Introduction

Welcome to the first edition of the AMPSA newsletter.

This year is a special year for Ampsa. Ampsa was founded in May 1986 and turned 21 in 2007!

Thanks to the support of all Ampsa's customers during the last 21 years, Ampsa's design software has reached a new level of maturity, and it fits the occasion that Version 9 of the Multimatch Amplifier Design Wizard has been released. It has many exciting new features, which include many enhancements to speed up the design of MMIC amplifiers, improvements to the small-signal models implemented to include modeling of the noise parameters of FETs, HEMTs and bipolar transistors, as well as a model customization feature. Users can use the last feature to customize the discontinuity models, parallel-plate capacitor models and the spiral inductor model provided in Multimatch to extend the accuracy of the software up to millimeter-wave frequencies. Because of the strong discontinuity effects often present, the customization feature is also useful to power amplifier designers, even at lower microwave frequencies. Customers have also found a new use for the multiple passband feature (which has been present in Multimatch for many years) to design matching networks to control the fundamental frequency load-line of power transistors, as well as the harmonic terminations. The power parameters implemented in Multimatch continues to provide accurate load-pull information for power transistors (biased in class A or class B mode) based on a minimum of information. In addition to the new features implemented, many improvements were also made to the Multimatch user interface. All of these add up to getting high quality amplifiers designed much quicker than ever before.

Multimatch is typically used as a front-end to the simulation software available from one of the major software vendors and exporting Multimatch designs into these formats continues to have a high priority. The AWR Microwave Office (TM) export features present in Multimatch were enhanced for MMIC designs in Version 9. Options to export Multimatch designs in Agilent ADS (TM) IFF format or Sonnet Software (TM) EM format will soon be in place. The artwork created in Multimatch can also be exported in DXF or HPGL format.

Designing Amplifiers The Smart Way

Pieter Abrie

There are many very smart amplifier designers out there, so why Multimatch? The answer to this is that smart designers usually get even smarter with Multimatch. In fact, there is a learning curve for novice designers, before they start to get the most out of Multimatch. Therefore, being a smart designer gives you a head start! There is very little in Multimatch that is irrelevant. The tools provided serve a purpose and assist you on the road you must travel to get to a smart design. At the same time, many tasks that are performed manually with other tools are automated in Multimatch. This improves efficiency and creates the freedom to explore many alternatives.

Fortunately, all of this does not leave the designer with nothing to do! There are many options and different paths that can be taken in the software, and it is unlikely that any two designers will come up with identical designs. Design flair and intuition may also prompt manual changes, which can then be optimized in Multimatch.

The first thing to understand about Multimatch is that it finds solutions to matching problems and modification problems by doing synthesis-based systematic searches to get a number of seeds, which are then optimized. This approach increases the probability that the solutions found will be close to the optimum and also provides perspective in that several solutions are provided to each problem instead of only one. It also finds solutions much faster than normally possible. Because the solutions provided are close to optimum, Multimatch designs generally tend to be less sensitive that those obtained by following alternative routes.

The second point is that the solutions synthesized are practical. Provisions are made for pads and connecting lines, as well as typical parasitics where required. It is important to realize that the solutions synthesized are also adequate for MMIC implementations. This follows because parallel-plate capacitors can be closely modeled as ideal capacitors sandwiched between transmission-lines. Similar rules apply to spiral inductors on substrates with low loss. By specifying adequate pads, one can ensure that the solutions obtained can be transformed accurately to distributed form. Any discrepancies can usually be resolved in Multimatch with minor optimization of the networks synthesized after replacing the lumped components with their distributed equivalents. Component losses are also introduced at this stage (losses are still ignored when matching networks are synthesized).

The next point is that matching is only part of the solution in getting to a good design. The other part is device modification, that is, adding resistive feedback and/or loading sections to a transistor before doing the matching in order to make it easier to match, remove inherent gain slopes, improve stability, move optimum gain points closer to optimum noise or optimum power points, etc. This pre-conditioning step is vital in many cases. Similar to matching, a routine has been implemented to solve this type of problem in Multimatch.

Designing an amplifier that oscillates is not a good thing. Many features have been implemented in Multimatch to assist in preventing this. These include the normal stability analysis, as well as loop-gain and reflection analysis. Multimatch designers have found the loop-gain analysis tool to be critical in checking that the adequate gain and phase margins are provided in each stage of the amplifier.

Lastly, having a good matching tool without knowing what to match to is limited. Several wizards are provided in Multimatch to automatically set up typical matching problems. These include power matching problems, gain leveling problems, noise figure and gain control problems , as well as indirect improvement of the input or output VSWRs.

While the software is linear, excellent load-pull contours can be generated for power transistors biased in class A or class B mode (power parameters). The point to grasp when controlling the power performance of a linear amplifier is that the power is limited first and foremost by clipping of the intrinsic current and/or intrinsic voltage waveforms in the limiting transistor in the amplifier chain. In general, clipping should occur first in the last (highest power) transistor in the chain, while sufficient headroom should be allowed in the drivers to prevent significant contribution to the third-order intermodulation distortion by these stages. Adequate features are implemented in Multimatch for these purposes. When narrowband amplifiers are designed, the multiple passband feature provided in Multimatch can also be used to design matching networks to control the fundamental tone termination, as well as the terminations at some of the harmonics. When the peak to average ratio of the power is high, a compromise between efficiency and linearity is required (some clipping of the signal must be allowed). The design can then be based on external load-pull data or a number of Multimatch designs can be done targeting different power levels above the average and below the peak. These designs can then be evaluated and optimized  experimentally or by using 3rd party software.

When MMICs are designed (especially at millimeter-wave frequencies), EM simulation of the final networks is usually essential. Making corrections to the design at this stage can be very time consuming, if not impossible. The answer to this is to add EM based models to the software. This option is now available in Multimatch. Users can customize the standard models used to provide EM level accuracy (an accurate simulator is required to generate the data required). When this is done, the difference between the design in Multimatch and the EM simulation is usually insignificant. When corrections are required, the quickest route is usually to optimize each Multimatch matching or modification network to fit the associated EM simulation, note any significant differences and then apply these in reverse to the EM simulation.

Training

Training

There is a lot to learn in Multimatch and training does help to make it easier. On-line training is available (recommended; GoToMeeting (TM)) and is inexpensive and convenient. On-site training can also be provided. A series of two day courses on Amplifier Design is also planned. Let us know if you are interested in such a course in your area.

A number of screen for screen PowerPoint (TM) examples of some standard designs are also provided to customers. These examples illustrate the design flow and accelerate the learning process.

Consulting Services

Consulting

The customization features in Multimatch can be set up to optimize amplifier design flows on a consulting basis.

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