Op-Amp Application Circuits

Key Takeaways

• The op-amp differential amplifier features low output resistance, high input resistance, and high open loop gain.

• In an inverting amplifier configuration, the op-amp circuit output gain is negative.

• All simple mathematical operations such as addition, subtraction, comparison, etc. are possible with op-amp application circuits.

In the computing field, operational amplifiers, or op-amps, represent amplifiers with the capability to perform mathematical operations. It is possible to perform math operations using various op-amp application circuits. Signals can be added, subtracted, amplitude-scaled, compared, etc. In this article, we will dive into what an op-amp is and look at a few of its application circuits.

An Overview of Op-Amps

In analog electronics, op-amps are frequently used components. They form one of the fundamental elements in analog electronic circuits. Op-amp application circuits can be found in signal conditioning systems, filtering applications, and wherever a mathematical operation is to be performed on the signals in an analog way.

If there are two inputs to an op-amp, say signal A and signal B, signal A is given to the non-inverting input terminal of the op-amp and signal B is given to the inverting terminal. The difference (signal A-signal B) can be amplified using an op-amp circuit. 

Op-Amp ICs

Op-amps are available as integrated circuit packages. Some examples are LMV 358, LMV 341, TSV 522, etc. As shown in the figure below, most op-amps consist of 5 terminals:

1. Positive side power supply terminal (V+)
2. Negative side power supply terminal (V-)
3. Non-inverting input terminal
4. Inverting input terminal
5. Output terminal

Stable Op-Amp Circuits

Open loop operation of op-amps is restricted, as it leads to instability. In an open loop configuration, it is difficult to control the input signals of the op-amp. The closed-loop operation of op-amps is preferred. The closed-loop operation can be performed in two configurations.

Configurations of Closed-Loop Op-Amp Operations

Op-Amp Application Circuits

As an example, let’s use a circuit that I built. An alternating voltage needed to be sensed and there was some control technique to be developed based on the sensed voltage. The control was implemented using an FPGA, so the sensed voltage needed to be given to the ADC of the controller. However, as FPGA only takes positive values, the mathematical operation of ‘summing’ needed to be performed on the signal to make it entirely positive. I used an op-amp-based summing amplifier, which added the voltages so all the negative values of sensed voltage turned positive. After passing through the op-amp circuit, the sensed voltage reached the ADC of the FPGA.

Summing Amplifier

The circuit above shows a summing amplifier based on an inverting amplifier configuration, as the three inputs are connected to the inverting input terminal through three resistors. Let’s assume the resistors R_in are equal. The output voltage V _out is proportional to the sum of the voltages  V₁ +  V₂ + V₃.

Op-amp summing amplifier output equation

As the input resistors are equal, the sum of given inputs is obtained in the output with the gain  . If it is required to obtain a weighted sum of the input signals, the appropriate resistor values can be connected to the corresponding input signals.

Similar to the above example and the corresponding circuit, there are numerous application circuits available in analog electronics. We will look at a few of them in the upcoming section.

Op-Amp Integration Circuits

All simple mathematical operations such as addition, subtraction, comparison, etc. are possible with op-amp application circuits. But, what if we need to integrate a signal? For integration, there is one circuit using an op-amp.

By replacing the resistive feedback in the inverting amplifier with a capacitor, the integration amplifier circuit can be constructed, as illustrated in the circuit diagram above. The integration amplifier circuit using op-amps gives an output that is proportional to the input voltage given with respect to time. In an integration circuit, the capacitor charges and discharges in response to the changes in the input voltage and produces the output.

Op-Amp Application Areas

Op-amps are versatile devices that can perform math operations not limited to summing and integration. Op-amp application circuits are extensively found in systems requiring filtering, signal conditioning, analog computation, analog instrumentation, etc.

Cadence offers simulation, analysis, and modeling tools for building analog circuits, including operational amplifiers. With Cadence’s OrCAD, it is possible to choose models of components, including op-amps, from the library and accurately design the system by assessing the model parameters. 

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