 LDMOS devices modeling

Under construction...

This page contains some notes about LDMOS devices modeling; while the concepts discussed are general, the main focus is on obtaining models for low/medium power amplifiers for the HF/VHF amateur radio bands.
At first, the small-signal model extraction is discussed and a method is presented that allows to obtain an estimate of the main parameters. The values obtained can then be used as a starting point for an optimization procedure, in order to obtain a model which fits even better the measured data provided as input. Of course, better could have different meanings, so first a measure of the model quality has to be defined; as discussed in  several different choices are possible.
Some practical examples of commonly used RF LDMOS are also presented on their own pages (see menu on the left): these model were optimized using the S-parameters Error Vector Magnitude (EVM) as error measure .

LTspice is a very popular free SPICE simulator from Linear Technology; while not specifically aimed at RF circuits simulation it can of course be helpful also in that area. For some of the most popular devices a (large signal) LTspice model based on the VDMOS device model has also been extracted; while in general its quality is not exceptional, it can nevertheless be used for a first design evaluation.

Later on also the extraction of a large-signal model, based on  and , will be presented.

Small-signal model

The following figure shows a typical MOS equivalent circuit The Z-parameters of this equivalent circuits can be easily determined after some tedious calculations; the general idea is first to compute the Y parameters of the intrinsic part and then add the contributions of the extrinsec resistors and capacitors 

Assuming the Z-parameters are    where There are a number of different methods to determine parameters values of the small signal model circuit ("parameters extraction"); one of the most general is described in . The Z-parameters above are rewritten as a ratio of polynomials whose coefficient can be determined by fitting these rational functions to the measured Z-parameters; Note that some of the Z-parameters equations may need to be multiplied by so that all are non-zero and . An important feature of this method is that a least-square fit can be determined directly, without any iterative optimization needing an initial starting point close enough to the optimal values to converge properly.

For the circuit above, still assuming , the Z-parameters are rewritten as    note that the denominator of all the expressions is the same. In the equations above  so that To be continued...

Large-signal model

Under construction...

References:

  B. Grossman, E. Fledell and E. Acar, "Adventures in Measurement: A Signal Integrity Perspective to the Correlation Between Simulation and Measurement," IEEE Microwave Magazine, vol. 14, no. 3, pp. 94-102, May 2013.  L. Vestling, "Design and Modeling of High-Frequency LDMOS Transistors," Ph.D. Thesis, ISBN 91-554-5210-8, Uppsala University, Feb.2002.  G. Dambrine, A. Capy, F. Heliodore and E. Playez "A new method for determining the FET small-signal equivalent circuit," IEEE Trans. Microwave Theory and Techniques, vol. 36, no. 7, pp. 1151-1159, 1988.  S. Lee, H.K. Yu, C.S. Kim, J.G. Koo and K.S. Nam "A novel approach to extracting small-signal model parameters of silicon MOSFET's," IEEE Microwave and Guided Wave Letters, vol. 7, no. 3, pp. 75-77, 1997.  C. Fager, J. C. Pedro, N. B. de Carvalho and H. Zirath "Prediction of IMD in LDMOS transistor amplifiers using a new large-signal model," IEEE Trans. Microwave Theory and Techniques, vol. 50, no. 12, pp. 2834-2842, Dec. 2002.  S. Lai, C. Fager, D. Kuylenstierna and I. Angelov "LDMOS Modeling," IEEE Microwave Magazine, vol. 14, no. 1, pp. 108-116, Jan.-Feb. 2013.