Especiall I would like to know why it's output resistance is high and why the drain voltage of the lower MOSFET M2 is stabilized, i.e. changes very little when the ouput voltage (Out) is changed.
Would this be a correct explanation?
- VG2 voltage determines the current going into the drain of M2. This current has to be the same for M1.
- Gate-source voltage of M1 (= VG1 – drain voltage of M2) is adjusted so that the drain current of M1 is equal to the drain current of M2.
- If output voltage (Out) is changed, the drain current of M1 would change due to the channel-length modulation of M1. But as the current of M1 has to be the same as the current of M2, the drain voltage of M2 (and thus the gate-source-voltage of M2) changes so that the currents are equal again. And this change is very small because the I_drain(U_drain_source) curve of a MOSFET is relatively flat and the I_drain(U_Gate_Source)-curve of a MOSFET is very steep. Therefore a big change of the Out voltage causes a very small change of the drain voltage of M2 (voltage is stabalized here). Therefore the current changes not much and the output resistance of the cascode is very high.