Hi,
Using math is not really my comfortzone so I calculated one example with the measured values of what I would like to show in a graph.
Calculating example 2000mV
Say we want stabilize an output voltage, which delivers initially 0,2030 A current.
The voltage drop over source drain is 2,0000 V and source gate is 2,2770 V at that moment.
The static resistance of the mosfet would be 9,8522 Ohm (2,0000 V/0,2030 A).
If 0,0010 A more current is drawn making a total of 0,2040 A, the voltage over the mosfet would raise to 2,0099 V (9,8522 Ohm * 0,2040 A), this is a 0,0099 V increase.
For small current differences this is equal to the resistance x amps, in this case also 0,0099 V.
Our circuit wants to maintain a constant voltage output, and therefore a constant Vsd.
It should therefore increase Vgs, but to what voltage?
In our sample points the next Vsg / current combination with a Vsd of 2,0000 V is 0,2110 A at 2,2810 Vsg.
That is a 0,0080 A difference, 8,0x more than the raise in amperage we assumed.
So we can say that required Vsg increment should be 8,0x less.
The Vsg increment is 0,0040 V for 0,0080 A, which would result in a 0,0005 V required increment for 0,0010 A.
So the increase of 0,0099 V can be counterbalanced with an increase of 0,0005 V on the gate, this is a gain of only 0,0508x.
The same calculations are done for a range of Vsg's which also correspond to a matching current.
In "IRLZ44N Characterization - AV curves 2000mV" we see the resistance curve which matches the data points. We also see the curve which shows how much Vsg should rise to counterbalance to get a steady voltage drop.
In "IRLZ44N Characterization - required gain 2000mV" we see the ratio between those. What it says is that between 0..1A one needs a total gain less than 1! (0.1) The example value can also be found in the graph.
I know that the 2.5V offset must be reached as well, and that there're rise and fall times. But I find this very interesting, and will use this knowledge in further exploration. I will then also measure how fast a low gain solution is.