Introduction

Welcome to the second AMPSA newsletter.

Since the previous newsletter Multimatch development was focused mainly on stepping up the quality of the software. This process included upgrading the code from Microsoft Visual C++ 6 to Visual C++ 2005 (Unicode/International Version) and enhancing many of the Multimatch Wizards to require less user input and to provide a smoother design flow. Vista^TM related changes were also made, and several new features were added.

Apart from the code enhancements, a series of screencast examples was also introduced in order to make learning Multimatch easier. Current examples include an introduction to impedance-matching with Multimatch, fitting a Multimatch model to a chip transistor and designing a feedback amplifier by using some of the features provided in the  Device-Modification Section. More examples will follow soon.

In the next phase of Multimatch development, interfaces to various other design tools will be created. An interface to AWR's Microwave Office^TM is already in place and the artwork can be imported into EM simulators by using the Multimatch DXF export features.

Why Multimatch?

Here are a few of the reasons why customers have invested in Multimatch:

• There is nothing else like Multimatch.
• You get the advantage of 22 years of continuous development and customer feedback.
• Multimatch designs are usually first-time-right. You switch on the power, and the performance is there.
• Multimatch designs are practical. While it is a synthesis tool, enough parameters are available to take most of the typical parasitics encountered in account. This also applies to MMIC designs: Inductors and capacitors with pads can be transformed to equivalent spiral inductors and parallel-plate capacitors with very little difference in the performance. (that is, on low loss substrates.) Powerful optimization features are also provided in Multimatch.
• You need very little information to design a high performance linear amplifier. Non-linear models are not required or used in Multimatch designs.
• You can use the Multimatch Load-Pull Wizard to find the optimum power terminations (P1dB; class A and B) for the transistors you plan to use.
• Multimatch amplifiers do not oscillate. The Modification Network Synthesis Wizard can be used to synthesize lossy networks to provide inherent stability, as well as other desirable properties. Loop gain and reflection gain analysis features are also provided.
• The design strategy in Multimatch is to provide solutions that are highly insensitive to component tolerances and variations. The design procedure typically first goes through synthesis of feedback and/or (frequency selective) resistive loading networks around the transistor (modification networks) which directly reduce the tolerance sensitivity. When reactive matching networks are synthesized, multiple solutions and tolerance sensitivity evaluation of these solutions are provided. This design approach substantially increases the chances that the design iteration will be successful when a yield analysis is done at the end of the full design cycle.
• Multimatch provides you with a systematic way to design amplifiers and impedance-matching networks (success runs on a narrow trail!). Once you have invested the effort to understand how the system works, you will design better amplifiers faster than you ever believed possible.
• Selecting appropriate transistors and creating good initial schematics are skills usually achieved only after years of experience. MultiMatch incorporates wizards and facilities which provide, in interactive mode, multiple initial solutions so that, even for a novice designer, it becomes possible to quickly approach optimum performance.
• Many of the tasks that you perform manually (like designing matching networks) are automated in Multimatch. You can use your time more productively to explore more options with Multimatch.
• You can very easily export your Multimatch designs to Microwave Office^TM or import the Multimatch DXF artwork into an EM simulator (CST^TM, Momentum^TM, Sonnet® Software's EM, etc.) to verify and fine-tune the performance of the Multimatch circuits you designed.
• The investment you need to make is only a fraction of the annual salary of an engineer.

Advantages of the Multimatch Transistor Model (MTM)

Pieter Abrie

Multimatch can be used to design Class A amplifiers, and in some cases Class B and Class AB amplifiers, using as a starting point only S-parameters and noise parameters data sets at a specified dc operating point. With a scarcity of large-signal models and the limited accuracy of many large-signal models, this puts Ampsa customers ahead of the competition.

The S-parameters and noise parameter on their own can be used to design only for gain, return loss, isolation and noise figure, and then only for the frequency range for which the data has been provided. Extrapolation of the S-parameters and the noise parameters at lower and higher frequencies usually provides erroneous results and, more importantly, can easily produce unstable amplifiers or straight-away oscillations. The S-parameters alone cannot be used to design for P1dB and IP3. To overcome the frequency and power limitations a unified transistor model was developed by Ampsa (the Multimatch Transistor Model (MTM)).

The FET/HEMT Modeling Wizard

The MTM incorporates three models – a linear (small-signal) model, a noise model and a power model. These models are defined in Multimatch by two Transistor Modeling Wizards - one for bipolar transistors and one for FETs/HEMTs.

The intrinsic small-signal model used for FETs.

The intrinsic small-signal model used for bipolar transistors.

The package parasitics allowed.

The small-signal model, if not readily available, is extracted inside the Transistor Modeling Wizard. The noise parameters are  calculated by using the model, as well as the noise fitting factors specified. These factors can be optimized with the small-signal model components to fit the measured S-parameters and noise parameters.

The power model consists of I/V-plane specifications and the power parameters.

The I/V-plane specifications consist of the dc operating point and four boundary lines defining the allowable intrinsic load line area. Hard clipping is assumed to occur at the boundary lines (only the fundamental tone is considered). It is also assumed that the output power is linear until the hard clipping happens.

The four boundary lines used to define the allowable class A intrinsic load-line area.

The power parameters map the intrinsic input (V_1i) and output voltage (V_2i), and the intrinsic output current (I_2i), of each transistor to the external voltages, and are calculated by using the small-signal model. (For more information on the power parameters refer to the Artech House book Design of RF and Microwave Amplifiers and Oscillators by Pieter L.D. Abrie.)

