About DC path

A capacitor is one of most important components in analog/digital circuit design in general. For example, a switched capacitor circuit is one of prevalent circuit topology in analog designs.
While we face many capacitor components in circuits, I am often heard that "DC path does not exist". This DC path implies that charges can go onto a capacitor. The heart insight of understanding DC path term is to recognize that there is no DC voltage drop if there is no DC path exists.

One of frequent uses of "DC path" term is when we talk about "ac coupling". Since a capacitor effectively blocks a DC signal, DC cannot pass through a capacitor. For example, there is typically a big fat C (capacitor) precedes a low-noise amplifier (LNA) input. This C effectivly block a DC signal and let an ac signal pass through without affecting a bias condition of the LNA. In other words, ac signal can go into the big fat C because the impedance of this big fat C at high frequencies is small so that ac signal has no problem of commuting through it. Since there is no DC path exist, this big fat C won't drop a bias (DC) voltage across it.

Remember if there is no DC path, there is NO DC voltage drop!

Another example - there is the rule called "Charge conservation rule", which is a good tool when we analyze a switched capacitor circuit transfer function. This rule can be applied when there is no DC path existence. If there is a DC path, the charges on the capacitor can go in and out depending on the voltage and capacitor value givne at that node. Also, it is important to recognize that this rule is applied to a node.