Master the Basics: What is Filter Signals

Controlling which parts of a signal pass through your circuit is essential in electronics. Whether you're working with audio, sensors, or power systems, signal filtering helps clean and shape your output.

In this post, we’ll cover how to use resistors, capacitors, and operational amplifiers (op-amps) to build low-pass, high-pass, and band-pass filters — perfect for hobbyists and electronics learners.

What Is a Passive RC Filter?

A passive filter uses only resistors and capacitors (no amplifiers). There are two common types:

• Low-pass filter: A resistor in series, followed by a capacitor to ground. Output is measured across the capacitor. This allows low frequencies to pass and blocks high ones.
• High-pass filter: Swap the resistor and capacitor. The output is now across the resistor. This lets high frequencies through and blocks low ones.

 Cutoff Frequency Formula:

Example: With a 10kΩ resistor and 100nF capacitor, the cutoff frequency is about 159 Hz.

Why Add an Op-Amp?

Passive filters are simple, but they can’t:

• Amplify your signal
• Drive loads with low impedance
• Handle complex multi-stage filters without signal loss

Solution: Use an op-amp to create an active filter.

Build a Low-Pass Active Filter

Use the same RC network as before, then add an op-amp configured as a:

• Unity gain buffer (voltage follower) — keeps signal strength intact
• Non-inverting amplifier — boosts signal power

Example: With two 10kΩ resistors, you'll double the signal (gain = 2 = ~6 dB).

High-Pass Filters Work the Same Way

Just flip the resistor and capacitor. The result:

• Frequencies above the cutoff pass through
• Frequencies below are blocked

You can buffer or amplify the high-pass output just like you did with the low-pass.

Important: All op-amps have bandwidth limits. Don’t expect infinite frequency response — always check the datasheet.

Combine Filters to Make a Band-Pass

• Want to let through only a specific frequency range? Combine a high-pass and a low-pass filter with a buffer between them.

Example Setup:

• Low-pass filter: 1kΩ + 100nF = ~1.6 kHz cutoff
• High-pass filter: 10kΩ + 100nF = ~159 Hz cutoff
• Add a final op-amp with gain = 11 (~20.8 dB)

This gives you a band-pass filter with a pass band from ~159 Hz to ~1.6 kHz — perfect for isolating audio or sensor data.

Practical Results

Use a function generator and oscilloscope to test your design.

• At 1 kHz input (within the pass band), the signal passes through and is amplified.
• At 10 Hz or 10 kHz (outside the pass band), the signal is heavily attenuated.

Key Takeaways

• Use RC circuits for basic filtering.
• Add op-amps to boost performance, drive loads, and reduce loss.
• Combine filters for more complex shaping — like band-pass or notch filters.
• Always consider op-amp bandwidth when filtering high frequencies.