Active Low-Pass Filter with Op-Amps

Overview

In signal processing, an active low-pass filter is essential for allowing low-frequency signals to pass while attenuating higher frequencies. This tutorial guides you through the design and assembly of an active low-pass filter using an operational amplifier (op-amp).

You will learn how to construct the filter, understand the role of each component, and observe its performance in processing signals. The filter operates by allowing frequencies below a specific cutoff value to pass through while effectively filtering out higher frequencies.

What You Will Learn

• How to configure an op-amp as a low-pass filter.
• The importance of resistor and capacitor values in determining the cutoff frequency.
• Real-world applications of low-pass filters in signal processing.

Components Required

To build the circuit, you will need the following components:

• 1 x Operational Amplifier (e.g., LM741 or TL082)
• 2 x Resistors (choose appropriate values based on design)
• 1 x Capacitor (choose based on required cutoff frequency)
• 1 x DC Power Supply
• Breadboard and jumper wires
• Signal generator and oscilloscope (for testing)

Theory

In an active low-pass filter, the combination of resistors and capacitors determines the cutoff frequency (fc) of the filter. The cutoff frequency is calculated as:

fc = 1 / (2πRC)

Here:

• R is the resistance (in ohms)
• C is the capacitance (in farads)

The op-amp amplifies the signal while maintaining a sharp cutoff at the specified frequency.

Circuit Diagram

The basic configuration for an active low-pass filter using an operational amplifier is as follows:

1. Place the operational amplifier (op-amp) on a breadboard.
2. Connect a resistor (R) between the input signal source and the non-inverting input (Vin) of the op-amp.
3. Connect a capacitor (C) between the non-inverting input and ground. This forms a frequency-dependent voltage divider.
4. Link the output of the op-amp to the load (or next stage of the circuit).
5. Power the op-amp by connecting the positive and negative voltage supplies to its power pins (Vcc and Vee).
6. Ensure the inverting input is properly grounded or connected, depending on the op-amp configuration being used.

Explanation: The resistor and capacitor form a frequency-dependent voltage divider, which determines the cutoff frequency. The op-amp amplifies the filtered signal and maintains the desired frequency characteristics.

Procedure

1. Place the op-amp on the breadboard.
2. Connect the resistor and capacitor in the low-pass filter configuration as shown in the circuit diagram.
3. Connect the input signal from the signal generator to the non-inverting input of the op-amp.
4. Power the op-amp using the DC power supply.
5. Connect the oscilloscope to observe the input and output waveforms.
6. Gradually increase the input frequency and observe the attenuation at frequencies higher than the cutoff.

Observations

As the input frequency increases, the output signal's amplitude decreases. Below the cutoff frequency, the signal passes through with minimal attenuation. Beyond the cutoff frequency, the filter attenuates the signal.

Expected Results

At the cutoff frequency, the output signal's amplitude will be reduced to approximately 70.7% of the input signal (-3dB). Frequencies below the cutoff will pass through with minimal attenuation, while frequencies above the cutoff will be attenuated.

Applications

Active low-pass filters are widely used in the following areas:

• Audio processing to filter out high-frequency noise.
• Signal conditioning in data acquisition systems.
• Prevention of aliasing in analog-to-digital converters.

Conclusion

By following this tutorial, you’ve learned how to design and implement an active low-pass filter using an operational amplifier. The key takeaway is understanding how the resistor and capacitor values influence the cutoff frequency and the overall performance of the filter.