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Make substitutions with care

12/6/1998 10:11 AM
Dave Stork
Make substitutions with care
I just wanted to point out that you can't necessarily substitute one op amp for another at will just because they use the same package and pinout. Besides the well-known input impedance difference between FET-input types like the TL0-series and bipolar inputs like the 553-series, there are more subtle differences between types including slew rate and GBP (gain-bandwidth product). As a fairly extreme example of this, I remember a bias servo circuit in an amp I worked on during my brief tenure as a hi-fi repair tech. The original part used was a 741 op-amp, which we didn't even keep in stock because it's such a "junky" part by modern hi-fi standards.I tried a couple of different bipolar-input op amps with no success. The next day, I brought in a 741 from my personal stock, installed it, and the circuit worked great. Apparently, this circuit was designed around particular characteristics of the 741 (possibly its relatively low slew rate) and wouldn't work at all with modern "upgraded" substitutes.
12/7/1998 7:29 AM
Paul Perry

When you are prototyping/troubleshooting, it can be very educational to substitute op amps with different characteristics.. some reverse output and lock up if the input hits rail, even for a microsecond. Others go into slew limited mode, as Dave says. Some have symmetric overdrive characteristics.. but (except for the rail to rail types) most don't. The extra gain/bandwidth on some makes some ckts unstable, with RF oscillations. And so on! Yes, I've "fixed" ckts in an emergency by "downgrading" to a 741!
12/18/1998 3:47 PM
Tim Rand
Re: Low noise opamp
Aaron one thing I've noticed with the 833 I use them in audio gear that the outputs tend to have a bit of oscillation in them (this is some LM833) but a 10pf cap between the output and the inverting input clears this up  
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