Adjustable Gain Cascode Low Noise Amplifier

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  • Author: Chin-Leong Lim
Tower mounted amplifiers (TMA) serve to lower the system noise figure (NF) in cellular base-stations. For performance reasons, the TMA is located as near to the receiving aerial as practical and has to be connected to the rest of the radio at the tower’s base by a long run of coaxial cable. The parameters critical to TMA performance are low NF for reasons of optimizing coverage area, high gain to overcome the cable loss, and high linearity as the TMA must operate reliably in a crowded RF spectrum. Additionally, the TMA is required to present a nonreflective input and output terminations, that is, a return loss (RL) of –20 dB, and yet exhibits sub-1 dB NF. LNA designs for TMA service typically rely on the hybrid-coupled balanced amplifier arrangement to satisfy this challenging combination of noise and RL requirements. Unfortunately, the balanced amplifier approach increases the cost, size and power consumption of the LNA. The MMIC device described henceforth (Avago Technologies MGA-632P8) represents an important milestone for cellular TMA designs as it simultaneously achieves both target RL and NF without relying on costly and spacehogging hybrid couplers (also known as quadrature hybrids). A means for varying the gain is provided to fulfill customers’ requirement for gain leveling between adjacent bands, e.g., at PCS and WCDMA.
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