The power parameters complement the S-parameter and noise parameter data and, with the I/V-plane constraints, provide a generalized way to estimate and control the linear output power that can be obtained from a transistor or a circuit. The output power can be controlled by using the Multimatch Load-Pull Wizard or the optimization features provided in the Analysis Module and the Device-Modification Module.

The hard-clipped linear output power has proven to be a very close estimate of the 1dB compression point (P1dB) of a transistor or a circuit. The estimate is usually slightly pessimistic because the clipping is softer in reality.

The Multimatch power model approach is similar to the Cripps’ load line approach but, because of the power parameters and the more realistic load-line boundaries, it can handle all the real life situations automatically. These include feedback, multiple parallel transistors, resistors in the transistor model, resistive loading, transmission line losses and even changes in the transistor configuration.

In a multi-stage amplifier the clipping effect of each transistor is considered separately (that is, the signal is increased until the intrinsic output current and/or voltage of the transistor considered touches a boundary line, while the other transistors are assumed to be ideal). The output power provided by each transistor is also referenced to the external load (that is, the maximum output power of each transistor is increased with the gain of the circuit section between the output port of the (modified) transistor and the external load). The limiting transistor is taken to be the one which clips first (lowest referenced output power). This process works well for the estimating the 1dB compression point of an amplifier.

The output power of each amplifier stage is referenced to the load.

Ideally, the referenced output power obtainable from each driver should be greater than the output power required. This will cause the output power to be limited mainly by the final (high power) stage. To provide control over this effect, power margins can be specified for each driver during optimization. Note that the margin should be higher in earlier stages.

The third-order intercept point of a transistor usually differs from the 1dB compression point by an amount that is more or less fixed. The difference will change somewhat with different transistor types, and can be increased by using linearization techniques. An estimate of the difference can be specified in the modeling section for each transistor (default 9.6 dBm) and this number will be used to estimate the third-order intercept point (TOI) for the complete amplifier (worst-case calculation - distortion components adding in phase). This TOI estimate and the power margins allowed for the drivers provide effective control in Multimatch over the contribution of the drivers to the amplifier distortion.

Harmonic tuning is often used in power amplifiers. When narrow-band amplifiers are designed, the specifications for a Multimatch matching network can be extended by using the multiple passband feature to set up targets for the second and third harmonics too. The data required for this purpose can be obtained by using the Multimatch Load-Pull Wizard. When the amplifier designed is analyzed, the intrinsic load termination presented to each transistor modeled can also be listed in Multimatch. Typically, the intrinsic load impedances should be low at the second harmonic and high at the third harmonic.

It is important to understand that while there are many demands on the performance required from modern (linear) power amplifiers, the extra linearity and efficiency can usually be obtained by fine tuning a Multimatch design in any of the popular circuit simulators available on the market, that is, if  accurate non-linear models are available for the transistors used.

It should also be noted that in many linear applications, excellent results can be obtained with only the MTM. In fact, this model will often provide better results than a design based on a large-signal model. This follows because large-signal models are much more complex and have to model the transistor at all bias points, with inevitable discrepancies at specific bias points (These discrepancies can sink a design at  millimeter-wave frequencies). These models are often also  developed for proper modeling of the transistor under highly non-linear conditions, with the consequence that the gain, reflection and P1dB are off under linear conditions.

Many first-pass high performance linear power amplifiers have already been designed by Ampsa customers. These also include power amplifiers for Wimax applications.

New Features

Many improvements were made to the software since August 2007. These include

• The code was upgraded to Unicode and safe versions of the various string functions were implemented (Visual C++ 2005). One of the advantage of Unicode is that foreign characters can now be used in path and file names.
• Vista^TM related changes were made to the software. At this point in time the software runs smoothly under Vista^TM.
• Some of wizards provided in the Device-Modification Section and the Impedance-Matching Section were enhanced to require less user input.
• The pads specified in the Device-Modification and Impedance-Matching Sections could previously be distorted by the microstrip compensation implemented for steps. The electrical lengths of these pads were increased in the Amplifier Design Wizard to allow the pads to come out as required, even with compensation.
• Copy, paste and undo features were added to all the tables provided.
• The Device-Modification Wizard in the Analysis Module was extended to allow selecting a sub-circuit in the schematic for modification. By doing this the artwork created for the sub-circuit is preserved and simulation features not present in the (simpler) Device-Modification Section can be used (Example: multiple substrates).
• The power performance of larger circuits can now be controlled. Previously current and voltage clipping in only 4 transistors were considered. The clipping in up to 16 transistors can now be taken into account. The intrinsic load impedance of the transistors for which Multimatch models were created can now also be listed.
• The Multimatch transistor model was extended to provide a better fit to the optimum noise impedance at low frequencies. Many changes were also made in the modeling section to ensure a smooth flow.
• The DXF export features were enhanced to allow Multimatch MMIC designs to be imported into EM simulators with less effort. (Airbridges and vias on the different layers can now be created automatically.)

Consulting Services and Training

If you need an amplifier and do not have the skills or the time to design it, Ampsa may be able to help. Contact us for a quotation. The customization features in Multimatch can also be set up to optimize your design flow on a consulting basis. Multimatch training can also be provided and is highly recommended.

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