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Why are two capacitors—0.1uF and 0.01uF—always placed in circuits?

Release time:

2021-10-15

The concepts of bypass capacitor and decoupling capacitor are common in circuit design, yet they can be tricky to grasp. To fully understand these terms, it’s helpful to return to their original English contexts. In English, “bypass” literally means taking a shortcut; in the context of circuits, it carries the same meaning—allowing a signal to bypass certain components or sections of the circuit. Meanwhile, “couple” in English refers to a pair or a matching set; by extension, it also conveys the idea of pairing or coupling. If a signal from System A induces a signal in System B, we say that Systems A and B are coupled—a phenomenon illustrated in the figure below.

　　The concepts of bypass capacitors and decoupling capacitors are common in circuit design, but they’re not easy to understand.

　　To understand these two terms, we need to go back to the English context.

　　In English, "bypass" means taking a shortcut. In electrical circuits, it carries the same meaning.

　　In English, the term "couple" originally refers to a pair or a pair of things. By extension, it can also mean pairing or coupling. If a signal in System A triggers a signal in System B, we say that Systems A and B are coupled—as illustrated in the figure below. Conversely, "decoupling" refers to reducing or weakening this coupling effect.

　　Bypass and Decoupling in Circuits

　　(1) Bypass

　　If the power supply is subjected to interference—typically high-frequency interference signals—it may cause the IC to malfunction. By connecting a capacitor C1 in parallel near the power supply, we take advantage of the fact that capacitors block DC current while presenting low impedance to AC signals. The high-frequency interference signals are shunted back to ground through C1, effectively bypassing the IC and diverting these interfering signals directly to GND via the capacitor. This is precisely the role of the bypass capacitor—C1.

　　(2) Decoupling

　　Since integrated circuits generally operate at relatively high frequencies, large current fluctuations can occur on the power supply lines when the IC is first powered up or when its operating frequency is switched. These interference signals can directly feed back into the power supply, causing it to fluctuate. Specifically, this effect is most pronounced near the VCC pin of the IC.

　　A capacitor C2 is connected in parallel with the power supply port. Since capacitors have energy-storage capabilities, they can provide instantaneous current to the IC, thereby reducing the impact of IC current fluctuations and interference on the power supply. In this context, C2 functions as a decoupling capacitor.

　　Why use two capacitors?

　　Returning to the question mentioned at the beginning of this article, why are two capacitors—0.1uF and 0.01uF—used?

　　The formulas for calculating capacitive impedance and capacitive reactance are as follows:

　　8581ed70cd564c2f80206bf429b3ab76.png

　　Capacitive reactance is inversely proportional to both frequency and capacitance—meaning the larger the capacitance and the higher the frequency, the smaller the capacitive reactance. You can think of it simply: the larger the capacitance, the better the filtering effect. So, if you already have a 0.1uF bypass capacitor, wouldn't adding another 0.01uF capacitor be a waste?

　　In fact, for a given capacitor, it exhibits capacitive behavior when the signal frequency is below its self-resonant frequency and inductive behavior when the signal frequency exceeds its self-resonant frequency. When two capacitors—0.1uF and 0.01uF—are connected in parallel, the resulting effect is equivalent to broadening the filter’s frequency range. (Source: Electronic Products World)

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