op amp question

[jim]

In spite of all the info my texts have on op amps,I'm still unclear on
one aspect. When I configure an inverting amp say with the old 741,
the gain is feedback resistance divided by input resistance. However
,several combinations of resistances give the same ratio. Aside from
input impedance,does the choice of resistances that give identical
ratios effect the gain bandwidth curve? And if not what would be
effected by these different choices? Thanks for any help. jim

 

[Joel Kolstad]

jim <road12fg@netscape.net> wrote:

In spite of all the info my texts have on op amps,I'm still unclear on
one aspect. When I configure an inverting amp say with the old 741,
the gain is feedback resistance divided by input resistance. However
,several combinations of resistances give the same ratio. Aside from
input impedance,does the choice of resistances that give identical
ratios effect the gain bandwidth curve? And if not what would be
effected by these different choices? Thanks for any help. jim

For an ideal op-amp, no. For a real op-amp, yes. The input of a real
op-amp often looks rather capacitive (if it's a FET based amplifier, you're
looking into what's often the largest gate in the device), so the
gain-bandwidth product as you use larger resistors will, at some point, be
reduced by the low pass filtering effect of your large resistor and the
input capacitance of the op-amp. Additionally, large resistors will
decrease the noise immunity of your inputs (as their impedances are no
longer necessarily large in comparison to those of induced noise sources).

Likewise, if you make the feedback resistor very small you could end up with
the op-amp's output becoming slew rate limited even for a 'reasonable' load,
and this can be deleterious as well (especially since slew rate limiting is
a non-linear effect). And as you're clearly aware, small resistors may
significantly load down whatever it is that's feeding the op-amp, which may
be undesirable.

The Art of Electronics has a good discussion of real-world op-amp
considerations, as does Op-Amps For Everyone, available as a free download
at TI's web site here:
http://focus.ti.com/docs/apps/catalog/resources/appnoteabstract.jhtml?abstractName=slod006b

---Joel Kolstad