How Low-Pass & High-Pass Circuits Work

 How RC Low-Pass and High-Pass Filters Work — Simple Explanation with Examples

Let me provide you with a basic understanding that will clear up any misunderstandings you may have regarding the capacitor in a high/low-pass filter.

Mechanism of High/Low Pass Filter:

‘Filter’ word is a bit misleading. It doesn’t require ON/FF-type action. Filter only reduces (voltage) gain for a certain frequency compared to the whole spectrum of the frequencies of interest.

A filter is basically a Voltage Divider: 

The filter works on a voltage divider principle. Let me clarify.

So, the output voltage reduces proportionally to the value of resistors R1 & R2. In simple words, the larger the R1 and the smaller the R2, Vout will reduce compared to Vin.

Can I use the above circuit as a low-pass filter? No, because resistance doesn't change its value proportionally to the frequencies. So our goal of reducing gain for a certain band of frequencies wont be fulfilled. We need to replace R1 or R2 with something that varies its value with respect to frequency.

Low-pass filter:

Let's replace R2 of the above circuit with a capacitor. Why a capacitor? because the capacitor changes its reactance (resistance/opposition) inversely proportional to the applied frequencies. High frequency means less opposition and vice versa.

So now, will this arrangement work as a low-pass filter? It will because for high frequencies reactance of the capacitor (equivalent to R2 of the voltage divider) will be less. Thus output voltage will be less for the high frequencies. At the same time reactance of the capacitor will be high for low frequencies, so the output voltage will be less affected.

This is how low low-pass filter reduces gain (output/input) for the high frequencies and passes low frequencies less affected.

Highpass filter:

Pl. apply similar logic as discussed above. For high frequencies reactance of the C will be less, and vice versa. So gain of the HPF is less affected for the high frequencies, and the gain is reduced for the low frequencies.

FAQ Set:

Q1. What are RC filters used for?

RC filters are used to control frequency components of signals — removing noise, shaping waveforms, or separating audio frequencies.

Q2. Why are they called “low-pass” and “high-pass”?

Because a low-pass filter lets low frequencies pass and blocks high ones, while a high-pass filter does the opposite.

Q3. What components make up an RC filter?

Only two: a resistor (R) and a capacitor (C) connected in series or parallel, depending on the filter type.

Q4. How is the cutoff frequency calculated?

It’s given by fₐ = 1 / (2πRC), where R is in ohms and C is in farads.

Q5. What happens at the cutoff frequency?

The output signal amplitude falls to 70.7% (−3 dB) of its input — marking the transition between pass and stop bands.

Q6. How does a capacitor behave in a low-pass filter?

At low frequencies, the capacitor acts like an open circuit (passing signal), and at high frequencies, like a short (blocking signal).

Q7. How does a capacitor behave in a high-pass filter?

At high frequencies, the capacitor acts as a short (passing signal), and at low frequencies, as an open circuit (blocking signal).

Q8. Can RC filters be cascaded?

Yes. Connecting multiple RC stages increases the filter’s order, making it sharper and improving frequency selectivity.

Q9. What are typical applications of low-pass RC filters?

Used in audio crossover networks, DAC output smoothing, and sensor noise filtering.

Q10. What are typical applications of high-pass RC filters?

Used for coupling AC signals, removing DC offsets, and in microphone preamps.

Q11. How does the phase shift occur in RC filters?

RC filters introduce a phase shift between input and output — lagging for low-pass, leading for high-pass.

Q12. Are RC filters active or passive?

They are passive filters, meaning they don’t require external power — only resistors and capacitors.

Q13. What’s the main difference between RC and LC filters?

RC filters use resistors and capacitors (simple, compact), while LC filters use inductors and capacitors (better for higher frequencies).

Q14. Can RC filters be used in digital circuits?

Yes, but mostly for signal conditioning or noise reduction before analogue-to-digital conversion.

Q15. Why are RC filters important in audio systems?

They help separate frequency bands, remove unwanted hiss, and smooth audio signals for clean output.

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• Why does every power supply need a filter capacitor?

• Why decoupling capacitor used in Electronics circuits?

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