Design of a Ka-band MMIC filtering LNA with a Metamorphic HEMT Technology for a Space Application
Armengaud, V¹; Lintignat, J¹; Barelaud, B¹; Jarry, B¹; Laporte, C²; Babak, L^3
1XLIM; ²CNES; ³TSUCSR

LNAs are usually designed for ensuring the best noise figure associated with the highest gain in the useful band. However, such LNAs provide gain around the useful band. Reducing this out-of-band gain thanks to filtering and matching circuits presents different advantages : first, the out of band high gain rejection improves the device stability and reduces the risks of oscillation, specially when the die is packaged. Secondly, removing the out-of-band noise decreases the front-end noise power. Finally, this filtering MMIC LNA property helps to insulate Rx (27-31 GHz) and Tx (17-21 GHz) channels and to relax selective RF filters requirements. Consequently this device could reduce bulk and weight of the spatial system.

This paper presents first a way to selecting the best MMIC transistor whatever the technology. This procedure is based on a statistical Monte Carlo method. Random bias and physical parameters are tested at the central frequency (f0) and strong requirements lead to select a pool of solutions. This method allows to choose the optimum transistors from only one graph and whatever the technology. Such transistors provide a high and stable gain, good noise figure and return losses.

The second step of this procedure deals with the passive matching and filtering networks synthesis. Indeed, this LNA is composed by three stages and therefore four passive circuits. The input circuit is mainly designed to noise and input matching without filtering specific constraints. Then, the first inter-stages provides a high pass filtering response while the second inter-stages allows a pass-band behaviour. Finally, the output network is used to complete the low pass rejection.

To achieve the interstage filters, we use a new and reliable software (GENESYN) developed by the university of Tomsk (Russia). GENESYN uses genetic algorithm that imitates evolution of living organisms in nature based on chromosome crossover, mutation and selection. From optimal input and output transistor impedance and for every tested circuits, GENESYN calculates the gain response until filtering requirements has reached.

The final paper will explain the complete design process. A Ka band filtering LNA design example is proposed. Those simulated results will validate the design methodology. The MMIC realization is actually in process and the chip will be available in April 2008. Measurements could be presented at the conference.