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Fri Dec 24 00:10:36 EST 2010

OTA output current mirrors

One things I got wrong is the use of two current mirrors that take the
current from the two differential pair legs and bring it out.  See
explanation and CA3080 schematic here[1].  The schematic I'm working
with is the differential pair with current mirror load [2], and the
discrete 4-pole filter [3] (which has the PNP's upside down in the
schematic)..

So what's the difference?  Why the two extra current mirrors?  The
answer is probably to remove the dependency on the common mode
voltage.  I.e. if that's high, the differential pair resistors might
saturate trying to provide a low output voltage.  However, if by
design this isn't a problem (as in the 4-pole filter?) the current
mirror can be left out.

[1] http://www.uni-bonn.de/~uzs159/ota3080.html
[2] http://en.wikipedia.org/wiki/Long_tailed_pair#Long-tailed_pair
[3] http://www.jhaible.de/tonline_stuff/jh2040.gif



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