Here is a simplified model of the FET:


To create this simplified model, we have made a unilateral assumption, and set the scattering parameter S12=0, to remove any internal feedback path. This approximation is reasonable so long as S12 is small.

When you create a model of a two-port device using OptiMatch, it's treated as two separate network optimizations. First you optimize the input, and fix RI, L1, and CI, and then you optimize the output and fix Ro, Co, and Lo.

The source file used for the optimization of the input network contains the complex conjugate of S11, the input reflection coefficient, expressed in impedance form, while the load file provides a ground at each frequency.

The source file used for the output network optimization contains the complex conjugate of S22, the output reflection coefficient, expressed as an impedance. The load file can be created in two ways. First, you can connect an open, (a high resistance), in shunt with Ro, or second, you could consider Ro as a series element and ground it.

The required translation of reflection coefficient to impedance is easily performed using our Utilities+ software.

The s-parameters of the NE-38806 FET device are given below. They are followed by the corresponding conjugate in/out impedances.

NE-38806 FET Scattering Parameters

F(MHz)	S11	S21	S12	S22
4000	0.78 / -93	3.03 / 79	0.058 / 28	0.55 / -63
5000	0.71 / -118	2.80 / 56	0.054 / 14	0.52 / -82
6000	0.68 / -141	2.51 / 38	0.046 / 14	0.56 / -100

NE-38806 FET Complex Conjugate In/Out Impedances

F(MHz)	Zin*	Zout*
4000	11.586 +j 46.089	43.425 +j 61.020
5000	11.422 +j 28.879	32.408 +j 45.746
6000	10.670 +j 16.986	22.757 +j 36.569

We have provided four .IMP files for this example. Two for the input, and two for the output.

The files "SOURCE1" and "LOAD1" contain Zin* and a ground for the RLC input network. The files "SOURCE2" and "LOAD2" contain Zout*, (read it as ' Zout conjugate), and a shunt open for the output RC and L network.

If you have any doubt as to what is in these files, it would be a good idea to use the File Utility and list them out for inspection.

Let's optimize the input network first. Run OptiMatch and use Z0=11 ohms, (the approximate value of Re Zin). Choose "C," for a complex match, and then input the files "SOURCE1" and "LOAD1." Specify the three RLC elements in turn. Use start values of 10 ohms, 0.8 nH, and 0.8 pF. Do a constrained optimization. Restart the optimizer after it stops the first time, (do you remember how?, and continue until it stops after the 15th iteration.

If you entered the elements in RLC order, as we did, using element codes 3, 1, and 2, you should see the following:

OptiMatch online on 04-14-2005 @ 09:59:11     Source Filename  :  SOURCE1     Load Filename      :  LOAD1     Circuit Optimization with 3 variables and Z0=11 ohms     Constrained Optimization     Initial Analysis           VSWR( 1 ) = 4.117051           VSWR( 2 ) = 3.503351           VSWR( 3 ) = 3.556839     I       VAR            GRAD     1    10.000000    -2.884625E+01     2      0.800000    +4.245807E+01     3      0.800000    +5.935908E+02     ITN = 0     ERR F= 39.40965     ITN = 1     ERR F= 2.69107     ITN = 2     ERR F= 1.15128     ITN = 3     ERR F= 1.13142     ITN = 4     ERR F= 1.12028     ITN = 5     ERR F= 0.51161        .   .   .        .   .   .        .   .   .     ITN = 15                                            ITN = 15     ERR F= 0.27848                             ERR F (Start)  :  39.40965                                                                ERR F (Now)  :  0.27848     FunctionTermination                   Change            :  -14052 %     Final Analysis           VSWR( 1 ) = 1.034176           VSWR( 2 ) = 1.039928           VSWR( 3 ) = 1.066324     I       VAR            GRAD     1    11.225621   +7.874493E-03     2      0.680748   +2.213402E-02     3      0.62891     +3.965636E-02             Auto or Expert mode (A/E) [<Enter>=Quit] ? _

Press <Enter> to Quit.

Now let's optimize the output network. Run OptiMatch and use Z0=150 ohms, (the approximate value of Re Zout in parallel form). Choose "C," for a complex match, and then enter the file names "SOURCE2" and "LOAD2." Specify the three elements in turn. Use start values of 150 ohms, 0.6 nH, and 0.6 pF. Do a constrained optimization. Be careful about the order in which you enter the elements this time! The "LOAD2" file contains an "open" which must shunt Ro and Co; therefore, you should enter the elements in RCL order, this time, using element codes 6, 5, and 1.

The following is what you should see in Auto mode:

OptiMatch online on 04-14-2005 @ 10:14:21     Source Filename  :  SOURCE2     Load Filename      :  LOAD2     Circuit Optimization with 3 variables and Z0=150 ohms     Constrained Optimization     Initial Analysis           VSWR( 1 ) = 2.300164           VSWR( 2 ) = 2.642153           VSWR( 3 ) = 3.385585     I       VAR                 GRAD     1    150.000000    +7.674335E+00     2         0.600000    +2.534346E+01     3         0.600000    +1.501528E+02     ITN = 0     ERR F= 20.80125     ITN = 1     ERR F= 3.89718     ITN = 2     ERR F= 1.56101     ITN = 3     ERR F= 0.80433     ITN = 4     ERR F= 0.74717     ITN = 5     ERR F= 0.38551        .   .   .        .   .   .        .   .   .     ITN = 11                                           ITN = 15     ERR F= 0.25997                            ERR F (Start)  :  39.40965                                                               ERR F (Now)  :  0.27848     FunctionTermination                  Change            :  -14052 %     Final Analysis           VSWR( 1 ) = 1.045432           VSWR( 2 ) = 1.061462           VSWR( 3 ) = 1.019224     I       VAR            GRAD     1    165.758972   +2.011833E-03     2         0.397577   +2.166449E-03     3         0.559632   -6.476287E-02     Auto or Expert mode (A/E) [<Enter>=Quit] ? _

Here is the completed FET Model for the NE-38806: