Biasing MOSFET for switching purpose

[hervehman]

Hi all,

I am planning to Bias a complementary source follower MOS output stage for Switching purpose. The idea is to bias them just under Vgs threshold (from 10uA to 100uA.) so as to reduce "wriggling" during switchings.
In DataSheets one can find large Vgsth values like from 0.5 to 1V for a NMOS. My question is: In a fixed circuit, with fixed Vds, fixed temperature..., does that Vgsth varies much from one part to an other (of the same MOS, same brand...) ?? or the manufacturing process can produce quite identical parts each time ?

Thanks.

 

[ZekeR]

In general, you should not rely on MOSFETs matching well, unless you've binned them so they match. In a single production run, the MOSFETs will usually match okay; they might differ only by 50mV or so. However, if you obtain two MOSFETs from two different production runs, they might be 200mV off from each other. A difference this large would probably not be a good thing. IC designers get the luxury of knowing that their devices will match well; as a circuit board designer, you don't get this luxury unless you're willing to pay extra for it.

It might be better to use a "rubber diode" which is controlled through feedback by measuring the average current in the push-pull stage. The late Jim Williams built a circuit which does this in his article on building a 100A electronic load. There might be better ways to do it, but it worked.

 

[hervehman]

Thanks ZekeR,

I will test that some time with MOS from a single Reel, using 1 MOS for creating the DC bias.