[sdiy] Weird op amp features & rules of thumb

[Aaron Lanterman]

---> Grant's mentioning of the LM837 got me into looking at various op  
amp data sheets, all of which contain various specs. I wanted to make  
a mental catalog of weird features outside of the standard specs, just  
to make sure that if a circuit used a special weird feature, I paid  
attention to that in thinking about substituting op amps (and whether  
changes would be needed.)

So far I've come up with:

* 741, 4558, and 4136 have special circuitry to avoid the chip going  
boom if the output is short circuited. Grant noted that the 220 ohm  
output resistors Buchla uses on many Music Easel circuits was put  
INSIDE the feedback loop, and that Buchla was relying on this  
circuitry. So if another op amp is used, Grant suggests a typical 1K  
output resistor.

* If the voltage difference on the input pins exceeds a magic number,  
the TL07x/TL08x do a really nasty phase inversion where the + acts  
like - and the - acts like +. For typical negative feedback mode  
circuits where the pins try to stay at the same voltage, this probably  
isn't a problem, but if it's being used as a comparator (or in a  
voltage-starved feedback mode like the Buchla timbre circuits, in  
which the op amp can't keep the voltages the same), this could yield  
all sorts of vicious nastyness. So blindly plugging in a TL07x/08x for  
another op amp may be dangerous and should be carefully considered.

* The 5532 series has some back to back diodes between its inputs to  
provide some input protection. That means that it really should be  
only used in a negative feedback configuration, as it would make a  
crappy comparator, and in particular would NOT work at all in the  
Buchla timbre circuit, which uses negative feedback but needs to allow  
the inputs to separate from one another.

Any other weird op amp things out there that someone substituting op  
amps should be on the lookout for?

---> I'm struggling on when to choose a 5532 type chip vs a TL072 type  
chip. After some reading, I came up with the following rule of thumb:

Aaron's possibly nonsensical rule of thumb: In terms of noise, BJT  
input op amps are a better choice when being driven by sources with  
low output impedances (like other op amps), and JFET input op amps are  
a better choice when being driven by sources with high output  
impedances (OTAs, expo converters, etc.), and when you don't want the  
input current to be doing things like sucking electrons off your  
integration cap. (A corrollary of this is that if you really need  
minimal input current, you want a MOSFET input chip like the CA3140.)

---> Aaron's usage rule of thumb, based on reading books and stuff:

BJT input amps need that little resistor from the + terminal to ground  
when you're setting up an inverter. JFET input op amps don't. If  
you're doing a more complicated circuit, you generally just want the  
resistances seen at each of the input terminals to be more or less the  
same.

Are my rules of thumb on target or off base?

- Aaron