# bypass capacitors

#### jaygatsby

Joined Nov 23, 2011
182
I just read the pinned thread about bypass capacitors. This reminds me of a question I've had. In my digital class, our teacher gives us schematics with capacitors on Vcc and also elsewhere... they are off of Vcc or the other pins and they are always in parallel with a resistor. I have wondered what these were for... my first thought as an amateur is that they are for a resonant circuit but this makes no sense in this application. Are they bypass capacitors with pull up functionality parallel, or something?

Thank you

#### Wendy

Joined Mar 24, 2008
23,318
Schematics are the language of electronics. If you would care to sketch something and scan it as a .gif or .png file it would make your question a lot more clear, it doesn't have to be pretty.

#### Wendy

Joined Mar 24, 2008
23,318
Many cases the capacitor is to speed up the switch transition. The resistor will allow for a set DC bias, but if speed is important then a cap is used to help the leading edge make it through quicker.

The text book drawing you showed has a major defect in it, did you spot it? Hint, they have an output of a gate connecting directly to ground, which is not good.

#### Wendy

Joined Mar 24, 2008
23,318
OK, but you are studying the subject? The schematic make some sense, yes?

Since you will not try I'll point it out to you, maybe you can point it out to your professor....

There is a race between the chip switching fast enough or burning out.

This might even be on a test sometime.

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#### crutschow

Joined Mar 14, 2008
33,331
A capacitor used to decouple a power pin to ground does not have a resistor in parallel. The resistor in that application would serve no useful purpose.

#### jaygatsby

Joined Nov 23, 2011
182
A capacitor used to decouple a power pin to ground does not have a resistor in parallel. The resistor in that application would serve no useful purpose.
Why wouldn't the capacitor filter even though it's in parallel with the resistor? The capacitor could still charge and discharge toward the gate when power fluctuates unexpectedly, right?

#### crutschow

Joined Mar 14, 2008
33,331
Why wouldn't the capacitor filter even though it's in parallel with the resistor? The capacitor could still charge and discharge toward the gate when power fluctuates unexpectedly, right?
Correct. The capacitor still does filtering. But the resistor serves no purpose in decoupling power supply noise.