Favorite quad op-amps for synth design?

Jim Patchell patchell at teletrac.com
Thu Mar 23 15:49:56 CET 2000



Grant Richter wrote:

> > In practice these days though, the best tends to cost no more (and
> > sometimes less) than the worst. For example a 741 now costs more than an
> > LM833. Why would you want to use one? If a circuit specifies a couple of
> > 741s, or even a TL082, You can generally just whack in an 833 in it's
> > place.  (assuming you're designing the circuit board from scratch of
> > course) Unless, as someone previously pointed out, that the original
> > circuit was designed around the characteristics of that op-amp.
> >
>
> One note of caution regarding the 833/837 is the somewhat high
> input bias current of (gasp) almost a micro-ampere.
>
> These will not work so well in sample and hold applications that
> rely on the JFET input of the op-amp for microscopic bias currents.
>
> Weren't there some op-amps that had MOSFET inputs?
> Does the CA3140 or 3130?

    The CA3140 is a combination Mosfet/BJT opamp (with mosfet inputs).  This
opamp has all of the qualities of a mosfet front end (good and bad, high
impeadence, very low bias current, very low input current noise, high voltage
noise, input ofset drift not so great).  Also, the CA3140 was internally
compensated to unity gain, 10V/uS slew rate, I think about 5MHz unity gain
crossover.

    The CA3130 was all Mosfet.  It had a CMOS output, which means it will do
rail to rail, and came uncompensated.  As I recall, it was a little tricky to
compensate, but not teribly so.  Also, because of the CMOS output, max supply
voltage was about 15 volts.  I prefered to keep it down around 5 myself.  This
opamp also made a good voltage comparator that could be used with 5 volt logic
(which is the way I used to primarily apply it).

    -Jim





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