Why is the 'active load' in an CS-amp called a load?

[mulothecook]

Hi there.

Could somebody please explain to me why this circuit is referred to as 'common source amplifier with active load'? From my understanding a load would be something external that is attached between the output and the ground terminals. For example a resistor attached to a voltage source can be considered a load.

So then why is the current mirror termed 'load' when it is a part of the circuit itself?

M

 

[LvW]

The FET in common source configuration acts as a voltage-controlled current source.
This current source will produce a varying drain current as a result of a corresponding input voltage.
Because - in most cases - we want to realize a voltage amplifier, the output current (and its variations) must be converted into a corresponding voltage using a suitable "load resistance".
For this purpose we use either a static resistive load (resistor RD) or - for larger voltage gains - a dynamic active load (as shown in your circuit). The shown current mirror acts as current source with a very large internal dynamic resistance (remember: A current source is a voltage source with a very large internal source resistance).

 

[mulothecook]

Ah okay, thanks a lot. To put it in in simple words: the load is in fact a load, but with respect to the NMOS current source.