Consider the current mirror below (circuit taken from [Razavi]). I wonder about the \$ V_{DD} \$ in the circuit. No matter if its there or not, \$ V_{GS} \$ of M1 stays the same and therefore also \$ I_D \$ of M2, since the current charges up parasitic capacitances of M1 and causes a voltage across it. This I also verified by simulation, see the plot below (\net01 and \net02 are the \$ V_{GS} \$ of M1 with \$ V_{DD} \$ and without). Is there any reason why \$ V_{DD} \$ is connected there?
simulate this circuit – Schematic created using CircuitLab
EDIT:
I simulated the circuit until 100V, \$ V_{GS} \$ stays the same. Here is the circuit:
Best Answer
If I1 is a current 'source' then no, you wouldn't need the voltage source. But typically a current source is implemented using a voltage source and current regulation, so when I look at that drawing I assume I1 just regulates the current from the voltage source V_DD (even though the symbol is that of a current source).