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

[ASSI]

On Sonntag, 19. Februar 2006 02:40, Aaron Lanterman wrote:
> 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.

Ask about subthreshold circuits and log-domain filters, also sqrt-domain 
filters and (other) current-mode circuits.

> 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.

Try to get some Advanced Linear Devices ALD1103, ALD1106 and ALDALD1107 
for that experiment.

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

...until you come to the Miller capacitance and other assorted 
parasitics.

[...and the earlier post of why there is a buffer...]

Putting a buffer is one of the many ways of saying "let there be a bias 
current". The IC design books mostly seem to assume that the biasing is 
done inside the "magical" box, so they can get away with not showing a 
buffer or other means of biasing. The other advantage of the buffer is 
that the voltage range for the cap can be decoupled from the (small) 
input range of the following OTA.


Achim.
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