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Decoupling Capacitors and Bypass Capacitors Overview: Working, Applications and Differences

Author: Apogeeweb
Date: 14 Jan 2021
Emitter Bypass Capacitor


Capacitors are one of the passive components used the most. You can find them being used in many analog and power electronic circuits, from basic amplifier circuits to complicated filter circuits. While we have already learned the fundamentals of a capacitor and how it functions, there is a wide range of capacitor applications. Two application terminology that is commonly used when referring to a capacitor in a circuit is the Bypass capacitors and the Decoupling capacitor. We will learn about these two types of capacitors in this article, how they operate in a design and how to pick a capacitor to be used as a bypass capacitor or decoupling capacitor.


While the terms Bypass capacitors and Decoupling capacitors are used interchangeably, they have their distinctions. The primary objective of powering any system would be to provide the input power with very low impedance (relative to the ground). Bypassing is applied to circuits to achieve this condition. To grasp the distinction between the two kinds of capacitors, let's dig deep into them.


Ⅰ Introduction

Ⅱ Decoupling capacitor

Ⅲ Positioning a Decoupling Capacitor

Ⅳ Value of the Decoupling Capacitor

Ⅴ Bypass Capacitor

Ⅵ Emitter Bypass Capacitor

Ⅶ Cathode Bypass Capacitor

Ⅷ How to select the value for a Bypass Capacitor

Ⅸ Applications of Bypass Capacitor

Ⅹ Difference between Bypass and Decoupling Capacitor



 Decoupling capacitor

For isolating or decoupling two distinct circuits or a local circuit from an external circuit, decoupling capacitors are used to isolate or decoupling two distinct circuits or a local circuit from an external circuit, i.e. for decoupling AC signals from DC signals or vice versa.


The real truth is that the decoupling capacitor is used for both the reason and by providing pure DC supply, we may describe the Decoupling capacitors as the capacitor used to eliminate power distortion and noise and protect the system/IC.


When it comes to logic circuits, the decoupling process is really necessary. For example, consider a logic gate that can operate at a 5V supply voltage, it will read as a high signal if the voltage goes above 2.5V and it will read as a low signal if the voltage goes below 2.5V. Therefore, if there is a noise in the supply voltage, the logic circuit will cause highs and lows, so DC Coupling condensers are commonly used for logic circuits.


 Positioning a Decoupling Capacitor

The decoupling capacitor should be mounted in parallel between the power supply and the load/IC. The decoupling capacitor would have infinite reactance on DC signals as the DC power supply provides the power to the circuit and they will have no impact on them, so it has far less reactance on AC signals so that they can move through the decoupling capacitor and, if possible, they will be shunted to the field. In order to get shunted, the capacitor can create a low impedance path for the high-frequency signals, resulting in a clean DC signal.

Positioning a Decoupling Capacitor

The positioning includes two separate capacitors, a 10μF capacitance capacitor positioned away from the IC that is used to smooth out the changes in the power supply's low frequency and a 0.1 μF capacitor held closer to the IC that is used to smooth out the changes in the power supply's high frequency.


Electrolytic capacitors are the most used type of capacitors for low-frequency smoothing, and surface mount ceramic capacitors are the capacitors used for high-frequency smoothing.


 Value of the Decoupling Capacitor

Unlike Bypass capacitors, the value of a decoupling capacitor does not have many riles to select. There are certain criteria for choosing the value since decoupling capacitors are commonly used.

• Usually, the low-frequency noise decoupling capacitor value should range from 1 μF to 100 μF.

• Usually, the high-frequency noise decoupling capacitor should be between 0.01 μF and 0.1 μF.

The datasheet for the ICs is often supplied with the exact value of the capacitors to be used. For their efficient operation, the decoupling capacitors should always be connected directly to a low impedance ground plane.


 Bypass Capacitor

To avoid noise from entering the device by bypassing it to the ground, the Bypass capacitor is used. In order to eliminate both the power supply noise and the result of the spikes on the supply lines, the bypass capacitor is mounted between the supply voltage (Vcc) and ground (GND) pins. The capacitor can suppress both inter-and intra-system noises for various devices and different components.

Bypass capacitor

The capacitor shorts any form of AC signal to the ground during operation so that the AC noise in a DC signal is eliminated, resulting in a cleaner and pure DC signal. Let's look at the Emitter and Cathode Bypass capacitors, for instance.


 Emitter Bypass Capacitor

If a bypass capacitor is attached parallel to an emitter resistance, consider a Typical Emitter (CE) amplifier with an emitter resistance, the voltage gain of the CE amplifier increases and if the capacitor is removed, extreme degeneration is produced in the circuit of the amplifier and the voltage gain will be reduced.

Emitter Bypass Capacitor


 Cathode Bypass Capacitor

If a capacitor is connected across the cathode resistance and if the capacitor is sufficiently high, it will function as an audio frequency short circuit and remove negative feedback. It also functions as a DC open circuit and retains the bias of the DC grid.


