[sdiy] Weird op amp features & rules of thumb

[harrybissell at wowway.com]

The rules seem pretty good... especially the choice of the 5534 op-amp.

I'd say that if the input inpedance was more than maybe 5K tops, the
5534 would not be the best choice. You also need to feed the beast..

The resistor for bias current compensation (from the positive terminal
to ground) is needed if you care if the offset voltage is a little uhhhh...
off ?  This might be a little more critical in the 5534 especially in high DC
gain apps... something I'd avoid as you end up taking gain on the offset
voltage as well (it can get really large :^)

H^) harry




On Sat, 29 Nov 2008 03:04:56 -0500, Aaron Lanterman wrote
> ---> 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
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Harry Bissell & Nora Abdullah 4eva