Op Amps

brad sanders radioactive at geocities.com
Thu Jan 23 20:52:17 CET 1997

>1) I hear a lot of talk about noise levels of different op amps.  I 
>presume when most of you talk about noise levels, you are referring 
>to equivalent input noise voltage.

Yep. And noise current. To be technically correct, both are referred
to as noise *density,* as it is specified in root Hz. Of course,
there's no need to be so semantically accurate amongst friends...

>With these voltage levels, won't the thermal noise from a resistor be 
>much greater (in the uV) than any of the op amp noise levels?

Sort of. The problem is *input* noise voltage is multiplied by both
the root of the bandwidth (roughly 141 in audio) and the GAIN of the
circuit. So, in a phono preamp (for example) that 6nV/^Hz INPUT noise
voltage density becomes 600nV/^Hz on the output!

National Semiconductor had an EXCELLENT appnote on this in thier old
Linear Applications/ Tech manual. Unfortunately, I haven't seen it in
years. Look for an old (early 80s) copy of this - or perhaps it's on
their web site; appnote called "Calculating noise performance in op
amps" or something like that.

>2) What is the full info on the article by Jung (is that correct?) 
>about op amps for audio applications?

The first rev appeard in AUDIO in the late '70s ('78, I believe). It
was a cover feature, spread out over two parts. Should be easy to
find. Also, I've heard (but can't confirm) the latest rev of his "OP
Amp Applications" has a chapter or two which are an updated (late
'80s) version of the article.

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