Resistor and capacitor are the key ingredients of many electronic circuit. Here are some reason why:
Power Supply Filter
A capacitor will smooth (filter) the pulsating voltage from a power supply into a steady direct current (DC).
Digital logic circuit (which we'll find more about later in next article), can use lots of current momentarily when they switch from off to on or vice versa. This can cause very brief but substantial reductions in power applied to nearby circuits. These power spikes (or glitches, as they are sometimes called) can be eliminated by placing a small (0.1 uF) capacitor across the power leads of the logic circuit:
AC - DC Selective Filter
Often an electrical signal will be riding atop a steady DC signal. For example, the signal from a lightwave communication system may look like this when,
A capacitor will pass the fluctuating signal and completely block the steady DC level.
Two circuits that combine a resistor (R) and capacitor (C) are very important. They are the integrator and differentiator. Both these circuit are used to reshape an incoming stream of waves or pulses.
The product of an R and C in these circuits is called an RC Times Constant. For the circuits shown below, the RC time constant (in second) i9s at least ten times the interval between incoming cycles or pulses.
Here's a basic RC integrator
If the input pulse are speeded up, the output waveform (often called a sawtooth) will not reach their full height (amplitude). It's easy to design an amplifier that ignores waves with less than a desired amplitude. Therefore, the integrator can function as a filter which passes anly signals below a certain frequency.
Here's a basic RC differentiator
This circuit produces symmetrical output waves with sharp positive and negative peaks. It's used to make narrow pulse generators for television receivers and to trigger digital logic circuits.
More about R-C
You will often see reference to the RC time constant of a circuit. It's the time in seconds for a charging or discharging capacitor to go through 63.3% of the change in charge.