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This is a low pass filter:

schematic

simulate this circuit – Schematic created using CircuitLab

This a high pass filter:

schematic

simulate this circuit

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

schematic

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:

schematic

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?

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  • \$\begingroup\$ Similar thing to high pass filter occurs when load is capacitive. A load need not always be purely resistive. \$\endgroup\$ – AJN Jun 27 at 14:58
  • \$\begingroup\$ Yes it is similar but in most common cases the load is purely resistive. \$\endgroup\$ – McCarter Jun 27 at 15:03
  • \$\begingroup\$ 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. \$\endgroup\$ – Tom Kuschel Jun 27 at 16:32
  • \$\begingroup\$ @TomKuschel a resistor has almost 0 capacitance and inductance... \$\endgroup\$ – McCarter Jun 27 at 16:36
  • \$\begingroup\$ @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. \$\endgroup\$ – Tom Kuschel Jun 27 at 16:46
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“Low pass” means below a certain frequency, the filter will not change the input to output transfer. I guess you could say it “quits working” below a certain frequency, but I would not use this phrase.

“High pass” means below a certain frequency, it changes the input to output transfer ( making it smaller). So, I guess you could say, by definition, a high pass filter always “works”, ( that is, changes the transfer) at low frequencies.

So, if you use your definition of the word “works”, you are right! Most people don’t think of it this way, though.

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