Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

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

  1. Jul 27, 2016 #1
    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?

    gainstage.PNG

    Best regards,
    M
     
  2. jcsd
  3. Jul 27, 2016 #2

    LvW

    User Avatar

    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).
     
  4. Jul 27, 2016 #3
    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.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted



Similar Discussions: Why is the 'active load' in an CS-amp called a load?
Loading...