Electronic Design Software

Why should I use a capacitor to design electronic circuits?

The only thing that I understand about capacitors is that they are an open circuit in DC and they have impedance in AC circuits. I just don't understand the purpose of a capacitor. I know that it charges and discharges and provides a time constant when included with a resistor. Please explain the purpose of a capacitor, when included in circuits using transistors and diodes

Public Comments

1. You should use capacitors in your circuit if you want it (your circuit) to work. Capacitors have a reactance (*not* an impedance) that is infinite at DC and equal to 1 / jwC for capacitance C in farads, w in radians/sec. and j = √-1 The purpose of a capacitor is to block DC, pass AC, and/or act as one half of a resonant circuit (along with an inductor). Doug
2. Capacitors serve a couple of purposes beyond what you mention. They allow the ac component of a dc signal to pass, such as in a power supply, thereby smoothing the dc output. These are polarity-sensitive caps called electrolytics. The smaller, non-polarized caps are used as coupling devices for RF devices. There are, in fact, several uses for the device, and building functioning circuits without them would be almost impossible.
3. At the heart of understanding the function of a capacitor is understanding the voltage across a capacitor is ideally the time integral of the current into the capacitor divided by its capacity. Capacitors are useful in realizing linear circuits such as power supply filters (storing energy by integrating the half sine wave of the rectified mains and releasing it later), resonant circuits of oscillators (integrating the current formed by the collapsing magnetic field from an inductor to increase the voltage across the capacitor and then returning that energy to the inductor cyclicly). In both examples here I'm sure you've noted that the capacitor is an energy storage element. The form of the stored energy is charge. Storing energy in this form has multiple uses. Charge coupled devices (CCD) store information in the form of charges on capacitors as a form of analog memory. Coupled with photo diodes they become sensors for many digital cameras. Within a field effect transistor, the capacitor becomes the channel enhancing or depleting element.