Understanding the Basics of Low-Noise and Power Amplifiers in Wireless Designs
In a wireless design, two components are the critical interfaces between the antenna and the electronic circuits, the low-noise amplifier (LNA) and the power amplifier (PA). However, that is where their commonality ends.
Although both have very simple functional block diagrams and roles in principle, they have very different challenges, priorities, and performance parameters.
How so? The LNA functions in a world of unknowns. As the “front end” of the receiver channel, it must capture and amplify a very-low-power, low-voltage signal plus associated random noise which the antenna presents to it, within the bandwidth of interest. In signal theory, this is called the unknown signal/unknown noise challenge, the most difficult of all signal-processing challenges.
In contrast, the PA takes a relatively strong signal from the circuitry, with very-high SNR, and must “merely” boost its power. All the general factors about the signal are known, such as amplitude, modulation, shape, duty cycle, and more. This is the known-signal/known-noise quadrant of the signal-processing map, and the easiest one to manage. Despite this apparent simple functional situation, the PA has performance challenges as well.
In duplex (bidirectional) systems, the LNA and PA usually do not connect to the antenna directly, but instead go to a duplexer, a passive component. The duplexer uses phasing and phase-shifting to steer the PA’s output power to the antenna while blocking it from the LNA input, to avoid overload and saturation of the sensitive LNA input (for more on antenna fundamentals, see the TechZone article “Understanding Antenna Specifications and Operation”); it also directs the antenna signal to the LNA and not to the PA (Figure 1a). The Avago Technologies ACMD-7612 is one such duplexer (Figure 1b); it uses Avago’s Film Bulk Acoustic Resonator (FBAR) technology for this function.
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