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NATIONAL Instruments has released a new application note detailing how to use the harmonic balance simulation to design RF/microwave amplifiers when only S-parameter data of the transistor is available.
The paper describes a method to be used within the AWR Design Environment which can be used for low-noise, high-power, narrowband, and multi-octave bandwidth amplifiers whether or not nonlinear transistor models are available.
The origins of the technique is the 1983 Load-Line approach developed by Dr Steve Cripps of Chalmers University, which was further developed using the power parameters technique introduced by Pieter Abrie in the MultiMatch Amplifier Design Wizard.
It is widely held that S-parameters combined with harmonic balance (HB) alone cannot provide for the simulation of the power performance of transistors. However, the method set out in the paper allows designing and simulating amplifiers within NI AWR Design Environment for maximum power using HB simulation when the only available data is the S-parameters of the transistors. The method is widely applicable when nonlinear transistor models are not available and can also be very helpful even when the nonlinear models are available.
When Dr Cripps first published his original paper, HB simulation was not in use yet and load-pull measurements were the only option available for device characterisation. When HB simulations later became available, they were very slow. The Cripps Approach, therefore, offered a much simpler way to design for high power.
Unfortunately, his approach was never implemented in any of the general simulators. It was, however, implemented in a more advanced form in the specialised MultiMatch Amplifier Design Wizard developed by Pieter Abrie. The mapping functions of the power parameters lifted any restrictions associated with the transistor’s confi guration, including feedback, resistive loading, grounding node position, parallel chips/cells, reference plane issues, multistage, and more. There are interesting similarities with the noise parameters, such as series feedback, which allows for easier match for maximum power.
Today HB simulations are as fast as linear simulations were 20+ years ago. While accurate nonlinear transistor models have been developed, they are still not provided for many useful transistors.
The method showcased in this whitepaper is widely applicable, from low-noise amplifiers to high-power amplifiers and from narrowband to multi-octave bandwidth amplifiers.
It is also useful anywhere where it is important to know, and design for, the power deliverable by any of the stages of an amplifier when nonlinear transistor models are not available.
Their incorporation will provide unprecedented levels of productivity and
creativity in the design of matching networks from inside the general simulators.