There are three basic, linear analog circuit components: the resistor (R), capacitor (C) and inductor (L). These may be combined in four important combinations: the RC circuit, the RL circuit, the LC circuit and the RLC circuit with the abbreviations indicating which components are used. These circuits, between them, exhibit a large number of important types of behaviour that are fundamental to much of analog electronics. In particular, they are able to act as passive filters. This article considers the RC circuit, in both series and parallel as shown in the diagrams.
The simplest RC circuit is a capacitor and a resistor in series. When a circuit composes of only a charged capacitor and a resistor, then the capacitor would discharge its energy into the resistor. This voltage across the capacitor over time could be found through KCL, where the current coming out of the capacitor must equal the current going through the resistor. This results in the linear differential equation
When solved, it results in the exponential decay function
The equivalent resistance of a capacitor increases in relation to the amount of charge stored on the capacitor. If a capacitor is subjected to an alternating current voltage source, then the voltage of the capacitor would flip to the frequency of the AC voltage source. The faster the voltage of the AC voltage source flips, the less time charge would allowed to be stored on the capacitor, therefore reducing the capacitor's equivalent resistance. This explains the inverse relationship the equivalent resistance of a capacitor has with the frequency of the voltage source.
Sinusoidal steady state is a special case in which the input voltage consists of a pure sinusoid (with no exponential decay). As a result,
and the evaluation of s becomes
and the voltage across the resistor is:
The transfer function for the capacitor is
Similarly, the transfer function for the resistor is
Both transfer functions have a single pole located at
The magnitude of the gains across the two components are:
and the phase angles are:
These expressions together may be substituted into the usual expression for the phasor representing the output:
The current in the circuit is the same everywhere since the circuit is in series:
The impulse response for each voltage is the inverse Laplace transform of the corresponding transfer function. It represents the response of the circuit to an input voltage consisting of an impulse or Dirac delta function.
The impulse response for the capacitor voltage is
where u(t) is the Heaviside step function and
is the time constant.
Similarly, the impulse response for the resistor voltage is
where δ(t) is the Dirac delta function
These are frequency domain expressions. Analysis of them will show which frequencies the circuits (or filters) pass and reject. This analysis rests on a consideration of what happens to these gains as the frequency becomes very large and very small.
This shows that, if the output is taken across the capacitor, high frequencies are attenuated (rejected) and low frequencies are passed. Thus, the circuit behaves as a low-pass filter. If, though, the output is taken across the resistor, high frequencies are passed and low frequencies are rejected. In this configuration, the circuit behaves as a high-pass filter.
The range of frequencies that the filter passes is called its bandwidth. The point at which the filter attenuates the signal to half its unfiltered power is termed its cutoff frequency. This requires that the gain of the circuit be reduced to
Solving the above equation yields
Clearly, the phases also depend on frequency, although this effect is less interesting generally than the gain variations.
So at DC (0 Hz), the capacitor voltage is in phase with the signal voltage while the resistor voltage leads it by 90°. As frequency increases, the capacitor voltage comes to have a 90° lag relative to the signal and the resistor voltage comes to be in-phase with the signal.
The most straightforward way to derive the time domain behaviour is to use the Laplace transforms of the expressions for and given above. This effectively transforms . Assuming a step input (i.e. before and then afterwards):
These equations are for calculating the voltage across the capacitor and resistor respectively while the capacitor is charging; for discharging, the equations are vice-versa. These equations can be rewritten in terms of charge and current using the relationships C=Q/V and V=IR (see Ohm's law).
Thus, the voltage across the capacitor tends towards V as time passes, while the voltage across the resistor tends towards 0, as shown in the figures. This is in keeping with the intuitive point that the capacitor will be charging from the supply voltage as time passes, and will eventually be fully charged and form an open circuit.
These equations show that a series RC circuit has a time constant, usually denoted being the time it takes the voltage across the component to either rise (across C) or fall (across R) to within of its final value. That is, is the time it takes to reach and to reach .
The rate of change is a fractional per . Thus, in going from to , the voltage will have moved about 63.2 % of the way from its level at toward its final value. So C will be charged to about 63.2 % after , and essentially fully charged (99.3 %) after about . When the voltage source is replaced with a short-circuit, with C fully charged, the voltage across C drops exponentially with t from towards 0. C will be discharged to about 36.8 % after , and essentially fully discharged (0.7 %) after about . Note that the current, , in the circuit behaves as the voltage across R does, via Ohm's Law.
These results may also be derived by solving the differential equations describing the circuit:
Consider the output across the capacitor at high frequency i.e.
This means that the capacitor has insufficient time to charge up and so its voltage is very small. Thus the input voltage approximately equals the voltage across the resistor. To see this, consider the expression for given above:
Consider the output across the resistor at low frequency i.e.,
This means that the capacitor has time to charge up until its voltage is almost equal to the source's voltage. Considering the expression for again, when
The parallel RC circuit is generally of less interest than the series circuit. This is largely because the output voltage is equal to the input voltage — as a result, this circuit does not act as a filter on the input signal unless fed by a current source.
With complex impedances:
This shows that the capacitor current is 90° out of phase with the resistor (and source) current. Alternatively, the governing differential equations may be used:
For a step input (which is effectively a 0 Hz or DC signal), the derivative of the input is an impulse at . Thus, the capacitor reaches full charge very quickly and becomes an open circuit — the well-known DC behaviour of a capacitor.