Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker!mit-eddie!mit-amt!mit-caf!shers
From: shers@mit-caf.MIT.EDU (Alexander The Great                                                                     Sherstinsky)
Newsgroups: comp.lsi
Subject: Re: A Question On Biasing Circuits
Message-ID: <4029@mit-caf.MIT.EDU>
Date: 4 Mar 90 20:37:26 GMT
References: <4025@mit-caf.MIT.EDU> <5450@buengc.BU.EDU>
Reply-To: shers@mit-caf.UUCP (Alexander The Great                                                                     Sherstinsky)
Organization: Microsystems Technology Laboratories, MIT
Lines: 57

In article <5450@buengc.BU.EDU> bph@buengc.bu.edu (Blair P. Houghton) writes:
#In article <4025@mit-caf.MIT.EDU> shers@mit-caf.UUCP
#(Alexander The Great Sherstinsky) writes:
#>Vi   C
#>----||----*-- Vo
#>          |
#>          /
#>          \ R
#>          /
#>          \
#>          |
#>          GND
#[...and asks how to bias this highpass so that Vo is about Supply/2...]
#
#That's the AC model.  The DC model can look like:
#
#          Vdd
#          |
#          /
#          \ 2R
#          /
#          \
#Vi   C    |
#----||----*-- Vo  (Now Vo == Vdd/2)
#          |
#          /
#          \ 2R
#          /
#          \
#          |
#          GND
#
#Which is the answer.
#
#There's a problem if you use turned-off enhancement FET's
#for the 2R values, in that process control is very spotty
#(and of limited economic value) in CMOS, and the pullup's
#off-impedance may be an order of magnitude different from
#the pulldown's, and good luck predicting which way.
#
#Now, if you've got an _analog_ CMOS line on which to
#produce these chips, or a process engineer who doesn't
#mind a few days of knob-twiddling...
#
#				--Blair
#				  "Them's the breaks."

Thanks.  But this is what my main question is:  How to achieve the
biasing point reliably at the mid point (or nearby) between the supply
rails and get a huge equivalent resistance (1Gohm) would be very nice.

Alex
-- 
+-------------------------------+------+-----------------+---------------------+
|Alexander The Great Sherstinsky|me    |shers@caf.mit.edu|To become as refined |
|Alexander Semyon Sherstinsky   |myself|shers@caf.mit.edu|a person as possible.|
|Alex Sherstinsky               |I     |shers@caf.mit.edu|*********************|