Circuit Idea/Reinventing the Common-Source Amplifier
The rule of understanding edit
It is an interesting fact that things in life are understood through similar but simpler things. The paradox is that they are not understood as things themselves but through other things. For example, no matter how much you study the transistor device in Semiconductor engineering, it is not enough to understand transistor circuits. There is a need for something else that is not learned there, but long before that in electrical circuits and in life.
Building the common-source stage edit
Perhaps it is best to reinvent step by step the most elementary transistor amplifier stage to understand the meaning of things.
Converting the voltage into resistance edit
Regardless of all the complexities of the (MOS) transistor, generally speaking, it does one simple thing in amplifier circuits - introduces adjustable losses in the form of a voltage drop. So does the much simpler variable resistor in electrical circuits. They both do it the same way - by inserting resistance into the circuit and changing it. So the simple functional idea of a transistor is as a voltage controlled variable resistor (voltage-to-resistance converter). Indeed, there is a "small" difference - a resistor is a linear element with a constant resistance, whereas a transistor is a non-linear element with a "self-changing" (dynamic) resistance, but this is not essential to understanding the basic idea of the transistor amplifier stage. It is a concept, and details get in the way of understanding concepts.
Converting the resistance into current edit
Variable resistor: The resistance is only a parameter that can control some "true" electrical quantity, most simply the current. For this purpose, we (the "input voltage source":-) supply the variable resistor Rds with a voltage source Vdd and by changing the resistance we adjust the current Id = Vdd/Rds = 10 mA (Ohm's law) looking at the "ideal" ammeter.
To get a better idea of the resistor's behavior, let's measure and plot its IV curves for three resistance values - 1 kΩ, 2 kΩ and 10 kΩ. As you can see from the graphs, at 10 V the corresponding currents are 10 mA, 5 mA and 1 mA.
Web resources edit
A similar story from an answer to a related StackExchange EE question