Most of the time, when we study, evaluate or try to design electronic circuits, we often encounter the terms coupling, decoupling and bypass capacitor. But what are the differences between them? How can you recognize one, and how do they work?
Let’s start to know what a decoupling capacitor is. A decoupling capacitor is usually used in power supplies and power sections of your circuit. It is placed in parallel with the power source and the load.
Decoupling capacitors have two functions in a circuit. The first function of a decoupling capacitor is to act as a local electrical energy reservoir. One characteristic of a capacitor is that it opposes quick changes of voltage. With this, the capacitor can provide energy to keep the voltage stable whenever the input voltage suddenly drops. This allows the decoupling capacitor to smoothen the ripples from the input voltage or output voltage, providing a stable supply to a circuit or a component.
The second function of a decoupling capacitor is to filter AC noise. Another characteristic of a capacitor is that it has less reactance to high frequency signals allowing it to pass AC signals through it while it blocks DC signals. In the way that a decoupling capacitor is placed in the circuit, AC signals are routed to ground instead of passing into the load circuit. And that is how a decoupling capacitor filters these noises. Decoupling capacitors protect both the circuit from the electrical noise from the power supply, and the power source from electrical noise generated within the circuit. This way, the circuit or the component being supplied is accepting only a pure DC signal.
Commonly, two capacitors are placed in parallel to act as decoupling capacitors. One is a smaller value and the other is a larger one. The larger one stores most of the energy in the circuit and filters the lower frequency noise. It is usually an electrolytic capacitor, ceramic, or tantalum capacitor. The smaller capacitor, typically a ceramic capacitor, filters the higher frequency noise.
From the definition in the second function of a decoupling capacitor, the AC noise is routed to ground or bypassed to ground. Hence, decoupling capacitors are also called bypass capacitors.
In the above discussion of decoupling capacitors, we have learned how bypass capacitors route the noise to ground from power sources. Bypass capacitors can also be used in other sections of a circuit to filter out noise and improve the overall performance of the circuit.
One example circuit where a bypass capacitor is used is in a Common Emitter Transistor amplifier. Looking at its schematic, the common emitter amplifier has a bypass capacitor parallel to its emitter resistor. The emitter bypass capacitor, which is C2 in the figure below, provides an effective short to the AC signal around the emitter resistor, thus keeping the emitter at AC ground. With the bypass capacitor, the gain of the given amplifier will be at maximum and is equal to the value of RC/r’e, since RE is bypassed, where r’e is the resistance that appears between the emitter and base terminals of the transistor.
The bypass capacitor must be large enough so that its reactance over the frequency range of the amplifier is very small (ideally ) compared to RE. A good rule-of-thumb is that the capacitive reactance, XC, of the bypass capacitor should be at least 10 times smaller than RE at the minimum frequency for which the amplifier must operate.
Now that we have discussed the decoupling or bypass capacitor, let’s move on to the next topic, the coupling capacitor. While decoupling capacitors are connected in parallel to the signal path, coupling capacitors are connected in series to the signal path. In this way, a coupling capacitor filters DC signals instead of AC signals.
Coupling capacitors are widely used in amplifier circuits. For example, in single supply op-amp based amplifiers, where the non-inverting input is biased to a reference voltage or a virtual ground. This is done so that the ground level of your signal will be positioned so that the negative part of your signal will not be cut off. Biasing the non-inverting input means connecting it to a DC voltage half that of your op-amp’s power supply. The DC voltage introduced to your input signal will now also be carried into your output signal. When your output signal is connected to another circuit stage, the DC signal that it carries may cause performance instability or damage to the circuit. The DC voltage from your bias is removed by placing a coupling capacitor. Coupling capacitors are usually placed at the input and output of your circuit as shown below. They are also placed in between circuit stages.
The capacitor’s reactance increases as the frequency of the signal passing through it decreases. As the signal approaches DC the capacitor’s reactance becomes high enough that the capacitor acts as an open circuit, thus blocking the DC signal.
Now we have learned the differences of the decoupling or bypass capacitor and a coupling capacitor. We also learned their applications and how they function in a circuit. In summary, decoupling or bypass capacitor allows DC to pass through while blocking AC, while a coupling capacitor allows AC to pass while blocking DC. A decoupling or bypass capacitor is placed in parallel with the source and the load while a coupling capacitor is placed in series with the load.