Understanding Op-Amp Circuits: The 5 Configurations You Must Know

Operational amplifiers (op-amps) are the building blocks of analogue electronics. If you are studying electrical or electronic engineering at university, op-amp circuits will appear in multiple modules across your degree — from first-year introductions to advanced filter design and control systems.

As an analogue electronics tutor with 9+ years of experience, I find that students who master these five fundamental configurations can tackle virtually any op-amp problem that appears in exams. Let me walk you through each one.

The Ideal Op-Amp: Two Rules That Solve Everything

Before diving into specific circuits, you must internalise the two golden rules of an ideal op-amp (with negative feedback):

1. No current flows into the input terminals (infinite input impedance)
2. The voltage at both input terminals is equal (the op-amp adjusts its output to make this true)

With these two rules and Kirchhoff's current law, you can derive the gain and behaviour of any op-amp circuit. This is the approach examiners expect — and the one I teach all my students.

1. Inverting Amplifier

The inverting amplifier takes an input signal and produces an output that is amplified and phase-inverted (flipped). The gain is determined by the ratio of two resistors:

Gain = -R_f / R_in

The negative sign indicates phase inversion. This configuration is the most commonly examined op-amp circuit. Key points to remember:

• The input is applied to the inverting (−) terminal through R_in
• The non-inverting (+) terminal is connected to ground
• Feedback resistor R_f connects the output back to the inverting input
• Input impedance equals R_in (important for signal source loading)

2. Non-Inverting Amplifier

The non-inverting amplifier preserves the phase of the input signal while amplifying it:

Gain = 1 + R_f / R_in

Note that the minimum gain is 1 (when R_f = 0 or R_in = infinity), which is the special case known as a voltage follower or buffer. The non-inverting configuration offers very high input impedance, making it ideal when you do not want to load the source.

3. Differential Amplifier

The differential amplifier amplifies the difference between two input signals while rejecting any signal common to both inputs (common-mode rejection). This is crucial in instrumentation and sensor applications where noise affects both inputs equally.

With matched resistor ratios, the output is proportional to (V₂ - V₁), effectively subtracting one signal from another. This configuration is the foundation of instrumentation amplifiers used in biomedical and industrial measurement systems.

4. Integrator

Replace the feedback resistor with a capacitor, and the inverting amplifier becomes an integrator — the output is proportional to the integral of the input signal over time. This circuit is essential in:

• Analogue computing (solving differential equations)
• Waveform generation (converting square waves to triangular waves)
• PID controllers (the I component)
• Active filter design (second-order sections)

In practice, a large resistor is placed in parallel with the feedback capacitor to prevent DC drift — a detail that examiners love to test.

5. Comparator

A comparator operates without feedback (open-loop). It compares two voltages and outputs either the positive or negative supply rail depending on which input is larger. Comparators are used in:

• Analogue-to-digital conversion
• Level detection and threshold circuits
• Square wave generation from sinusoidal inputs
• Window comparators for range detection

Adding positive feedback creates a Schmitt trigger with hysteresis, which prevents noisy signals from causing rapid output switching. This is a very common exam question.

Common Exam Mistakes to Avoid

1. Forgetting the virtual ground. In an inverting amplifier, the inverting input is at 0V (virtual ground) — not actually connected to ground.
2. Confusing open-loop and closed-loop. The ideal op-amp rules only apply with negative feedback. Without feedback, the op-amp saturates.
3. Ignoring real-world limitations. At higher years, examiners test concepts like slew rate, bandwidth (gain-bandwidth product), input offset voltage, and CMRR.
4. Not labelling currents. Always draw and label current directions when analysing circuits. This prevents sign errors.

Need Help with Analogue Electronics?

Op-amp circuits are just one part of the analogue electronics syllabus. If you need help with transistor biasing, feedback analysis, oscillator design, or active filters, I offer specialist one-to-one online tuition. With a PhD in Electronic Engineering and years of tutoring experience, I can help you build the intuition and problem-solving skills needed to excel.