In Fig 1, Why would the lower transistor Q1 guaranteed to be in triode operating region ?
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Although I'm not certain how to prove that...
I wouldn't be surprised if it has to do with this cause and effect below.
1) Voltage at the transistor gates causes both to conduct (as long as V is above a certain threshold level.)
2) Current through the lower transistor produces a voltage.
3) This voltage appears at the source terminal of the upper transistor, in effect changing its G-S relationship.
4) This alters its conductivity which affects current through it.
5) Current in the upper transistor is identical to that in the lower. The transistors influence one another by the above steps.
6) Equilibrium state is reached.
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This simulation of the circuit illustrates. A picture is worth a thousand words.
A voltage sweep is applied instead of a fixed current source. This reveals behavior at very low current levels (near pinch-off region). Namely the exponential function of Amperes versus Volts.
Can we say the function definitely is square and square root? Or cube and cube root? Some effort might make it possible to bend response so it is mathematically accurate.