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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?


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


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    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.
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