Vabzter wrote on Feb 8th, 2007, 7:23am:Hi,
Thanks for the replies..But still I am not sure how to find the values of k(=un*Cox) for a perticular technology as we need these values for doing hand calculations..
BR
Vabzter
I don't worry about those things... You can do a lot by simply knowing that gm=2*I/Vod, where Vod=(Vgs-Vt). You don't need to know what K' or Vt is to tune Vgs-Vt.... if it is too big, make the devices wider or lengths shorter. You can get Vgs-Vt from the DC operating point information. Vdsat is close to the same quantity -- it is what I usually look at. Hand calculations that use more than Gm have limited use in the design process.... everything interesting is determined by parasitics or nonideal effects (plus Vt and K' change depending on operating point, sizes, etc).
Opamp design is pretty simple (ok, that is a gross exageration).... You give the diff pair high gm, and the load (current mirror) low gm. If you don't, you get a bad op-amp. End of story
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For a given current, all you can do is change Vod. 150mV is a reasonable target for Vod for the diff pair. Lower than that, the diff pair gets into weak inversion and wider widths offer only small improvements (i.e. diminishing returns). 2-400mV is a good target for the mirrors. Since Vod =~ Vdsat, the upper limit is usually set by the amount of headroom you have.
From that basic starting point you can play with the sizes to suit your particular situation (keeping current fixed and lengths as small as you can). If your diff pair is causing frequency response problems you need to make it smaller, which will likely decrease gm (because Vod is increased). Similarly, making the mirrors smaller (increasing Vod) will
increase their frequency response (lower Cgs), but it may make other problems - like headroom or an inability to drive cap loads. If you cannot fix things by changing Vod, your only other choice is to increase current (or change topologies).
That said, the first thing I do when I get a new process is to plot the Vt vs. Width for various lengths of devices. You get the Vt information from the DC operating point (just diode connect the devices and put 1uA thru them and do a parametric anlysis sweeping width and length). You will find that Vt is a strong function of length and width for smaller geometries. It is also instructive to plot gm/I vs Vgs-Vt for various sized devices, as well as Vt vs bulk-source voltage. Armed with a few of these plots you can get most of the information you need. Note that, in some sense, this is determining K' and other parameters, but since they are functinos of device sizes it put them in a more usable format.
rg