Microwave Engineering Europe - April 2009 - (Page 12)

12 MMICs/RFICs Figure 11: Distributed ampliﬁer, simpliﬁed schematic. at around +30 dBm input, is a real and repeatable feature [5]. A number of limiters have been tested to destruction. The input power level, which was required to cause destruction, was around +39 dBm at 5 GHz and around +37.5 dBm at 17 GHz. Following destruction, the limiter continued to provide protection but the small signal insertion loss had increased to a level, which would necessitate replacement of the module for continued operation of the system. Conclusions (Limiter) Details of the design, fabrication and evaluation of a dual channel limiter MMIC have been presented. It exhibits very low insertion loss (< 0.8 dB) from 0.5 to 20 GHz. The saturated output power (at 10 dB compression) of the limiter is between 15 and 16.5 dBm across the entire band and it can handle CW power levels of 4 W. Amplitude and phase match between the channels is within 0.1 dB and 1°. For additional information see reference [6]. Dual channel (LNA) Introduction (LNA) The most appropriate topology for a broadband LNA is a distributed ampliﬁer. Each amplifying stage employs multiple transistors, the capacitive input and output impedances of which are absorbed into a low pass ﬁlter structure (sometimes also referred to as an “artiﬁcial transmission line”). The approach is well suited to a monolithic implementation as, provided that the transistors are very similar, the ampliﬁer performance is relatively insensitive to process variation. Figure 12: Simulated LNA performance, including bonding parasitics. Figure 13: Simulated LNA NF and K, including bonding parasitics. Figure 14: Photograph of the dual channel LNA MMIC. Microwave Engineering Europe ● April 2009 ● Circuit design (LNA) The simpliﬁed schematic of a distributed ampliﬁer is depicted in Figure 11. A DC blocking capacitor at the RF input is followed by a low Figure 15: Photograph of a dual channel LNA module. pass ﬁlter structure comprising a ladder network of shunt capacitors and series inductors. It can be seen that the central shunt capacitors of the ﬁlter have been replaced with transistors, in a similar manner to the design approach adopted for the limiter. The ﬁlter structure is terminated in a 50 Ω load, ensuring a good broadband input match. The RF input signal travels along the Figure 16: Measured versus simulated gain and input ﬁlter structure exciting the input of each transistor. Another low pass match of the LNA. ﬁlter structure is used to combine the outputs of the transistors resulting in broadband ampliﬁcation. The 0.25 µm gate length Pseudomorphic High Electron Mobility Transistor (PHEMT) process — Triquint Semiconductor Texas was selected for implementation of the LNA. With a practical microwave distributed ampliﬁer IC, the value of the series Figure 17: Measured versus simulated output match of inductive elements are small enough that they can be realised using the LNA. www.mwee.com http://www.mwee.com

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Microwave Engineering Europe - April 2009 News Contents Comment MMICs/RFICs: MMICs for Broadband Receiver Applications Antennas: Mutual Coupling Reduction in Compact Arrays for Wireless Sensors via a Pre-fractal Defected Ground Structure Repeatable Characterization of Distortion Caused by Nonlinearities in Wideband Communication Systems Products Calendar

Microwave Engineering Europe - April 2009

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