# Operational Amplifier Equations For Circuit Design

Follow article### Introduction

An operational amplifier (op-amp) is an analogue circuit block that takes a differential voltage input and produces a single-ended voltage output. How to solve op-amp circuits? Look at the following op-amp equations by several main operational amplifier applications circuit.

An operational amplifier has two input pins and one output pin. Its applications in a host of different circuits where their attributes of gain, input impedance, output impedance, differential input and so on. The major op-amp formulas used to calculate are shown as follow:

*Fig 1. Voltage Follower*

Note: Buffer High Impedance Signal and Low Impedance Load

*Fig 2. In-phase Op-Amp*

Note: In-phase Signal Amplification

*Fig 3. Reversed-phase Op-Amp*

Note: Amplify and Invert Input

*Fig 4. Voltage Subtractor, Differential Amplifier*

Note: Amplify the voltage difference and suppress the common-mode voltage

*Fig 5. Voltage Adder*

Note: Summation of Adding Voltage Values

*Fig 6. Low-pass Filter, Integrator*

Note: Limit Signal Bandwidth

*Fig 7. High-pass Filter, Differentiator*

Note: Eliminate DC, Amplify AC Signal

*Fig 8. Differential Amplifier*

Note: Drive Differential Signal to Analog-to-Digital Converter From A Differential or Single-ended Signal Source

*Fig 9. Instrumentation Amplifier*

Note: Amplify the Low-Level Difference Signal and Suppress the Common Mode Signal

*Fig 10. Single-State Op-Amp Noise*

Note:

RTO NOISE=NG×RTI NOISE

RTI=Converted to the Input

RTO=Converted to the Output

*Decibel Formula (equivalent impedance)*

*Johnson-Nyquist Noise Formula*

*Ohm's Law (DC circuit)*

*Fig 11. Closed-loop Frequency Response (voltage feedback amplifier)*

*Resistance Formulas*

*Reactance Formulas*

*Impedance Formulas (in series)*

Note:

RL in series

RC in series

LC in series

RLC in series

*Voltage and Impedance Formulas (parallel connection)*

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