Master the Basics: Operational Amplifier Characteristics

Operational amplifiers (Op-Amps) are essential components in modern electronics. Whether you’re amplifying signals, building filters, or designing analog computing circuits, understanding op-amp characteristics is key to creating reliable, effective systems. This post breaks down the basic traits of a popular and accessible op-amp—the LM358—to help you get started.

Why the LM358?

The LM358 is a go-to op-amp for hobbyists. It's low-cost, easy to use, and great for low-power applications. It contains two op-amps in a single 8-pin package, making it perfect for breadboarding and small projects.

Power Supply Versatility

One of the LM358’s best features is its flexible power supply options. You can use it with:

• Single supply mode: 3V to 32V
• Dual supply mode: ±1.5V to ±16V

This makes it adaptable for both battery-powered and wall-powered applications.

High Gain Amplification

The LM358 boasts an open-loop voltage gain of 100 dB, which means it can amplify even a tiny 100 μV difference between input pins into a full 10V output (assuming sufficient supply voltage). However, keep in mind that real-world op-amps can't output beyond their supply rails.

Frequency vs Gain

As input signal frequency increases, gain decreases. For the LM358:

• Gain starts dropping after DC (0 Hz)
• At around 0.7 MHz, the op-amp provides unity gain (gain = 1)

This limit is called the unity gain bandwidth, and it tells you how fast your input signal can be while still being amplified effectively.

Package & Pinout

In the 8-pin PDIP package:

• Pins 1–3: Op-Amp 1 (Output, Inverting, Non-inverting)
• Pins 5–7: Op-Amp 2
• Pin 4: Ground (or negative rail)
• Pin 8: VCC (positive rail)

Always refer to the datasheet for accurate pin configuration and connection.

Electrical Characteristics

Input Offset Voltage: Typically 3 mV. This means even with identical voltages at both inputs, you might see a slight difference internally. In circuits with high gain, this can become noticeable.

Input Bias Current: Around 20 nA. This is the small current that flows into the input terminals to keep the internal transistors operating.

Output Current: The LM358 can source or sink limited current. While not ideal for driving high-power loads directly, it works great for signal processing or buffering.

Practical Tips

• Don’t forget the power rails, even if your schematic doesn’t show them.
• The LM358 does not have offset null pins, so you can’t manually tune out the offset voltage.
• It’s not rail-to-rail, meaning the output won't reach exactly 0V or VCC. Keep this in mind for designs that need full-range output.

Key Takeways

The LM358 may not be the most powerful op-amp on the market, but it’s one of the most accessible and forgiving for new designers. Its versatility, simplicity, and dual op-amp package make it a fantastic starting point for any electronics project. As you grow more confident, you can explore specialized op-amps with lower offset, higher bandwidth, and rail-to-rail output.