Why use an OTA for capacitive loads?

Discussion in 'General Electronics Chat' started by jaydnul, Nov 21, 2017.

  1. jaydnul

    Thread Starter Member

    Apr 2, 2015
    As I understand it an OTA is basically an op-amp without a buffer, i.e. it has high output resistance. Why then are these better for driving purely capacitive loads? Wouldn't the RC time constant only increase with a bigger Rout, making it take longer to charge?

  2. KeepItSimpleStupid

    Distinguished Member

    Mar 4, 2014
    It basically comes down to the first of two axioms:

    The voltage across a capacitor cannot change instantaneously.


    The current in an inductor cannot change instantaneously.
  3. Veracohr

    Well-Known Member

    Jan 3, 2011
    The output of an OTA is a current source. The time constant of an RC circuit fed by a voltage source increases with higher resistance because the current decreases. When your output is a current source, you set it to whatever level you want.
  4. jaydnul

    Thread Starter Member

    Apr 2, 2015
    Ok I see. Looking into drain of a mosfet provides a high resistance. But more importantly, it will act just like a current source and change the voltage to keep that current constant. While looking into the source will look like a voltage source where the current will be variable. Got it, thanks!