Op amp ltspice


  • How do I add op amp to LTspice?
  • Build an Op Amp SPICE Model from Its Datasheet – Part 1
  • Simulating the ideal OP amp
  • Simulation of Ideal Operational Amplifier using LTspice.
  • LTspice Modeling of Universal Operational Amplifier
  • How do I add op amp to LTspice?

    Our second tutorial will focus on simulating an ideal op-amp, using the LTspice simulation tool. But what is an ideal op-amp? The figure below shows an ideal op-amp, the input impedance is denoted, as Rin and does not allow any current to flow into it.

    Ideally, the op-amp input impedance must be infinite, and the output impedance of op-amp denoted as Ro. The Ro ensures that the output voltage of the op-amp remains the same for any value of load resistance connected. Ideally, the output impedance of op-amp must be zero.

    When we look into the open-loop voltage gain of an ideal op-amp, it is infinite. Then the difference between the two inputs can be applied by an ideal op-amp to infinity. For an ideal op-amp, there is zero output voltage for zero input voltage then the input offset voltage is zero.

    An op-amp that starts to work as an amplifier at a particular frequency range is known as bandwidth. The bandwidth of an ideal op-amp is infinite, and it can amplify both DC and AC signals. The output voltage of op-amp changes immediately, with a change in the input voltage. Go to File, click on New Schematic.

    In this example, we will simulate our ideal op-amp using a voltage-controlled current source. Click on the component icon and select the voltage-dependent current source. The inverting input at the top and the non-inverting input at the bottom, in our voltage-dependent current source, and press escape to come out of placing node.

    Right-click on the voltage-controlled current source and enter the value of gain as 1e6. Next, click on the resistor and place it on the schematic, change the value of the resistor to 1 ohm, and select the ground symbol and place it on the schematic window. Connect the voltage-dependent current source and resistor through the wire, select the wire icon, and complete the wiring.

    In this example, we must define the port type as an input or output. Label the input as Vin placed at inverting input and then Vnin at the non-inverting input. The output terminal positioned at the resistor R1, click the wire and connect the output terminal with the node of resistor R1. Click on Netname and define the label as Vout and select the port type as output. Save the schematic, click on the file and select save as in this example saved as Opamptut.

    The difference across the input terminal across the voltage-controlled current source will generate a current that causes the output to flow through the 1-ohm resistor. In this example, we will convert this circuit into a symbol. How to create a symbol in LTspice? The next section will guide you on how to create a symbol in LTspice. Click on Hierarchy and then select create a new symbol you will get a new schematic.

    Save the symbol as Opamptut, this symbol will correspond to the schematic which we had done earlier. Now click on the draw and then click on Line. We will draw the Op-amp symbol and add the input and output pin to our symbol. Name the pin as, Vin for inverting input terminal and Vnin for non-inverting input terminal, and the output pin as Vout. Verify the pin table for our circuit, click on view and select pin table. Connect the pins to the Op-amp symbol through drawing lines, which corresponds to the schematic.

    Next, we simulate the Inverting configuration of an Op-amp. Open a new schematic. Use the symbol which we had created previously by selecting the component.

    Add a resistor R1 to the schematic, this resistor act as a load and ground the non-inverting terminal input and the voltage source to the inverting input.

    Also, add resistor R2 and R3, as shown in fig below. Let us change the value of the resistance value of R2 to ohms and R3 to ohms. The first source of sweep will be V1, the start value to be 0, and stop value as 1 with 1mv increment.

    Click on to the node between V1 and R2 the input of the inverting terminal and the output terminal. The figure below shows the DC sweep. Now let us make a few changes to our voltage source, right-click on voltage, and click on advanced. Define the stop time as 5us, time to start saving data 0, and maximum time stop 10ns.

    Label the input as Vin, and Output as Vout. Run the simulation. When we move the cursor on the circuit in the schematic, select the node on the inverting input and the output.

    We get the inverted output plot. LTspice also gives us the option of copying the entire circuit. For example, we need a non-inverting circuit. Click on a new schematic, go back to inverting circuit schematic and click on the Copy icon and left-click and drag until you cover the entire circuit. Save the schematic as a non-inverting circuit. Now we must make a few changes in your circuit, we will place the input voltage to the non-inverting input. To distinguish between the Vin, and Vout we can label those using the label net.

    End of tutorial.

    Build an Op Amp SPICE Model from Its Datasheet – Part 1

    Nastase Why do you need to build your own Op Amp model? Then why should you know how to build one? Well, not everything has a model and that is why, sometimes, you have to build your own. Also, it may be necessary to study a circuit to see what happens if you change the Op Amp slew rate or bandwidth, offset, and so on. Sometimes the manufacturer own model does not work, as a user found out and posted a question in this forum.

    I told him that the model has a bug and advised him to build his own. No matter the reason, building your own model is fun and rewarding and can only add to your overall understanding on how an Op Amp works. One note of caution. The model described here is a behavioral model.

    This means that the model will mimic the op amp functionality, but will not have any transistor or any other semiconductor SPICE models. We can build the model around it. It was developed by Electronics Workbench and now owned by National Instruments. Figure 1 Input and Output Resistance The input resistance can be defined in the datasheet of an op amp as a group of three resistors: Rin1, Rin2 and Rin. The ADA does not have a specification for the input resistance.

