Op-Amp

Ideal operational amplifier

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Libraries:
Simscape / Foundation Library / Electrical / Electrical Elements

Description

The Op-Amp block models an ideal operational amplifier (op-amp). The figure shows the implementation schematic.

Implementation schematic of the Op-Amp block

The block implementation is based on the following assumptions:

The ideal op-amp gain A is assumed to be infinite
Then, for finite output, must have V1 = 0
Ideal op-amp also implies current from p to n is zero (i1 = 0)

These assumptions result in the following equations for the block:

equations
    v1 == p.v - n.v;
    v1 == 0;
    i1 == 0;
end

For more information, click the Source code link in the Description tab of the block dialog box.

The block has no parameters. However, you can initialize the Current into output node variable prior to simulation. For more information, see Block-Level Variable Initialization.

Variables

To set the priority and initial target values for the block variables prior to simulation, use the Initial Targets section in the block dialog box or Property Inspector. For more information, see Set Priority and Initial Target for Block Variables.

Nominal values provide a way to specify the expected magnitude of a variable in a model. Using system scaling based on nominal values increases the simulation robustness. Nominal values can come from different sources, one of which is the Nominal Values section in the block dialog box or Property Inspector. For more information, see Modify Nominal Values for a Block Variable.

Examples

Band-Limited Op-Amp

How higher fidelity or more detailed component models can be built from the Foundation library blocks. The model implements a band-limited op-amp. It includes a first-order dynamic from inputs to outputs, and gives much faster simulation than if using a device-level equivalent circuit, which would normally include multiple transistors. This model also includes the effects of input and output impedance (Rin and Rout in the circuit), but does not include nonlinear effects such as slew-rate limiting.

Open Model

Finite-Gain Op-Amp

How higher fidelity or more detailed component models can be built from the Foundation library blocks. The Op-Amp block in the Foundation library models the ideal case whereby the gain is infinite, input impedance infinite, and output impedance zero. The Finite Gain Op-Amp block in this example has an open-loop gain of 1e5, input resistance of 100K ohms and output resistance of 10 ohms. As a result, the gain for this amplifier circuit is slightly lower than the gain that can be analytically calculated if the op-amp gain is assumed to be infinite.

Open Model

Op-Amp Circuit - Differentiator

A differentiator, such as might be used as part of a PID controller. It also illustrates how numerical simulation issues can arise in some idealized circuits. The model runs with the capacitor series parasitic resistance set to its default value of 1e-6 Ohms. Setting it to zero results in a warning and a very slow simulation. See the User's Guide for further information.

Open Model

Op-Amp Circuit - Inverting Amplifier

A standard inverting op-amp circuit. The gain is given by -R2/R1, and with the values set to R1=1K Ohm and R2=10K Ohm, the 0.1V peak-to-peak input voltage is amplified to 1V peak-to-peak. As the Op-Amp block implements an ideal (i.e. infinite gain) device, this gain is achieved regardless of output load.

Open Model

Op-Amp Circuit - Noninverting Amplifier

A noninverting op-amp circuit. The gain is given by 1+R2/R1, and with the values set to R1=1K Ohm and R2=10K Ohm, the 0.1V peak-to-peak input voltage is amplified to 1.1V peak-to-peak. As the Op-Amp block implements an ideal (i.e. infinite gain) device, this gain is achieved regardless of output load.

Open Model

Ports

Conserving

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+ — Positive terminal
electrical

Electrical conserving port associated with the op-amp positive terminal.

- — Negative terminal
electrical

Electrical conserving port associated with the op-amp negative terminal.

out — Output terminal
electrical

Electrical conserving port associated with the output node of the op-amp.

The port name is not visible in the block icon, but you can see this name in the underlying source file by clicking the Source code link in the Description tab of the block dialog box.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2007a