High Incremental Drain impedance (from the book The art of electronics)

[khatus]

Hello guys, I don't understand the meaning of this line

"The FET has a high incremental drain impedance, giving roughly constant current for VDS greater than a volt or two"

from field-effect transistors chapter of the book "The art of electronics".
Here what does it means by "high incremental drain impedance" Can anyone help me to understand it?

[TimFox]

"Incremental impedance" or resistance is simply the derivative (slope) dV/dI of the function V(I) near the operating point of the device.
The operating point is the voltage and current (in this case V_ds and I_d) when the input signal is 0.
For FETs, BJTs, and vacuum pentodes operating as amplifiers, if you make a small change in V about that point, you get a very small change in I, hence the derivative is high, and the incremental resistance is high.

[tggzzz]

"Incremental impedance" or resistance is simply the derivative (slope) dV/dI of the function V(I) near the operating point of the device.
The operating point is the voltage and current (in this case V_ds and I_d) when the input signal is 0.
For FETs, BJTs, and vacuum pentodes operating as amplifiers, if you make a small change in V about that point, you get a very small change in I, hence the derivative is high, and the incremental resistance is high.

Another name is the "differential resistance", unsurprisingly. Occasionally it is known as "dynamic resistance", especially in the context of AC coupled amplifiers.

Although not relevant to this thread, the OP might find it useful to keep in mind some devices can demonstrate negative differential resistance. Tunnel diodes are the classic example, but there are others.