From: "Jeff Verive"
Subject: Re: Help analysing a CMOS ring oscillator
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Wed, 15 Jan 2003 11:22:01 -0600
NNTP-Posting-Date: Wed, 15 Jan 2003 11:22:02 CST
Of course, I understand the relationship between gain-bandwidth product and
miller capacitance, which the original poster was having a hard time
modeling. No doubt infinite DC gain does little good if unity gain is to be
had at a crappy low frequency, but one can estimate miller capacitance as an
input capacitor nearly equal to Cf(1-A), where A is the gain of the circuit
with the feedback reactive element Cf removed (i.e. the DC gain between gate
and drain when the feedback [miller] capacitance is removed, not the overall
BTW, the same approach is used in textbooks by Mauro, Schilling and Belove,
to name a few. The original poster could have modeled this in many ways in
SPICE to determine the effective miller capacitance, but did not seem to be
willing to do this, despite my urgings. Instead, he was looking for a
lumped-parameter model that would be useful as a first-order approximation.
In the end, DC gain in a simple inverter establishes an upper bound for AC
gain (where Cgd is the chief contributor to miller capacitance). If the
source had some bypass capacitance, or if bootstrapping, cascode
connections, or source follower stages are used, AC gain improvements and
reductions in miller capacitance are relatively easy to accomplish, but we
have to remember that the original poster was trying to analyze a simple
CMOS ring oscillator, in which none of these tricks are commonly used.
Kevin's not on the suds, though I'm sure he could have used some by now (we
all could). For some reason, after some dozens of replies, the original
poster's intent gets lost, and I try to keep the basic problem in mind.
> Kevin must still be on the suds ;-) What he's saying, but can't put
> his finger on the succinct word, is that it's GAIN-BANDWIDTH that
> matters, NOT DC GAIN.
> ...Jim Thompson
> | James E.Thompson, P.E. | mens |
> | Analog Innovations, Inc. | et |
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> | Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
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