# RC low pass filter difference with RC high pass filter

This is a low pass filter: simulate this circuit – Schematic created using CircuitLab

This a high pass filter: simulate this circuit

If we add a load resistor to the high pass filter it becomes like this: simulate this circuit

The total resistance of the R1 with the RL will be lower than the smallest resistance(R1 or RL).

If we add a load resistor to the low pass filter it becomes like this: simulate this circuit

The total impedance of the C1 with the RL will be lower than the smallest impedance(C1 or RL).

If we start from a frequency when the impendance of C1 is smaller than the impendance of RL then the strength of the signal passing through RL will depend on frequency.

However if the impendance of the load resistor becomes less than the impedance of the capacitor then the strength of the signal passing through RL will not depend anymore on frequency

(in both cases i am reffering to voltage not current passing through RL resistor)

Low pass filters stop working below below a certain frequency and high pass filter always work?

• Similar thing to high pass filter occurs when load is capacitive. A load need not always be purely resistive.
– AJN
Jun 27 '20 at 14:58
• Yes it is similar but in most common cases the load is purely resistive. Jun 27 '20 at 15:03
• In a theoretical world this may be true, but the properties of all components including the circuit layout change with increasing frequency, so this approach is no longer generally valid for high or very high frequencies. E.g. in practise, a load is never completely purely resistive, this can be shown for each frequency with capacitive and inductive components. Jun 27 '20 at 16:32
• @TomKuschel a resistor has almost 0 capacitance and inductance... Jun 27 '20 at 16:36
• @McCarter as I said: never - just look at the connecting wires and how the restor is built. The wires are inductive elements, and there are parasitical capacity everywhere. Therefore a resistor is never 0 capacitance and 0 inductance. Look at the circuit with a frequency of 1 GHz and above. Jun 27 '20 at 16:46