Designers Typically use an available gain design to enable a gain-versus-noise tradeoff for lownoise- amplifier (LNA) designs. To make the correct tradeoff, however, they need to know about the system in which the amplifier will be used. A white paper by Agilent's Ken Payne details a method of designing an LNA for an IEEE 802.11b receiver application that uses the firm's Advanced Design System (ADS) computer-aided-engineering (CAE) software tools to remove much of that guesswork from the design process. Titled "Practical RF Amplifier Design Using the Available Gain Procedure and the Advanced Design System EM/ Circuit Co-Simulation Capability," the paper is 62 pages long.
This example application contains an Avago ATF54142 PHEMT covering 2.40 to 2.48 GHz. The goal of this effort is to avoid multiple printed-circuit-board (PCB) layouts, thereby saving design cost and time. The paper illustrates the LNA's entire design including the construction of the PCB and measurement results. In the section on "Available Gain Design Procedure," for example, the author explains that a family of circles known as available gain circles are constructed. They provide a specific amount of mismatch at the device input. An infinite number of source terminations forming the circle allow the selection of mismatch at the device input. To construct an available gain circle, one should locate the center of the circle on a Smith chart and draw the circumference from a calculated radius. The chosen LNA had both the nonlinear model and measured S-parameters available. A quick validation was recommended using an Amplifier Design Guide to set up the simulation. That simulation was performed from 100 MHz to 6 GHz in 10-MHz steps and stored in a dataset. Later, the results were compared to the nonlinear model S-parameter simulation. The S-parameter data was measured with a commercial vector network analyzer (VNA) and an Intercontinental Microwave (ICM) transistor text fixture, which is used with the appropriate midsection and ICM TOSL-3001 calibration kit. Before calibration is performed, the TOSL-3001 calibration coefficients are loaded into the VNA. The Avago measured S-parameters, nonlinear-model-generated S-parameters, and lab-measured S-parameters were all found to be in close agreement.
The first-pass amplifier exceeded most of the requirements without having to adjust the circuit components to meet the objectives. The author credits the use of the EM/Circuit Co-Simulation capability in ADS for the first-pass design success. With careful attention to layout and simulation results, the engineer can speed the design process. Interestingly, the entire PCB does not have to be simulated with the EM simulator to obtain good results. By putting the whole PCB in the simulator, one may improve the resultsbut at the expense of significantly longer simulation time.
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