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

[Aaron Lanterman]

On Sun, 19 Feb 2006, [ISO-8859-1] René Schmitz wrote:

> Your BJT OTA inputs aren't really hi Z in the same sense that opamp 
> inputs are. Basically the input current is a function of the input

It makes me wonder how the situation changes if we started building OTAs 
out of CMOS instead... when I have time (which is rare this month) I've 
been trying to sit in on an analog VLSI design class, that it's all CMOS. 
I haven't seen a BJT all semester.

It might be fun to try building a filter with a four-CMOS-transistor OTA 
instead of the usual four-BJT-transistor OTA. I've never looked into 
whether MOSFETS come in matched pairs, or for that matter how tightly 
matched things on say the 4007 are. The CMOS would give you a different 
type of distortion instead of a tanh type of BJT distortion, which could 
be interesting.

I've heard there's this guy named Rene that's looked into doing weird 
things with CMOS... ;)

Or, you could mix and match maybe? Use CMOS for the diff pair and BJT for 
the current mirror, that would give you high-Z on your OTA input.

You could try either (4-CMOS or 2-BJT/2-MOS) as a current switcher in a 
triangle core OTA...

Things with high input impedance are just so much easier to analyze...

- Aaron

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