Negative resistance from MOSFET circuit
Say you increase the voltage at \$V_{DS}\$. This increases the voltage on Q2's gate, increasing the conductivity of its channel, lowering the voltage at \$V_{GS}\$. This decreases the current through Q3's channel. So by increasing the voltage \$V_{DS}\$ you've decreased the current that the \$V_{DS}\$ supply must provide, which is the definition of a negative resistance load.
Of course this all depends on Q1, Q2, and Q3 being in the appropriate operating modes, so there will be a limited range of voltages over which the negative resistance behavior can be observed.

negative current is not proportional to voltage.

If you apply a positive voltage to a circuit branch and it sends current back out towards you (&quot;negative current&quot;) then that branch is delivering power to you, not absorbing power from you.
This circuit doesn't do that.
But it does have a region where \$\frac{dV_{DS}}{dI_{DS}}&lt;0\$, which is what the authors are referring to as negative resistance (and also what we call negative resistance in the case of common negative-resistance elements like glow tubes and Esaki diodes).