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Design Notes

EM Optimization of a Power Amplifier Output Matching Network

This project shows the output matching network of a power amplifier, and demonstrates how EM optimization can be used to ensure the desired matching impedance at the operating frequency and its harmonics.

Overview

Power amplifier output matching networks are difficult to simulate accurately with standard circuit element models, because they tend to be large, and include closely spaced discontinuities.  Such structures support non-TEM mode waves, particularly at higher frequencies such as harmonics of the amplifiers operating frequency.  Matching at harmonics can significantly affect amplifier performance, so EM simulation should be used to ensure accurate modeling of the network at higher frequencies.

This example focuses only on the EM optimization task: to make the EM simulation results match the desired impedance values at the output fundamental, 2nd, and 3rd order harmonics.

The circuit element model parameters were already optimized to achieve the desired matching impedance at the fundamental, 2nd, and 3rd order harmonics of the output.  Then the network was set up for extraction, so that it can be simulated and optimized using Axiem.

To start the optimization, click the Start button in the Optimizer window.  Depending on computing power, the optimization may take as long as 2 minutes or more.  The Discrete Local Search optimizer is ideal for this type of problem as no points are ever simulated twice.  Additionally, using this optimizer eliminates the need for discrete variables to set the parameter values.  This exercise is left to the user. 

Desired_Gamma

In this schematic, the desired gamma values at the fundamental, 2nd, and 3rd order harmonics are entered into a properly terminated HBTUNER element as parameters.  These values may be supplied by the manufacturer of the transistor, or may be arrived at by using the HBTUNER or HBTUNER2 element, and simulated load-pull analyses.

Simulated_Gamma

This schematic is the output matching network.  The circuit element model parameters were already optimized to achieve the desired matching impedance at the fundamental, 2nd, and 3rd order harmonics of the output.  Then the network was set up for extraction, so that it can be simulated and optimized using Axiem. 

The equations in this schematic have been set up to keep the layout geometries on the same 20mil grid as the extracted EM structure.  The “res” variable sets the resolution, and the “grid” variable is an array (or vector) of numbers, from 0 to 1.2” in 20mil increments.  The dimension variables all point to specific indices of “grid”, so they can only take on values that are in that vector.  Finally, “res” is also used to specify the X_Cell_Size and Y_Cell_Size parameters in the EXTRACT block, which sets up the EM structure.

You can disable the EXTRACT block and simulate to see the results using the models, and re-enable it to see the results EM simulation.  The results match closely at the 2GHz fundamental, but differ at the harmonics.

Difference Between Simulated and Desired_Gamma

This graph uses the SMODEL measurement to show the difference between the s11 of the “Simulated_Gamma” and “Desired_Gamma” schematics.  Although it is displayed in dB, this is the difference between the complex values, not just the magnitudes.  The optimization goal is to make the difference lower than -50dB.

Match at Harmonics

Phase and Magnitude

These graphs show the gamma of the two schematics for comparison, on a Smith chart and a rectangular graph, respectively.

Schematic - Simulated_Gamma

Schematic - Desired_Gamma

EM Structure - Output_Extract_Doc

Graph - Match at Harmonics

Graph - Phase and Magnitude