Introduction

The LM358 operational amplifier can be configured to create active filters that allow certain frequencies to pass while attenuating others. This experiment will demonstrate how to design and implement a second-order low-pass filter using the LM358. Low-pass filters are commonly used in audio processing, signal conditioning, and noise reduction applications.

Materials Required

• LM358 Operational Amplifier IC
• Breadboard and jumper wires
• Resistors (R1 and R2)
• Capacitor (C1)
• Signal generator (for input voltage)
• Multimeter or oscilloscope (for measuring output voltage)
• Power supply (5V to 12V)

No Ads Available.

Circuit Diagram

The following circuit diagram illustrates how to configure the LM358 as a low-pass active filter:

Pin Configuration of the LM358

• Pin 1 (OUT1): Output of the first op-amp
• Pin 2 (IN1-): Inverting input of the first op-amp
• Pin 3 (IN1+): Non-inverting input of the first op-amp
• Pin 4 (VCC+): Positive power supply
• Pin 8 (VCC- or GND): Ground or negative power supply

Steps for the Experiment

1. Place the LM358 IC on the breadboard.
2. Connect the VCC+ (Pin 4) to the positive terminal of the power supply (5V to 12V) and connect VCC- (Pin 8) to ground.
3. Connect the non-inverting input IN1+ (Pin 3) to the input voltage source (signal generator).
4. Connect a resistor (R1) from the output OUT1 (Pin 1) to the inverting input IN1- (Pin 2).
5. Connect another resistor (R2) from the inverting input IN1- (Pin 2) to ground.
6. Connect a capacitor (C1) from the inverting input IN1- (Pin 2) to ground.
7. Connect the output OUT1 (Pin 1) to a multimeter or oscilloscope to measure the output voltage.
8. Apply a sinusoidal input signal and vary the frequency to observe the filter response.

Explanation

This configuration of the LM358 acts as a second-order low-pass filter. The output voltage is influenced by the combination of resistors and capacitor, allowing low frequencies to pass while attenuating higher frequencies. The cutoff frequency (f_c) can be calculated using the formula:

f_c = 1 / (2πRC)

Where:

• R is the equivalent resistance (R1 and R2 in parallel for feedback).
• C is the capacitance of the capacitor connected to the inverting input.

Results and Observations

After applying the input signal, you should observe the following:

• At frequencies below the cutoff frequency, the output voltage will closely follow the input voltage.
• As the input frequency approaches and exceeds the cutoff frequency, the output voltage will decrease significantly, demonstrating the filter's effectiveness.

Measure the cutoff frequency and compare it to the theoretical value calculated using the formula above.

Applications of Active Filters

Active filters have various applications in electronics, including:

• Audio signal processing and tone control
• Noise reduction in communication systems
• Signal conditioning for sensors and transducers
• Data acquisition systems and analog signal processing

Conclusion

In this experiment, we configured the LM358 Operational Amplifier as an active low-pass filter. The op-amp successfully attenuated high-frequency signals while allowing low-frequency signals to pass, demonstrating its utility in filtering applications.