Op-Amp Series – Part 1: Operational Amplifier Basics

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Operational Amplifier Basics

Operational amplifiers (“op-amps”) are one of the most important building blocks in electronics. They're used everywhere from audio circuits and sensors to filters, oscillators, and control systems.

Before building more advanced circuits, we need a solid grounding in how an op-amp works and how to wire its simplest configuration: the voltage follower. The final setup can be seen in the picture below.

Voltage Follower

In this post you’ll learn:

  • What an op-amp is
  • The Golden Rules of op-amps
  • Why negative feedback stabilises a circuit
  • How to build a voltage follower using an LM358
  • How to generate a safe test signal using only a voltage divider
  • How to verify the behaviour using a multimeter or oscilloscope

This is the foundation for everything related to operational amplifiers.

What Is an Operational Amplifier?

An operational amplifier is a high-gain differential amplifier with:

  • Two inputs

    • Non-inverting (+)

    • Inverting (–)

  • One output

  • Very large internal gain (often 100,000× or more)

Its output is determined by the difference between the inputs:

Vout=AOL(V+−V−)V_\text{out} = A_{OL}(V_+ - V_-)

Because the open-loop gain is so large, even a tiny input difference would slam the output into a supply rail.

That’s why almost all practical op-amp circuits use negative feedback, which stabilises the behaviour and sets a predictable gain.

The Golden Rules of Op-Amps

These are the golden rules or characteristics of op-amp behaviour.

1) Infinite Open-Loop Gain (Ideally)

The op-amp tries to amplify the difference between its inputs by an enormous factor.
Real op-amps typically have a gain of 20,000 to 200,000.

This is what allows negative feedback to “force” the inputs to match.

2) No Current Flows Into Either Input

The input impedance is (ideally) infinite:

I+=I−=0I_+ = I_- = 0

Meaning:

  • The op-amp does not load the previous stage
  • Input current is negligible

This rule is used when deriving gain formulas.

3) With Negative Feedback, the Input Voltages Become Equal

When negative feedback is present and the op-amp is not saturated:

V+=V−V_+ = V_-

For example:

  • If the non-inverting input is at 2.5 V, the inverting input will also be at 2.5 V
  • In a follower, the output moves until both inputs match exactly

This is the core principle behind all linear op-amp circuits.

Meet the LM358 – Perfect for Beginners

The LM358 is ideal for single-supply experiments:

  • Works on 5–30 V
  • Inputs can go down to ground
  • Output can go very close to ground
  • Very stable and forgiving

Important limitations:

  • Inputs cannot sense voltages near the positive rail
  • Outputs cannot reach the positive rail (typically VCC – 1.2 V)

As long as your input stays below the top of the valid range, the LM358 behaves very predictably.

Building the First Circuit

The voltage follower is the simplest op-amp configuration and a perfect first experiment.

It has:

  • Unity gain (gain = 1)
  • Very high input impedance
  • Low output impedance
  • Output voltage matches input voltage exactly

It’s used to buffer weak or high-impedance signals.

Parts Required

  • Breadboard
  • LM358
  • 12V bench supply

  • Two resistors (for the voltage divider)

    Any values where R1 = R2
  • Example: 10 kΩ + 10 kΩ
  • Multimeter (or oscilloscope)
  • A few jumper wires

Creating the Input Signal Using a Voltage Divider

Instead of a function generator, we’ll make a simple input voltage using two resistors:

Vin=R2R1+R2×VCCV_\text{in} = \frac{R2}{R1 + R2} \times V_{CC}

Vin=6 VV_\text{in} = 6 \text{ V}

This stays safely within the LM358’s input common-mode range.

Circuit Diagram

Voltage Divider Wiring

  • One end of R1 → +12 V
  • Other end of R1 → junction node (this is your input)
  • One end of R2 → junction node
  • Other end of R2 → GND

The junction between R1 and R2 becomes Vin.

LM358 Voltage Follower Wiring

Using op-amp A of the LM358:

  • Pin 8 → +12 V
  • Pin 4 → GND
  • Pin 3 (non-inverting +) → Vin (voltage divider output)
  • Pin 1 (output) → Vout
  • Pin 2 (inverting –) → Pin 1 (feedback connection)

That single feedback wire is what makes a voltage follower.

Circuit wiring

Understanding Why the Voltage Follower Works

Apply the Golden Rules:

Rule 3 — With negative feedback, the input voltages are equal

V+=V−V_+ = V_-

But:

  • V+=VinV_+ = V_\text{in}

  • V−=VoutV_- = V_\text{out}

So:

Vout=VinV_\text{out} = V_\text{in}

Rule 2 ensures:

No current flows into either input → your divider is not loaded.

Rule 1 explains why:

The huge internal gain forces the output to whatever value makes

V+−V−=0V_+ - V_- = 0

This is the essence of op-amp behaviour.

Measuring the Behaviour

You can verify the voltage follower using:

Multimeter

  • Measure the divider node: ≈ 6.00 V
  • Measure the op-amp output: ≈ 6.00 V

They should match within a few millivolts.

Measuring the output voltage

Measuring the input voltage

Oscilloscope

  • Connect CH1 → Vin
  • Connect CH2 → Vout
  • The traces should overlap perfectly

No phase shift, no delay, no distortion.

Oscilloscope Readings

Oscilloscope connections to input and output

What You Learned in Part 1

You now understand:

  • What an op-amp is
  • The Golden Rules
  • Why negative feedback forces stable behaviour
  • How to build a voltage follower using an LM358
  • How to generate a test input using simple resistors
  • How to measure and verify the circuit

This is the essential foundation for all linear op-amp circuits.

Next Up: Part 2 — The Non-Inverting Amplifier

In the next post, we’ll move to the non-inverting amplifier, where we introduce controlled gain and your first real op-amp calculation.

Hope you have enjoyed this post and visit again.
Thanks for reading,
Matty






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