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?

Thanks

 

It basically comes down to the first of two axioms:

The voltage across a capacitor cannot change instantaneously.

and

The current in an inductor cannot change instantaneously.

 

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.

 

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!