RC and RL Lowpass and Highpass Filters

The simplest RC (Resistor Capacitor) and RL (Resistor Inductor) chains are first-order filters; Such filters usually only implement either the LPF (Lowpass) or HPF (Highpass) they contain active resistances and one reactive element: capacitance or inductance, connected in series or parallel.

There are a total of six options for including elements R and L, R and C.

The first four must operate from a voltage source, and the last two must operate from a current source.

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

The simplest RC RL chains of the order of the first type (Fig. 1, 4, 6) are high-frequency filters with a frequency response roll-off outside the bandwidth of 6 dB/octave (or 20 dB per decade). And the simplest RC and RL chains of the second type (Fig. 2, 3, 5) are low-pass filters with a frequency response roll-off outside the bandwidth of 6 dB/octave (or 20 dB per decade).

The cut-off frequency for these circuits is determined by the half-power transmission level or by a drop in frequency response of 3 dB from the maximum.

To calculate the cut-off frequency (-3 dB level) of RC and RL chains, use our interactive online High Pass Filter Calculator and Low Pass Filter Calculator.