LNA Suits WCDMA Base Stations At 1.95 GHz

Sept. 17, 2012
This two die solution on a single laminate carrier uses a cascade amplifier in the first gain stage to achieve a better noise figure and linearity than other silicon based solutions.

Numerous base stations elevate the placement of the first low-noise amplifier (LNA) so that it is near the antenna, rather than at the ground units. With this approach, signal conditioning is performed before the signal is transferred over a long cable from the top of the base station to the ground. The requirements for these circuits are determined by the base-station receiver’s demands for both sensitivity and intermodulation. Typically, the receiver calls for a sub-1-dB noise figure, as well as an output third-order intercept point (OIP3) and output 1-dB compression point (OP1dB) higher than +40 and +15 dBm, respectively. A silicon-integrated LNA for WCDMA cellular-infrastructure applications was recently demonstrated by Jos Bergervoet, Domine M.W. Leenaerts, Gerben W. de Jong, Edwin van der Heijden, Jan-Willem Lobeek, and Alexander Simin from NXP Semiconductors.

This LNA, which is designed for the WCDMA band 2 (1.92 to 1.98 GHz), achieves a noise figure of 0.9 dB at +27°C and 1.2 dB at +65°C. The team obtained a 0.1-dB noise improvement by implementing the first gain stage using a cascode rather than a two-stage topology. The result was an OIP3 of +40 dBm (+38 dBm) at +27°C and +37 dBm (+36 dBm) at +65°C. Both design approaches provided input and output return loss beyond 20 dB.

To cope with large input signals, both a bypass mode and variable attenuation were included in the design. The bypass path is realized with two switches on the first die, which allow the signal to be fed directly onto the second die. There, the variable RF attenuator can attenuate the signal before it is passed to the output stage. In this way, both the linearity requirements and high OP1dB are handled.

The two-die approach helps to maintain a low noise figure. To achieve OIP3 beyond +30 dBm, a large current is needed in the output stage. The resulting high power dissipation causes the die’s temperature to rise, negatively affecting the noise figure. This monolithic microwave integrated circuit (MMIC), which is mounted on a single laminate, consumes just below 200 mA from a 5-V supply. It is implemented in silicon germanium:carbide (SiGe:C) BiCMOS. The solution does not require any off-chip components for matching. See “A 1.95 GHz Sub-1 dB NF, +40 dBm OIP3 WCDMA LNA Module,” IEEE Journal Of Solid-State Circuits, July 2012, p. 1672.

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