Side-by-side comparison
| Parameter | LPF | HPF |
|---|---|---|
| Passes frequencies | Below cutoff fc (low frequencies) | Above cutoff fc (high frequencies) |
| Blocks frequencies | Above fc (stopband is high frequency) | Below fc (stopband is low frequency) |
| Transfer function (1st order) | H(s) = ωc / (s + ωc) | H(s) = s / (s + ωc) |
| −3 dB cutoff formula (RC) | fc = 1/(2πRC) | fc = 1/(2πRC) |
| Roll-off (1st order) | −20 dB/decade above fc | −20 dB/decade below fc |
| DC response | Passes DC (0 Hz) | Blocks DC (gain → 0 at f = 0) |
| Key parameter | Cutoff frequency fc | Cutoff frequency fc |
| Passive realisation | R in series, C to ground | C in series, R to ground |
| Active IC example | Sallen-Key LPF using TL072 | Sallen-Key HPF using TL072 |
| Typical application | Anti-aliasing, audio bass, power supply ripple removal | AC coupling, rumble filter, differentiator base |
Key differences
An LPF lets through DC and low frequencies, attenuating everything above fc at −20n dB/decade (n = filter order). An HPF is the mirror image — it blocks DC and passes everything above its cutoff, realised by simply swapping R and C positions in the RC network. A BPF combines both, passing only a band around centre frequency f0; its bandwidth BW=f0/Q means high Q gives a narrow, selective passband. A 2nd-order multiple-feedback BPF built with an LM324 can achieve Q of 10–50, giving a 200 Hz bandwidth around a 1 kHz centre. All three appear together in an ECG amplifier front-end — HPF at 0.5 Hz, LPF at 150 Hz, notch (twin-T) at 50 Hz.
When to use LPF
Use a low-pass filter when high-frequency noise or aliasing must be removed from a signal — for example, a 1 kHz Sallen-Key LPF using a TL072 before the ADC input of a temperature data logger sampling at 5 kHz.
When to use HPF
Use a bandpass filter when only a specific frequency range carries the wanted signal — for example, a multiple-feedback BPF centred at 1 kHz with Q=10 to isolate DTMF tones in a telephone line interface circuit.
Recommendation
Match the filter type to the spectral location of your wanted signal: LPF for baseband signals, HPF to block DC offset or low-frequency drift, BPF when the signal is centred around a specific frequency. The cutoff or centre frequency formula must be the first thing you calculate — fc=1/(2πRC) for first-order RC types.
Exam tip: Examiners frequently ask you to draw the frequency response (Bode magnitude plot) for all three types on the same axes and label the −3 dB point, the roll-off slope, and the passband — practise this as a single diagram.
Interview tip: Interviewers expect you to swap component positions in an RC network to convert an LPF to an HPF and to calculate the new cutoff frequency when given R and C values — do this mentally in under 30 seconds during the interview.