From: Robert Baer
X-Mailer: Mozilla 4.75 [en] (Win98; U)
Subject: Re: Opamp GBW tempco ?
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
Date: Fri, 13 Sep 2002 19:01:35 GMT
NNTP-Posting-Date: Fri, 13 Sep 2002 12:01:35 PDT
Organization: EarthLink Inc. -- http://www.EarthLink.net
fred bartoli wrote:
> Kevin Aylward a écrit dans le message :
> > "fred bartoli" wrote in message
> > news:email@example.com...
> > >
> > > Kevin Aylward a écrit dans le message :
> > > dnGf9.1410$ZB3.firstname.lastname@example.org...
> > > >
> > > > "fred bartoli" wrote in message
> > > > news:email@example.com...
> > > > > Maybe a question for Kevin or Jim ?
> > > > >
> > > > > For my 100kHz IF amplifier that has to have a really (really mean
> > :
> > > > <0.01d)
> > > > > stable phase rotation at the (constant) IF frequency, I'm
> > evaluating
> > > > the
> > > > > sensitivity of all the components.
> > > > > One point I don't have answer is the tempco of opamps.
> > > > > The ones that I want to use are in the 100/200MHZ range, bipolar
> > input
> > > > (low
> > > > > voltage noise).
> > > > >
> > > > > Any order of magnitude for this tempco ?
> > > >
> > > > If the data sheet don't spec it, I reckon you have to bank on 20% to
> > 50%
> > > > or so at least as an open loop gain variation over temperature. Some
> > > > might be better, it depends on how well all the gm stages have been
> > > > biased with a PTAT current.
> > > >
> > >
> > > That is about 0.2% to 0.5% per °C and is pretty large regarding my
> > needs. So
> > > that means that I probably will have to measure it and account for in
> > the
> > > amplifier, and the situation may not be so bad if this tempco is
> > fairly
> > > repeatable.
> > > Do you have some ideas about that ?
> > >
> > It might take a bit of digging. Off hand, I have not looked much at
> > temperature variations of the Early effect.
> > Note that the GBW is also highly variable from unit to unit. e.g. a 741
> > might be quoted at typically 1Mhz, but it might be 0.47MHz to 1.5Mhz.
> > The open loop dc gain might easily be a vary by factor of two.
> > The whole idea of the high open loop gain is that on closed loop the
> > variations are reduced tremendously.
> Indeed, but my app is 100kHz centered. Also, what I was interested for is
> the temperature dependance, the case to case dependance being calibrated out
> once for all.
> So, unless I'm missing something, Early effect will not have first order
> effect on gain at these frequencies, but rather the main pole setting
> capacitor, which I guess is pretty stable, and the input stage gm, which is
> dependant on bias current stability as you stated, and also on temperature.
> I'm well ready to admit the 20%-50% range you gave over the full temerature
> range, but one thing I find amazing is that it implies the same change in
> bias current, which I find not good at all, even for a moderately careful
> design. Did I missed something ?
> > Kevin Aylward
> > firstname.lastname@example.org
> > http://www.anasoft.co.uk
> > SuperSpice, a very affordable Mixed-Mode
> > Windows Simulator with Schematic Capture,
> > Waveform Display, FFT's and Filter Design.
> > >
It is rather silly to use an operational amplfier for IF work is a
semi-discrete design, when a single transistor can do the job (i am
speaking of a single stage).
On the other hand, if one designs an opamp circuit conservatively,
there is plenty of phase margin and any GBW changes are completely
irrelevant, as in the case you specified; 0.1Mhz is a DC frequency
compared to the closed loop bandwidth of those 100MHz opamps.
Furthermore, even a 20 percent change is irrelevant compared to that
Like the difference of Bill Gates seeing a $10 bill on the sidewalk VS
seeing a $20 bill.
Not even pocket lint.