Why a transistor (MOS) changes the output current (Ids) with changes of a load resistor?

Question

I am a newbie, and I can not understand, why current of the MOS changes if I change the resistor.

Basically, why transistors so depend on load? And how I can calculate changes..

Regards, Paddy

Answer 1

I guess you are aware that drrain current is function of voltage across GATE -SOURCE.

By KVL, 5V = VGS + (drain current voltage across 10KOhm) So when you change your R, the voltage across it changes which in turn affects the voltage across Gate-Source. This VGS decides Id via the resistor R.

Hope you get it.

Answer 2

Your question suggests that you think it is a property of the transistor that Ids changes with the value of the load resistor.

However you disregard the fact that it also depends on HOW you use the transistor. In your circuit, assuming the transistors are NMOS (when using PMOS, no current would flow), the transistors appear to be connected as "diodes" and in this configuration the MOSFET sort of behaves as a normal silicon diode although with a less steep current increase over voltage. So the high dependence of Ids is to be expected for the circuit you show.

Answer 3

Basically, why transistors so depend on load?

Beacuse the load (and other resistors) define transistor's bias conditions. Your gate voltage is always 5V, while your source voltage is Ids*10k Ohm, and Ids is a function of voltage between gate and source. You see that you'll have some mutual influence between Ids and Vgs - the bigger the Vgs, so bigger will be Ids which will in turn reduce Vgs and you'll have a stabilized bias of transistor.

And how I can calculate changes..

By looking up any textbook on transistor or any electronics course there is.