[tooler]

I'm working on the Szekeres amp right now and I'm trying to determine a good guess for how much resistance I should dial in for R3 so that Vds is half my power supply voltage.

To do this, I figured I could dust off the circuits book I never thought I'd use and plug in some values for the DC analysis equations. However, all the equations I have in this book require Vgs, which usually requires some information about the transconductance and threshold voltage of the MOSFET. I looked at a data sheet for the IRF513, but didn't find exactly what I needed. The threshold voltage Vtn (listed as Vgs(th)) is a range and doesn't list a typical value. I don't see anything I could use for Kn, which is in A/(V^2). Where do I get useful information for the equation,

Vgs = Kn/2(Vgs-Vtn)^2

Or should I use some other equations?

I'd like to know how to apply my worthless book knowledge to real-world situations like this. I'm really curious and I think it will help me in the future.

P.S. I got a weird feeling of deja-vu that I asked this before, but nothing turned up in a search on here or HeadWize.

 

[Budgie]

I can't help with the math, but I suspect it will always be inaccurate due to the variations in the individule mosfets. I used a ten turn pot, of about the value as the two resistors together, that make up the voltage divider. If I rememeber correctly, it is not a critical issue, if you are using output coupling caps. Close but not perfect is good enough. I hope that is some help.

 

[tooler]

The reason I ask is that with a 14.2V supply, I had R3 up around 400k and Vds was still around 7.8V. I wanted to learn how I could figure out if that is way off by myself, rather than just ask the board.