Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
Subject: Re: Help analysing a CMOS ring oscillator
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Date: Tue, 14 Jan 2003 16:58:41 -0000
NNTP-Posting-Date: Tue, 14 Jan 2003 16:58:42 GMT
Jim Thompson wrote:
> On Tue, 14 Jan 2003 08:22:08 -0000,
> "Kevin Aylward" ,
> In Newsgroup: sci.electronics.design,
> Article: ,
> Entitled: "Re: Help analysing a CMOS ring oscillator",
> Wrote the following:
>> Jeff Verive wrote:
>>> "Kevin Aylward" wrote in message
>>>> Nope. DC gain is not relevant at all. The oscillator will oscillate
>>>> when the gain is around 1. This is way off from the DC gain. The HF
>>>> gain is gm.Xc, where Xc is the effective capacitance load.
>>> Gee, and where does that AC gain come from? Yes, you can oscillate
>>> with unity gain, but the fact is that an inverter stage can be
>>> analyzed first as a DC circuit (even Spice uses successive DC
>>> operating point interactions to determine AC gain).
>> Indeed. You need to be able to set up its bias point in order to
>> calculate things such as gm, however, this is not relevant to the
>> point that DC gain itself is not important in calculating the main
>> characteristics of an oscillator. One can have a Dc gain of 20db,
>> 40db, 60db, etc, without it effecting the oscillating frequency at
>> all. That is there is no direct relation to oscillation and dc gain
>> at all.
> 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.
Well, yes and no. sort of. Its the characteristics around the unity gain
point that really matters. The GBW on its own don't tell the whole
story, you need to know how the -1 point is actually enclosed. But as
far as stability is concerned, the dc gain don't tell you anything.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.