Amplifier - Model amplifier in RF systems

Library

Elements

Description

The Amplifier block models a linear or nonlinear amplifier within the SimRF™ circuit-envelope simulation environment. For more information about simulating RF systems, see the example, Reduce Computations by Using RF Simulation Techniques.

To model linear gain, the amplifier implements the relation

between the input voltage V_i(t) = A_i(t)e^jωt and the output voltage V_o(t) = A_o(t)e^jωt at each carrier ω = 2πf in the SimRF environment. Specify the linear gain coefficient using either an S-parameter array or a data file.

To model nonlinear gain, the amplifier implements the relation

between the input voltage and output voltages. The block derives the polynomial internally from specified IP2 or IP3 for the amplifier:

Input IP2 and input IP3 are the ratio of OIP2 and OIP3 to a₁². The remaining polynomial coefficients a₂ and a₃ are defined by the input modulation A_i,_IP₂ and A_i,_IP₃:

Models that contain SimRF Amplifier and Mixer blocks generate files at update time. Before you can successfully update and run models with these blocks, you have to set up a compiler by running mex -setup.

By default, SimRF software generates files in the current MATLAB^® folder. However, you can change the output location for these files by specifying a cache folder in the Simulink^® Preferences dialog box. To specify a cache folder:

Open the Simulink Preferences dialog box (File > Preferences).
Specify a location for the Simulink cache folder parameter.

For more information about the Simulink interface, see Simulink Preferences Window.

Dialog Box and Parameters

Treat as linear component

Select this check box to model a linear amplifier using an S-parameter array or a data file. Clear this check box to model a nonlinear amplifier. By default, the check box is cleared.

Source of amplifier gain

Specify the source parameter of the amplifier gain as:

Available power gain — The block uses the value of the Available power gain parameter to calculate a₁. Available power gain can be measured by implementing a conjugate match between the output and load.
Open circuit voltage gain — The block uses the value of the Open circuit voltage gain parameter to calculate a₁. Open circuit voltage gain can be measured with an open circuit at the output of the amplifier
Data source — The block calculates an available power gain using the value of S₂₁ from the data source with the highest magnitude:

Specify S₂₁ values using either Data file or S-parameters, depending on the value of Data source.

The default value of this parameter is Available power gain.

Available power gain

When Source of amplifier gain is Available power gain, specify the available power gain of the amplifier. Specify the units from the corresponding drop-down list. If you specify the units as None, the gain must be positive. The default value of this parameter is 0 dB.

Open circuit voltage gain

When Source of amplifier gain is Open circuit voltage gain, specify the open circuit voltage gain of the amplifier. Specify the units from the corresponding drop-down list. If you specify the units as None, the gain must be positive. The default value of this parameter is 0 dB.

Source of input and output impedance

When Source of amplifier gain is Data source, specify the source of the input and output impedance as either User-specified or Data source.

Input impedance (ohms)

When Source of input and output impedance is User-defined, specify the scalar input impedance of the amplifier. The default value of this parameter is 50 Ω.

Output impedance (ohms)

When Source of input and output impedance is User-defined, specify the scalar output impedance of the amplifier. The default value of this parameter is 50 Ω.

Data source

When Source of amplifier gain is Data sourcespecify the data source as either Data file or S-parameters.

Data file — Specify an AMP or Touchstone^® .s2p file for the Data file parameter. If you specify an AMP file, the block ignores noise and nonlinearity data in the file.
S-parameters — Provide S-parameters, Frequency, and Reference impedance (ohms) data for the amplifier.

S-parameters modeling

Choose how to model the device represented by your S-parameters.

When you select Time domain (rationalfit), the block uses a rational function approximation during simulation. The block uses the RF Toolbox™ function rationalfit to produce the approximation. This approach captures transient behavior, but the rational function must be a good fit for the data in order to produce accurate results. Use the Visualization tab to compare the approximation against your data.
When you select Frequency domain, your S-parameter data is represented exactly in the frequency domain during simulation. This approach produces accurate steady-state results but does not capture transient behavior.

The default value of this parameter is Time domain (rationalfit).

Source of cubic polynomial

Specify how the block calculates the nonlinear effects of the amplifier as:

Derived from intercept points — Specify the IP2 and IP3 parameters, from which the block defines a cubic polynomial.
Cubic polynomial coefficients — Specify the Cubic polynomial coefficients parameter to define the cubic polynomial directly.

Intercept points convention

Specify the IP2 and IP3 parameters as either Input or Output intercept points.

IP2

Specify the second-order intercept point of the amplifier. The default value is inf. This parameter applies only when you set Source of cubic polynomial to Derived from intercept points.

IP3

Specify the third-order intercept point of the amplifier. The default value is inf. This parameter applies only when you set Source of cubic polynomial to Derived from intercept points.

Noise figure (dB)

Specify the noise figure of the amplifier. The default value of this parameter is 0 dB.

To model noise in a SimRF model with a Noise, Amplifier, or Mixer block, you must select the Simulate noise check box in the SimRF Parameters block dialog box.

Cubic polynomial coefficients

Specify the coefficients of the polynomial a₀ + a₁V₁ + a₂V₂² + a₃V₃³. The elements of the vector are [a0,a1,a2,a3]. Trailing zeroes can be omitted; for example, if a₃ = 0, [a0,a1,a2] defines the same polynomial as [a0,a1,a2,0]. The default value of this parameter is [0 1], corresponding to the polynomial a₁V₁.

Specifying a polynomial overrides the available power gain or open circuit voltage gain since the polynomial contains the term a₁.

Ground and hide negative terminals

Select this check box to internally ground and hide the negative terminals of the block. Clear the check box to expose the negative terminals, allowing you to connect them to other parts of your model. By default, this check box is selected.

Examples

In the Demos section:

The demo Validating IP2/IP3 Using Complex Signals verifies the nonlinear modeling capabilities of the Amplifier block.
The demo Impact of an RF Receiver on Communication System Performance performs quantitative noise analysis of the noise from an RF cascade.

In the SimRF User's Guide:

The section Create a Low-IF Receiver Model uses an amplifier in an IF receiver with specified gain and noise figure.

References

Gonzalez, Guillermo. Microwave Transistor Amplifiers: Analysis and Design.

Kundert, Ken. "Accurate and Rapid Measurement of IP₂ and IP₃".

Pozar, David M. Microwave Engineering.

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