# Selecting Vgs and Vds for MOSFET Amplifier

Hello All,

I just finished a college course on electronic circuits (opamps, mosfets, bjts, etc) and I'm trying to put some of that knowledge towards designing a basic MOSFET amplifier circuit. Right now I'm at the stage of selecting DC biasing points and I'm having a bit of difficulty selecting these values.

http://www.datasheetcatalog.org/datasheet/irf/jantx2n6770.pdf

Above is the datasheet for the part I am using in my multisim simulations. Is there any way of determining what my Vgs DC biasing point for saturation region should be just based on the datasheet. I ask this because I don't know whether or not it is common practice in design to always create a Vgs vs Vds curve to find the appropriate Vgs DC bias point. From what my professor told me we should be able to look at a datasheet and find out what we need to set our Vgs DC to but I don't know how to do that based on the information given in the datasheet.

Any help with this would be greatly appreciated. I'm just having some difficulty bringing the classroom knowledge into the real world where all of the specific device parameters aren't explicitly listed.

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In real world design we don't use Vgs vs Vds curve to find the appropriate Vgs DC bias point. For all single stage amplifiers we usually select Vds = 0.5Vdd.
And we find Vgs in real world measurement. And sometimes we use this equation as first approximation method.

$I_D = K*(V_{GS} - V_{GS(th)})^2$

then form this equation

$\Large K = \frac{I_D}{(V_{GS} - V_{GS(th)})^2}$

And so for example from IRF510 datasheet

http://inst.eecs.berkeley.edu/~ee40/su10/labs/Booster/IRF510.pdf

That the average value of Vgs(th) is equal to Vgs(th) = (2 + 4) / 2 = 3V
And from Figure 5 that, for Vgs = 5V we have Id = 1A

K = Id/(Vgs - Vgs(th))² = 0.25 = 1A/(5V - 3V)^2 = 0.25.

So we have all the data that we need for now.

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That makes a lot of sense, thank you. I do have one more question about this though. I was reading over my textbook and they mentioned using the Vds vs Id graph (fig 5 in your example) to also determine where the Vds and Vgs values needed to be by using a load line in the form:

id = (Vdd/Rd) - (Vds/Rd)

And by finding the intersecting of this load line with the appropriate Vds vs Id curve for the Vgs you want to find biasing characteristics. Is this method employed at all or is it mainly the one you have described?

We don't use Vds vs Id curve also. Why? Well the answer is quite simple.
The datasheet Vds vs Id graphs show typical value. Also in our circuit we will use different Vds and Id value range than it shows in the datasheet. So this Vds vs Id graph are not very useful in real life.