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

[Don Tillman]

   > Date: Sat, 18 Feb 2006 19:36:47 -0500 (EST)
   > From: Aaron Lanterman <lanterma at ece.gatech.edu>
   >
   > 1) Maybe the OTAs we typically use, 3080, 13700, discrete 4-BJT,
   > whatever, don't have high enough input impedance to avoid having
   > the next stage load down the previous one?

Hey Aaron,

Besides the input impedance, there's the bias current.  Y'know the
differential pair at the input of the OTA?  Those transistors are
passing some current that's proportional to the OTA's Iabc current.
Okay, but for them to work you need to supply a little bias current to
their bases, 1/Q times their running current.  There's really no
source of that without a buffer.

   > (One thing that makes me think this is the IC books are all
   > CMOS, and a CMOS-based OTA would probably have higher input
   > impedance, so maybe they could get away with it)

The gain of a CMOS-based OTA is not propotional to the programming
current like bipolar OTA's, but to the sqrt() of the programming
current.  Of course you could put two stages in series to get the job
done.

  -- Don

-- 
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com