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Parameterize an Op-Amp from a Datasheet

The Triangle Wave Generator example model, also described in Build and Simulate a Simple Circuit, contains two op-amps, parameterized based on a datasheet for an LM7301. The National Semiconductor datasheet gives the following data for this device:

Gain97dB = 7.1e4
Input resistance39MΩ
Slew rate1.25V/μs

The Band-Limited Op-Amp and Finite-Gain Op-Amp blocks have been designed to work from manufacturer datasheets. Implementing detailed op-amp device models, derived from manufacturer SPICE netlist models, is not recommended, because it provides more accuracy than is typically warranted and slows down simulations. The simple parameterization of the Simscape™ Electrical™ op-amp blocks allows you to determine the sensitivity of your circuit to abstracted performance values, such as maximum slew rate and bandwidth. Because of this behavior-based parameterization, you can determine which specification of op-amp is required for a given application. A circuit designer can later match these behavioral parameters, determined from the model, against specific op-amp devices.

Based on the datasheet values above, set the Band-Limited Op-Amp block parameters as follows:

  • Gain set to 7.1e4

  • Input resistance, Rin set to 39e6Ω

  • Output resistance, Rout set to zero. The value is not defined, but will be small compared to the 1000Ω load seen by the op-amp.

  • Minimum output, Vmin set to the negative supply voltage, -20V in this model

  • Maximum output, Vmax set to the positive supply voltage, 20V in this model

  • Maximum slew rate, Vdot set to 1.25/1e-6 V/s

  • Bandwidth, f set to 4e6 Hz

Note that these parameters correspond to the values for +-5 volt operation. The datasheet also gives values for +-2.2V and +-30V operation. It is usually better to pick values for a supply voltage below what your circuit uses, because performance is worse at lower voltages; for example, the gain is less, and the input impedance is less. You can use the variation in op-amp parameters with supply voltage to suggest a typical range of parameter values for which you should check the operation of your circuit.