Special op amp "tricks"

--- Yes.

1. Consider the opamp inputs holy and _ALWAYS_ have the ends of the resistors connected to the opamp inputs as close to the opamp as you can get them. Sacrifice almost everything for that.

Ditto for what's driving the resistors through which the opamp is being driven, but the opamp input end of the resistors takes precedence.

1. Always roll off any bandwidth you don't need.

--- Unless otherwise indicated, the textbook stuff will mostly be sunshine on a cloudy day.

-- JF

Is this mainly for HF stuff or does it matter. What is the purpose of this too? lead inductance?

huh? Not sure what you mean here? How can you do this if the signal is, say, comming from a guitar or mic?

And this is because of oscillations?

Thanks, Jon

--
It\'s for everything.

Amen to #1. Possibly that could be expanded to include the feedback loops -- keeping them as small as possible and putting the terminating leads as close as possible to the input pins also works miraculous wonders.

In addition to #2, a couple more lesser caveats might be in order:

• Always bypass the power supply for anything faster than an LM358 (and sometimes even then), and keep the AC impedance of the bypasses to GND as small as possible. Memorize the venerable Analog Devices appnote AN-202, "An IC Amplifier User's Guide to Decoupling, Grounding, and Making Things Go Right for a Change". This appnote was written in the days when dinosaurs ruled the earth and AD still made modular op amps, but its advice is still just as valid.
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• Always look carefully at fault conditions, and make absolutely sure the input voltages will never exceed absmax under any circumstances. Again, careful planning with GND impedances is necessary. An op amp input can be grievously damaged, but still kinda function, causing no end of headaches.

Textbook stuff is great as far as it goes. But the OP might just want to open up good audio equipment, especially older stuff from the seventies and eighties, and look carefully at their schematics. The additional practical methods to make things work well and reliably are right in front of him, if he takes the time and effort to learn from them.

Cheers Chris

Thanks again, I'll try to keep that stuff in mind next time I play with them but chances are I'll forget ;/

Jon

yeah, I never thought to do that. But then again, its one thing to see what they were doing and totally different to understand why they did it.

Thanks, Jon

I've seen op amps used in parallel to reduce noise. Neat trick.

Dave.

One trick that you are likely to need for audio use is the ability to drive a cable. Many op-amps are poor at this due to the capacitance of the cable. The trick is to put 100 ohms between the amp output and the cable, and connect the amp feedback resistance from the cable side of the resistor instead of from the amp output. You also need a small feedback cap (10-50 pF) wired in the usual way from output to - input.

Best regards.

Bob Masta D A Q A R T A Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Signal Generator Science with your sound card!

--
Oh, don\'t worry, get bit on the ass enough times and it\'ll become
part of your most easily accessible _permanent_ memory! ;)

Thanks guys. I'll try to keep all this stuff in mind but atleast I know some of the details.

Jon