What does a resistor do in parallel with a capacitor re smoothing, coupling etc?

Thread Starter

Coach77

Joined Jul 27, 2015
3
Hello, novice electronics guy here. I'm reading about RC circuits and am having trouble finding a clear explanation of why resistors are put in parallel with capacitors for AC circuits. On my online travels, I came across a page about coupling caps and it mentions a high-range resistor alongside a capacitor means a lower-value cap can be used. Why is this the case? Sorry if this has been covered previously. Cheers j
 

MrAl

Joined Jun 17, 2014
7,755
Hello there,

Capacitors mostly pass AC current while resistors pass both AC and DC. Both of these depend on the values too though as to how much of that they actually pass.

One of the things that makes a coupling capacitor work is that it maintains a nearly constant DC voltage across it while coupling AC signals from one part of the circuit to another part of the circuit.
Capacitors discharge however when there is a resistor in parallel. If the resistor has a small value then the capacitor can not stay charged long enough to make a good coupling for AC, so a larger value resistor would allow a smaller value capacitor to be used.

This would be mainly when the designer wanted to save maybe one resistor by using the previous stage DC output to help bias the following stage input. That's not always the way it is done as the following stage might be biased with it's own set of resistors, but then the value of the cap required depends on the values of those bias resistors anyway.
 
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