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Design Notes

Small Signal MESFET Modeling with Noise

This project demonstrates the use of the SMODEL measurement and the optimizer to fit a generic Small-Signal FET model to measured s-parameter and noise data of a MESFET.

Overview

The “MESFET” schematic shows the general topology of the small signal FET which includes noise currents and resistors. Alternatively, the Pospieszalski noise model (FETN) could be used; however, this schematic representation offers more flexibility in that the user can modify it quite easily.

A set of S-Parameters for the device was imported into the project thereby allowing us to optimize the model to fit the data. Several parameters of the model were made tunable and optimizeable within a specified range. The SMODEL measurement is used to measure the error between the s-parameter data to the model and an optimization goal was defined to minimize the error between the two.

Measurements

Comparative s-parameter measurements are presented in the “MESFET” schematic. Additional plots include Noise Resistance (Rn) and Minimum Noise Figure (NFMin). The difference between the measured and modeled s-parameters is plotted in the "S_Parameter Error between Model and Data" graph. An optimization goal is defined such that this error is minimized (closer to zero) as the accuracy of the model parameters is improved.

Implementation

The project is saved in an un-optimized state and includes the "MESFET_Model_Solution" schematic as a solution circuit. Simply optimize the circuit (Simulate > Optimize > Start) and observe the reduction on the error measured by SMODEL. Also there is improved agreement between measurement and model of the s-parameter and noise parameter plots.

Schematic - MESFET

Graph - MESFET S11_S22 Model vs Data

Graph - S_Parameter Error Between Model and Data

Graph - MESFET S21 Magnitude Model vs Data