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Amplifier

Model amplifier in RF systems

Elements

Description

Use the Amplifier block to model a linear or nonlinear amplifier, with or without noise. Defining the amplifier gain using a data source also defines input data visualization and modeling. Use the Main tab parameters to specify amplifier gain and noise using data sheet values, standard s2p files, S-parameters or circuit envelope polynomial coefficients.

The amplifier is implemented as a polynomial, voltage-controlled voltage source (VCVS). The VCVS includes nonlinearities that are described using parameters listed in the Nonlinearity tab. To model linear amplification, the amplifier implements the relation Vout = a1*Vin between the input and output voltages. The input voltage is Vi(t) = Ai(t)ejωt, and the output voltage is Vo(t) = Ao(t)ejωt at each carrier w = 2πf in the SimRF™ environment.

Nonlinear amplification is modeled as a polynomial (with the saturation power computed automatically). It also produces additional intermodulation frequencies.

Dialog Box and Parameters

Main Tab

Source of amplifier gain

Specify the source parameter of the amplifier gain as:

• Available power gainAvailable power gain parameter is used to calculate the linear voltage gain term of the polynomial VCVS, a1. This calculation assumes a matched load termination for the amplifier.

• Open circuit voltage gainOpen circuit voltage gain parameter is used as the linear voltage gain term of the polynomial VCVS, a1.

• Data source — Linear voltage gain term of the polynomial VCVS is calculated from the specified data source options:

for the maximal value of S21.

When using the data source option, S11 and S22, are used as the input and output impedances. The data sources are specified using either Data file or Network-parameters or Rational model, depending on the value of Data source.

• Polynomial coefficients — The block implements a nonlinear voltage gain according to the polynomial you specify. The order of the polynomial must be less than or equal to 9. The coefficients are ordered in ascending powers. If a vector has 10 coefficients, [a0,a1,a2, ... a9], the polynomial it represents is:

Vout = a0 + a1Vin + a2Vin2 + ...  + a9Vin9

where a1 represents the linear gain term, and higher-order terms are modeled according to [2].

For example, the vector [a0,a1,a2,a3] specifies the relation Vo = a0 + a1V1 + a2V12 + a3V13. Trailing zeroes are omitted. If a3 = 0, then [a0,a1,a2] defines the same polynomial as [a0,a1,a2, 0]. The default value of this parameter is [0,1], corresponding to the linear relation Vo = Vi.

The default value of this parameter is Available power gain.

Available power gain

When you set the Source of amplifier gain parameter to Available power gain, you can specify the available power gain of the amplifier. Specify the units from the corresponding drop-down list.

The default value of this parameter is 0 dB.

Open circuit voltage gain

When you set the Source of amplifier gain to Open circuit voltage gain, you can specify the open circuit voltage gain of the amplifier. Specify the units from the corresponding drop-down list.

The default value of this parameter is 0 dB.

Input impedance (ohms)

When you set the Source of amplifier gain to Available power gain, Open circuit voltage gain, or Polynomial coefficients, you can specify the scalar input impedance of the amplifier.

The default value of this parameter is 50  ohms.

Output impedance (ohms)

When you set the Source of amplifier gain to Available power gain, Open circuit voltage gain, or Polynomial coefficients, you can specify the scalar output impedance of the amplifier.

The default value of this parameter is 50  ohms.

Data source

When you set Source of amplifier gain to Data source, you can specify the data source as either Data file or Network-parameters or Rational model.

• Data file — Name of a Touchstone file with the extension.s2p . The block ignores noise and nonlinearity data in imported files.

• Network-parameters — Provide Network parameter data such as S-parameters, Y-parameters, and Z-parameters with corresponding Frequency and Reference impedance (ohms) for the amplifier.

• Rational model — Provide values for Residues, Poles, and Direct feedthrough parameters which correspond to the equation for a rational model

In this rational model equation, each Ck is the residue of the pole Ak. If Ck is complex, a corresponding complex conjugate pole and residue must also be enumerated. The example, Model an RF Filter Using S-Parameter Data, shows how to use the RF Toolbox™ rationalfit function to create an rfmodel.rational object. This object has the properties C, A, and D. You can use these properties to specify the Residues, Poles, and Direct feedthrough parameters.

Noise figure (dB)

Specify the noise figure of the amplifier. The default value of this parameter is 0 dB, which implies that no noise is added to the system by this block.

You can model noise in a SimRF model with a Noise, Resistor, Amplifier, or Mixer block. To do so, in the Configuration block dialog box, verify that the Simulate noise check box is selected (default).

Ground and hide negative terminals

Select this option to internally ground and hide the negative terminals. Clear this to expose the negative terminals. By exposing these terminals, you can connect them to other parts of your model.

By default, this option is selected.

Nonlinearity Tab

Nonlinear polynomial type

Specify either an Even and odd order or Odd order polynomial nonlinearity. The default value is Even and odd order.

• When you select Even and odd order, the amplifier can produce second- and third-order intermodulation frequencies in addition to a linear term.

