator. But it seemed I should be able to analyze such a simple circuit by hand, based on information available in the datasheet. I guess not! >> [...] >> Why not measure it? [quoted text clipped - 3 lines] > But it seemed I should be able to analyze such a simple circuit by > hand, based on information available in the datasheet. I guess not! Yes you can. I already pointed out where the basic equations can be found. For the simple model, the device is either in linear (ron) region or constant current (saturation) region. The formula for the gm in these regions are available. Have you tried google? In saturation the gm varies as sqrt(I). If you know it at one current, then it is known at all currents. Do we need to hold you hand as well? Kevin Aylward 431infoEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. >>> [...] >>> Why not measure it? [quoted text clipped - 11 lines] > In saturation the gm varies as sqrt(I). If you know it at one current, > then it is known at all currents. Do we need to hold you hand as well? Hi, Kevin. Yes, some handholding would be welcome; that's why I posted to s.e.b., rather than s.e.d. The model you cited earlier was: .MODEL MM PMOS(LEVEL=1 IS=1e-32 +VTO=-3.73073 LAMBDA=0.0109168 KP=7.97276 +CGSO=1.08608e-05 CGDO=1e-11) With some Googling I find many references to a 1968 paper by Shichman and Hodges, in IEEE J. Solid State Circuits. But I can't seem to find the actual formula itself. (I don't happen to have access to a technical library, so I don't have the journal itself at hand.) Would you be able to point me to an online reference that shows the formula that Spice is using for this model? And, should I believe that this "Level 1" model (which does not include the subthreshold region) will be a good fit to the relatively low Id and Vds in my scenario? After all, the whole reason for the question is that the region I'm interested in is outside of the range shown in the datasheet. Thanks, -walter >>>> [...] >>>> Why not measure it? [quoted text clipped - 15 lines] > Hi, Kevin. Yes, some handholding would be welcome; that's why I > posted to s.e.b., rather than s.e.d. I was trying to avoid that. I am on holiday. > The model you cited earlier was: > [quoted text clipped - 14 lines] > question is that the region I'm interested in is outside of the range > shown in the datasheet. Yes. The subthreshold region is not accounted for at all in the Level 1 model. The level 1 model is "not bad" for the two main regions, that is satuation (constant current with Vds)and ohmic (linear with Vds). Satuation region: Id = W/L . (Kp/2) . (1 + lambda.Vds).(Vgs-Vt)^2 Linear region: Id = W/L . (Kp/2) . (1 + lambda.Vds).Vds.(2(Vgs-Vt) - Vds)) From gm = dI/dVgs Satuation region gm: let K = W/L . (Kp/2) . (1 + lambda.Vds) then: gm_satuation = 2.sqrt(K.I) Linear region gm: let K = W/L . (Kp/2) . (1 + lambda.Vds).Vds then: gm_linear = 2K Note 1: Most spices will assume a default W=L=100u if not specified, i.e. 1 for the ratio. Note 2: I just had to redo the sums myself, so any errors in the above are mine alone:-) In the subthreshold region, the relevent formular is: Id = Io.exp(Vgs/Vc) i.e. the same form as a bipolar, with Vc, a constant. The gm is therefore = I/Vc. For the bipolar Vc is Vt=KT/q, or 25mV (gm=40.I). The gm of a mosfet is *always* less than that of a bipolar in subthreshold, say 4 times less, i.e. a Vc of say, 100mv. Kevin Aylward 431infoEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. > The gm of a mosfet is *always* less than that of a bipolar in > subthreshold, say 4 times less Do you mean " The gm of a mosfet in subthreshold is *always* less than that of a bipolar, say 4 times less " ? Graham >> The gm of a mosfet is *always* less than that of a bipolar in >> subthreshold, say 4 times less > > Do you mean " The gm of a mosfet in subthreshold is *always* less > than that of a bipolar, say 4 times less " ? Indeed. I see your on the ball over the holidays Graham. Kevin Aylward 431infoEXTRACT@anasoft.co.uk http://www.anasoft.co.uk SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design. > [...] > [quoted text clipped - 44 lines] > *always* less than that of a bipolar in subthreshold, say 4 times less, > i.e. a Vc of say, 100mv. Thanks, Kevin! I'm on holiday too - that's why I've got time to be thinking about this stuff. I appreciate the handholding. Over in s.e.d, Tony Williams posted a link to an online text (http://ece.colorado.edu/~bart/book/book/contents.htm) that gives a derivation of the Vc term in subthreshold region. -walter >>> [...] >>> Why not measure it? [quoted text clipped - 11 lines] >In saturation the gm varies as sqrt(I). If you know it at one current, >then it is known at all currents. Do we need to hold you hand as well? His specified Vds of 1 volt may change things a little. I'd just try a part to be sure. John |
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