Op-Amp Inverting Amplifier Circuit Design & Calculator Guide

1. OP AMP Resistor Calculator Overview

The OP AMP Resistor Calculator is a professional design tool used to determine the correct resistor values (R2, R3, and R4) for an inverting operational amplifier circuit.

To use this tool effectively, you typically need to define the following input parameters:

• Gain: The desired amplification factor.
• R1: The input resistance value (typically in $k\Omega$).
• Voltages: $V_{out}$ (Output Voltage), $V_p$ (Positive Power Supply), $V_n$ (Negative Power Supply), etc.

This tool simplifies the design process by automatically computing the feedback and stabilization resistors needed to achieve specific gain and bandwidth targets.

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2. What is an Inverting Op-Amp?

The Inverting Amplifier is one of the most fundamental and widely used operational amplifier circuits. It is simple to construct, requiring only a few discrete components.

Key Characteristic: As the name suggests, the output voltage signal is inverted relative to the input voltage signal. This means there is a $180^{\circ}$ phase shift between input and output.

• If the input is positive, the output is negative.
• If the input is negative, the output is positive.

Formula

The voltage gain ($A_v$) of an inverting amplifier is determined by the ratio of the feedback resistor ($R_f$) to the input resistor ($R_{in}$):

$A_v = - \frac{R_f}{R_{in}}$

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3. Inverting vs. Non-Inverting Op-Amp

Below is a visual comparison of the two most common amplifier configurations.

3.1 Inverting Amplifier Configuration

How it works: In this configuration, the Non-Inverting Input (+) is connected directly to the ground. The input signal is applied to the Inverting Input (-) through a resistor ($R1$).

The Concept of "Virtual Ground":

• Because the open-loop gain of an ideal op-amp is infinite, the voltage difference between the two input terminals ($V_+$ and $V_-$) is assumed to be zero.
• Since the Non-Inverting terminal is grounded ($0V$), the Inverting terminal is also maintained at virtually $0V$. This is known as a Virtual Ground.
• This feature makes the analysis of the circuit straightforward, as the current flowing through the input resistor must equal the current flowing through the feedback resistor (since no current flows into the op-amp inputs).

3.2 Non-Inverting Amplifier Configuration

How it works: In a Non-Inverting Amplifier, the input signal is applied directly to the Non-Inverting Input (+). The feedback comes from the output to the Inverting Input (-).

Key Differences:

1. Phase: The output is in phase with the input ($0^{\circ}$ shift).
2. Input Impedance: This configuration offers very high input impedance because the signal goes directly into the op-amp gate, making it ideal for interfacing with high-impedance sensors.
3. Gain Formula: $A_v = 1 + \frac{R_f}{R_{in}}$ Unlike the inverting configuration, the gain here is always greater than or equal to 1.

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4. Related Calculation Tools

Here are other essential calculators often used alongside Op-Amp design:

• LED Series Resistor Calculator: Used to calculate the necessary resistance value to protect LEDs in a series circuit.
• Resistor Color Code Calculator: Quickly identifies the tolerance and resistance values for 4, 5, and 6-band through-hole resistors.
• SMD Resistor Code Calculator: Decodes the 3 or 4-digit markings found on Surface Mount Device (SMD) resistors.
• Parallel and Series Resistor Calculator: Computes the total equivalent resistance for complex resistor networks connected in series or parallel.