    As such, the input resistance is very high, up to ohms. Having these resistors in the model will not make too much of a difference, therefore the model will work very well with or without them for this particular op amp.

    However, other op amps, built with different technologies, will have lower input resistance values and they will make a difference in the SPICE model. The output resistance is not specified as well for this op amp. Because of that we will estimate a 20 ohm series resistor with the op amp output. Figure 2 The Input Capacitance As in the case with the input resistance, the input capacitance of an op amp can be specified as three input capacitors, Cin1, Cin2 and Cin.

    The ADA does not have these capacitances specified in the datasheet. Figure 3 The Offset Voltage If we power an op amp with a bipolar power supply and connect the inputs to ground, the output will not go to zero volts. It will likely rail up or down due to the offset voltage. This voltage is present in any op amp at its inputs. The output rails up because the op amp high gain, multiplied by the input offset voltage, makes a theoretical voltage of tens of volts, so the output will be limited by the available power supply level.

    To simulate this voltage, all we have to do is add a DC voltage source in series with one of the inputs, say the non-inverting input, as in Figure 4. Figure 4 This model can now be connected in a circuit. Figure 5 shows a non-inverting summing amplifier made with our ADA model. Figure 5 In the next article I will show you how to add the gain bandwidth product and how to add poles in this model.

    It was developed by Electronics Workbench and now owned by National Instruments. Figure 1 Input and Output Resistance The input resistance can be defined in the datasheet of an op amp as a group of three resistors: Rin1, Rin2 and Rin. The ADA does not have a specification for the input resistance. As such, the input resistance is very high, up to ohms. Having these resistors in the model will not make too much of a difference, therefore the model will work very well with or without them for this particular op amp.

    However, other op amps, built with different technologies, will have lower input resistance values and they will make a difference in the SPICE model. The output resistance is not specified as well for this op amp.

    Because of that we will estimate a 20 ohm series resistor with the op amp output. Figure 2 The Input Capacitance As in the case with the input resistance, the input capacitance of an op amp can be specified as three input capacitors, Cin1, Cin2 and Cin. The open-loop gain of many electronic amplifiers is exceedingly high by design — an ideal operational amplifier op-amp has infinite open-loop gain. What is closed loop gain in op amp?

    To achieve stable operation, op-amps are used with negative feedback. The gain of an op-amp without feedback is called the open-loop gain whereas the gain of an op-amp with a feedback circuit is called the closed-loop gain. The open-loop gain halves when frequency doubles. How do you control the gain of an op amp? Closing the open loop by connecting a resistive or reactive component between the output and one input terminal of the op-amp greatly reduces and controls this open-loop gain.

    Op-amps can be connected into two basic configurations, Inverting and Non-inverting. What happens if op amp is configured as a closed loop inverting amplifier? What happens if op-amp is configured as a closed loop inverting amplifier? What do you mean by virtual ground in op amp? A virtual ground is a node of a circuit that is at a steady reference potential, without being directly connected to the reference potential.

    It is a concept that made for easy explanation and calculation purpose as voltage is approximately zero. In an ideal Op-amp. What is the difference between real ground and virtual ground?

    Real ground is when a terminal is connected physically to the ground or earth. What is principle of virtual ground? In electronics, a virtual ground or virtual earth is a node of a circuit that is maintained at a steady reference potential, without being connected directly to the reference potential. What is virtual short condition? Virtual short is a circuit node with a steady voltage that is not directly connected to a voltage reference such as ground or a DC voltage source.

    The concept of virtual short is commonly used for op-amps. Figure 1 shows a negative feedback amplifier using an op-amp. What is the output of non-inverting amplifier?

    Simulating the ideal OP amp

    Ideally, the op-amp input impedance must be infinite, and the output impedance of op-amp denoted as Ro. The Ro ensures that the output voltage of the op-amp remains the same for any value of load resistance connected. Ideally, the output impedance of op-amp must be zero. When we look into the open-loop voltage gain of an ideal op-amp, it is infinite.

    Then the difference between the two inputs can be applied by an ideal op-amp to infinity. For an ideal op-amp, there is zero output voltage for zero input voltage then the input offset voltage is zero. An op-amp that starts to work as an amplifier at a particular frequency range is known as bandwidth.

    Simulation of Ideal Operational Amplifier using LTspice.

    The bandwidth of an ideal op-amp is infinite, and it can amplify both DC and AC signals. The output voltage of op-amp changes immediately, with a change in the input voltage. Go to File, click on New Schematic. In this example, we will simulate our ideal op-amp using a voltage-controlled current source.

    Click on the component icon and select the voltage-dependent current source. The inverting input at the top and the non-inverting input at the bottom, in our voltage-dependent current source, and press escape to come out of placing node. Right-click on the voltage-controlled current source and enter the value of gain as 1e6.

    LTspice Modeling of Universal Operational Amplifier

    Next, click on the resistor and place it on the schematic, change the value of the resistor to 1 ohm, and select the ground symbol and place it on the schematic window. Connect the voltage-dependent current source and resistor through the wire, select the wire icon, and complete the wiring.

    In this example, we must define the port type as an input or output. Label the input as Vin placed at inverting input and then Vnin at the non-inverting input. The output terminal positioned at the resistor R1, click the wire and connect the output terminal with the node of resistor R1.


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