MOSFET current mirror: Saturation mode?
If \$V_{GS}=V_{DS}\$, and \$V_T&gt;0\$, you can change the saturation requirements of \$V_{DS}\ge V_{GS}-V_T\$ to \$V_{DS}\ge V_{DS}-V_T\$. Subtracting \$V_{DS}\$ from both sides gives you \$0\ge-V_T\$, which can also be written as \$V_T\ge0\$. This is why this configuration is always in saturation as long as you meet the other saturation criteria of \$V_{GS}&gt;V_T\$.