questions from a newbie (Op Amps again)

[Tim Daugard]

2) Opamps differ in many ways. Important specs include

>Gain Bandwith Product... This means that an op-amp can be used for a low
>frequency signal with very high gain, or a high frequency at low gain
>(1 * 1000  =  1000 * 1) again, bigger is sometimes better

A recent piece of paper from TI argues against this point. For audio
applications they said that smaller is better. Wide gain bandwidth means
more noise gets amplified while not actual having any benefit for the
signals being used. Noise can add to distortion and modulate signals or at
least continue to be fed down the path.

To reduce noise they recommended an input filter to restrict inputs to the
frequency range desired and tailor the feedback network to keep the gain in
the band of frequency desired.

Reasonable or just an engineer who had to write some thing for the latest
advertising piece?

>Single vs Dual supply... some opamps need a plus/minus supply, some are
>happy with one...

I don't see the difference in this. The best I can figure, A dual supply op
amp will put out a signal balanced around the arbitrary center  (+V-V)/2
while the single supply op amp isn't optimized for balance. ??

>Output Current... Can it drive the load you will be using ???

I have never looked at this spec. Every thing I've done so far has had a
reasonable input impediance. I hooked the output of an LM324 to a 10Hz low
pass filter today and it seems to be working okay - I'm not sure what the
impediance is - I forgot to check - should be on the order of 100 to 200
ohms.

>Output Short Circuit current protect... will it blow up if you shory it
>out... This can be real important in a modular synth, where you cannot
>predict what will be connected...

So far I have hung a 1K resistor off all outputs to avoid problems with
this. What is everyone else doing?


Tim Daugard