[sdiy] Why do we need the buffers? - gm-C filter question

[René Schmitz]

Hi Jürgen, Aaron and all,

JH. wrote:
> Even with BJTs, you can design OTA filters without buffers. The Zin of each
> stage changes with Iabc, so you have a tracking input impedance. 

Thats true.

> You just have to put Zin into the equatation, and you can
> design a polygon filter just as well as with buffers.

But the bias current sucked into the next stage would create offsets, 
since the direction of this will be always the same, so your filter caps 
see a current difference of this error current and shift their voltage.

> No, I think the main drawback is parasitic Cs.

Interesting point. Then again why not simply interpose a unity gain 
buffer. That should lower the capacitances.

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159