clifto wrote:
Has anyone played with any of the new high-bandwidth op amps in IF
amplifier circuits? I've got an idea I want to experiment with, and
not a whole lot of analog design ability (though I do better with
building blocks than with compensating discretes), and now that
GBw products are cheap above 1 MHz it might look practical to
pursue. I have no standards for comparison since I know nothing
about parameters like noise figures and the like in standard
discrete circuits.

GBw products above 1MHz have been around for a long time in op amps, if
you don't mind dropping the bucks then you can even get GHz numbers.
There's very little stopping anyone from using even the old
high-bandwidth op amps in IF's. But there's not much need to.

In IF strips for FM receivers, most op-amps do really bad things when
they hit their limiting amplitudes. Google for "phase reversal" with
"op amp".

"Straight" op-amps are not easily amenable to AGC action, but I've done
some playing around with for example LED/CdS optocouplers as very
linear controllable-loss blocks for AGC action and they work fine up
through the low MHz. Above a few MHz I think capacitance across the CdS
photocell gives too much leakage in typical receiver use, although if
you only want a limited amount of AGC I think you're good to go.

It is MOST interesting to use chips like AD603's in IF's if you want
AGC action. Many many app notes and construction articles on the web
will show you examples of use.

Are you perhaps really talking about RC active filters to replace
crystal or mechanical IF filters? I have always been VERY PESSIMISTIC
about this, usually even few percent tolerance capacitors (not cheap)
cause your passband/ultimate rejection to be way way worse than what
you expect. Different filter topologies have different tradeoffs, but
RC active filters when you want the Q to be 100 and want even mediocre
ultimate rejection are economically unfeasible in every analysis I've
done.

Tim.