 How to select the value for a Bypass Capacitor

The capacitor reactivity applied to the circuit should be parallel to 1/10th or less of the resistance. We all know that the current always takes a low resistance course, so the capacitor should have a lower resistance if you want to shunt the AC signal to the field. You can measure the capacitance value of the bypass capacitor to be used using the formula.

Formula 1

Let's remember that you need to find the capacitance of the capacitor connected across the resistance resistor 440 with the above bypass capacitor formulae, we understand that the reactance is always 1/10th of the resistance, so the reactance is 44 and the normal frequency of the Indian electrical network is 50Hz, so the bypass capacitor value can be calculated as

Formula 2

73μF should be the capacitance of the capacitor around the 440 x resistor. You will find out the importance of condensers that can be used in a circuit using the same thing.


 Applications of Bypass Capacitor

The bypass capacitors, some of the notable applications where they are used, are almost used in both analog and digital circuits to eliminate unnecessary signal from the supply voltage.

They are used to create a consistent sound between the amplifier and the loudspeaker.

• Used when converting to DC/DC

• Used in coupling and decoupling signals

• Used in Filters for High Pass(HP) and Low Pass (LP)


 Difference between Bypass and Decoupling Capacitor

There is not much difference between the two types of capacitors when you look at the reason they are used for. Surprisingly, the decoupling capacitors are often called Bypass capacitors much of the time. This is because often they are shunted to the ground.


The bypass capacitor is designed to shunt the noise signals while the decoupling capacitors are intended to smooth the signal by stabilizing the distorted signal, some of the few visible distinctions between the bypass capacitor and decoupling capacitors. We can only use a single electrolytic capacitor for shunting the signal, but we would need two separate types of capacitors for calming the signal.



1. How do coupling and bypass capacitors differ?

A coupling capacitor goes between the output, usually a collector of a transistor or drain of a FET to the input of the next stage, the base of another transistor or gate of another FET. It should be a high enough value to have a reactance below the impedance at the lowest desired frequency.

A bypass capacitor is to conduct the frequency to ground, typically on a power supply to minimize noise or ripple or can be across the emitter/ source resistor to ground to increase gain. In RF circuits there are often two bypass capacitors in parallel. A ceramic capacitor in the 1nF to 0.1uF range, and an electrolytic or tantalum in the 1 to 1,000uF range.


2. Is there a difference between a decoupling capacitor and a by-pass capacitor?

Decoupling is used to decouple noise or other transients from say, a power supply IC output. This is usually a ceramic capacitor of low value.

Next, a bypass capacitor is usually an electrolytic capacitor used to bypass a resistor which is mainly used to set the DC biasing of the amplifier. This cap is used to avoid the negative feedback of the signal and to improve the gain of the amplifier.


3. What is the function of an output decoupling capacitor?

A typical function is to convey an audio frequency ac signal from amplifier output to a speaker, whilst blocking the DC supply from the speaker voice coil.

Also used for coupling audio/radio frequency signals between stages of an amplification chain, whilst isolating dc between stages to simplify biasing, etc.

A typical function of a DEcoupling capacitor is to try to isolate unwanted signals from power supply rails or between stages in multi-stage af/rf/whatever circuitry.


4. What is coupling decoupling and bypass capacitor explain with an application example?

While decoupling capacitors are connected in parallel to the signal path and are used to filter out the AC component, coupling capacitors, on the other hand, are connected in series to the signal path and are used to filter out the DC component of a signal. They are used in both analog and digital circuit applications.


5. What is the significance of bypass capacitors define their functions and applications?

A Bypass Capacitor is usually applied between the VCC and GND pins of an integrated circuit. The Bypass Capacitor eliminates the effect of voltage spikes on the power supply and also reduces the power supply noise. The name Bypass Capacitor is used as it bypasses the high-frequency components of the power supply.


6. What is the purpose of the bypass capacitor?

Bypass capacitors are used to maintain low power supply impedance at the point of load. Parasitic resistance and inductance in supply lines mean that the power supply impedance can be quite high. As frequency goes up, the inductive parasitic becomes particularly troublesome.


7. What is the use of coupling and decoupling capacitor?

Coupling capacitors allow AC components to pass while blocking DC components. Decoupling capacitors are used in electronic circuits as energy reservoirs to prevent quick voltage changes. Bypassing capacitors clean DC signals by shunting unwanted AC components to the ground.


8. What is the effect of a coupling capacitor?

Coupling capacitors (or dc blocking capacitors) are used to decouple ac and dc signals so as not to disturb the quiescent point of the circuit when ac signals are injected at the input. Bypass capacitors are used to force signal currents around elements by providing a low impedance path at the frequency.


9. What is the purpose of using decoupling capacitors in PCB?

The decoupling functions as a reservoir and acts in two ways to stabilize the voltage. When the voltage increases above the rated value, the decoupling capacitor absorbs the excessive charges. Meanwhile, the decoupling capacitor releases the charges when the voltage drops to ensure the supply is stable.


10. Where should a bypass capacitor be placed?

The ideal location to place bypass capacitors is as close as possible to the supply pin of the component. By placing the bypass capacitor very close to the power supply pin, it reduces the impact of the current spikes during the switching. It also provides a low impedance path to the ground for AC noise signals.

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