[sdiy] Newbie alert: opamp resistor values

[David G. Dixon]

Ian, Harry,

Thanks for your excellent and informative responses.  It was pretty much as
I figured.  I typically use 100k for unity-gain applications, just because I
have a ruddy great pile of them (literally) on my workbench!  I guess opamps
make things just too easy!

Also, Ian, thanks for your advice from yesterday re: comparators and JFET
switches.  Most of your advice worked well, but the 1M self-biasing resistor
and gate diodes were a bust.  For the sample-and-hold, I found it best to
put a fairly large resistor (1M to 2M) between the comparator output and the
gate, and just leave it at that.  This is not the case when shorting an
integrating capacitor (where the source is held permanently at virtual
ground), but seems to be critical when firing a sample-and-hold, where too
much juice to the JFET's gate alters the sampled voltage.  In that case, I
actually like my summing-amp biasing method better (and it's not really more
complicated -- either method requires one opamp and a couple of resistors).
On the integrator JFET, however, it is also important to bear in mind that
the integrator's output must be strictly positive (i.e., only a negative
source current into the inverting input) to use a JFET (this is buried in
the fine print of a figure in Horowitz and Hill, and definitely bears
keeping in mind).

Cheers,
Dave


David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C.  V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
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-----Original Message-----
From: Ian Fritz [mailto:ijfritz at comcast.net] 
Sent: Thursday, January 08, 2009 10:26 AM
To: David G. Dixon; 'sdiy'
Subject: Re: [sdiy] Newbie alert: opamp resistor values

At 10:34 AM 1/8/2009, David G. Dixon wrote:
>What are the basic guidelines for selecting absolute resistor values for
>opamp circuits?  For example, say one needs a unity-gain inverter or
summer,
>and is using a generic FET-input opamp (TL07x, say).  What determines
>whether one uses 10k, 100k, or 1M for the two resistors, or something
>inbetween?  Now, what if input impedance is not an issue (i.e., the
incoming
>voltage is well buffered)?

Generally not too critical.   100k is standard modular input impedance, so 
that often determines the two values.  Otherwise, too large an impedance 
gives noise/offset issues, too small draws more power than necessary.  I 
say anything 33k - 330k is fine, but I'm sure you'll get other opinions.

Another consideration is to think about other folks building your 
circuit.  It's convenient to have as small a number of values as is 
practical.  So don't use 33k one place and 47k another if they could be the 
same.

Ian