• When you select Odd order, the amplifier generates only odd order intermodulation frequencies.

The linear gain determines the linear a1 term. The block calculates the remaining terms from the specified parameters. These parameters are IP3, 1-dB gain compression power, Output saturation power, and Gain compression at saturation. The number of constraints you specify determines the order of the model.

The preceding figure shows the graphical definition of the nonlinear amplifier parameters.

Intercept points convention

Specify either an Input-referred or Output-referred convention. Use this specification for the intercept points, 1-dB gain compression power, and saturation power.

The default value is Output.

IP2

When Nonlinear polynomial type is Even and odd order, specify the second-order intercept point of the amplifier.

The default value is inf dBm, which corresponds to an unspecified point.

IP3

Specify the third-order intercept point of the amplifier. The default value is inf dBm, which corresponds to an unspecified point.

1-dB gain compression power

When Nonlinear polynomial type is Odd order, specify the 1-dB gain compression point. The 1-dB gain compression point must be less than the output saturation power.

The default value is inf dBm, which corresponds to an unspecified point.

Output saturation power

When Nonlinear polynomial type is Odd order, specify the output saturation power. The block uses this value to calculate the voltage saturation point used in the nonlinear model. In this case, the first derivative of the polynomial is zero, and the second derivative is negative.

The default value is inf dBm, which corresponds to an unspecified point in the polynomial model.

Gain compression at saturation

When Nonlinear polynomial type is Odd order, specify the gain compression at saturation. This parameter cannot be set unless Output saturation power is specified.

The default value is inf dBm.

Modeling Tab

Setting Source of amplifier gain to Data source activates the Modeling Tab.

Modeling options

Modeling domain. Select Frequency domain or Time domain (rationalfit) from the drop-down list.

When modelling using Frequency domain, Visualization tab plots only the data defined in Data Source.

When modelling using Time domain, Visualization tab plots the data defined Data Source and the rationalfit values .

For the Amplifier and S-parameters blocks, the default value is Time domain (rationalfit). For the Transmission Line block, the default value is Frequency domain.

Fitting options

This field displays when you select Time domain (rationalfit) as the Modeling options. Valid values are Share all poles, Share poles by columns, and Fit individually.

For the Amplifier block, the default value is Fit individually. For the S-parameters block, the default value is Share all poles.

Relative error desired (dB)

This field displays when you select Time domain (rationalfit) as the Modeling options. Enter the desired relative error in decibels (dB). The default value is -40.

Rational fitting results

These fields display the results of rational fitting when you select Time domain (rationalfit) as the Modeling options.

Visualization Tab

Setting Source of amplifier gain to Data source activates the Visualization tab.

Source of frequency data

Frequency data source. When Source of frequency data is Extracted from data source, the Data source must be set to Data file. Verify that the specified Data file contains frequency data.

When Source of frequency data is User-specified, specify a vector of frequencies in the Frequency data parameter. Also, specify units from the corresponding drop-down list.

For the Amplifier and S-parameters blocks, the default value is Extracted from source data. For the Transmission Line block, the default value is User-specified.

Plot type

Specify the type of plot that you want to produce with your data. The Plot type parameter provides the following options:

• X-Y plane — Generate a Cartesian plot of your data versus frequency. To create linear, semilog, or log-log plots, set the Y-axis scale and X-axis scale accordingly.

• Polar plane — Generate a polar plot of your data. The block plots only the range of data corresponding to the specified frequencies.

• Z smith chart, Y smith chart, and ZY smith chart — Generate a Smith® chart. The block plots only the range of data corresponding to the specified frequencies.

The default value is X-Y plane.

Parameter #

Specify the S-parameters to plot. From the Parameter1 and Parameter2 drop-down lists, select the S-parameters that you want to plot. If you specify two parameters, the block plots both parameters in a single window.

The default value for Parameter1 is S11. For the Amplifier and S-parameters blocks, the default value for Parameter2 is None. For the Transmission Line block, the default value for Parameter2 is S22.

Format #

For X-Y plots, format the units of the parameters to plot from the Format1 and Format2 drop-down lists. For polar plots and Smith charts, the formats are set automatically.

The default value is Magnitude (decibels).

Y-axis scale

Scale for the Y-axis.

The default value is Linear.

X-axis scale

Scale for the X-axis.

The default value is Linear.

References

[1] Gonzalez, Guillermo. "Microwave Transistor Amplifiers: Analysis and Design", Englewood Cliffs, N.J.: Prentice-Hall, 1984.

[2] Grob, Siegfried and Juergen Lindner. "Polynomial Model Derivation of Nonlinear Amplifiers, Department of Information Technology, University of Ulm, Germany.

[3] Kundert, Ken. "Accurate and Rapid Measurement of IP 2 and IP 3", The Designers Guide Community, Version 1b, May 22, 2002. http://www.designers-guide.org/analysis/intercept-point.pdf.

[4] Pozar, David M. "Microwave Engineering", Hoboken NJ: John Wiley & Sons, 2005.