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Re: bypass cap?

Posted by patentguy on January 15, 2000 at 14:22:49:
I think the real confusion is that engineers and techs take for granted the knowledge accumulated over the years when discussing these techniques and forget that the terminolgy is the same for different kinds of bypasses, even though amongst themselves there is agreement based on the context (got that?).
In this instance (op-amp applications) a bypass of the op-amp is the placement of a capacitor between the op-amp power supply pins and the ground plane. This is done to provide a path for AC radio-frequency (RF) garbage that might be contaminating the DC power supply rails. RF hash can cause op-amps to mis-behave in a lot of ugly ways, but generally because they have a large open-loop bandwidth they can oscilate when the gunk gets inside the power rails inside the op-amp via the supply pins.
In power supplies a bypass capacitor of small value is usually is placed in parallel with a large capacitor to provide sonic benefits. The bypass in this case is usually of a film type and doesn't really contribute to the capacitance or energy storage of the power supply, its purpose is to provide a lower series resistance path, and the benefit of a faster response to time-varying voltages( an audio signal). There is a lot more to the subject, and I am hardly an authority, you might check out Richard Marsh's papers available at reliablecapacitors.com, for in-depth review of the variety of way capacitor bypasses are implemented, and other intelligent stuff. Did this help?