Low Noise Amplifier

LNA is the first gain block in the receive(Rx) chain of a transceiver. It has a significant impact on the noise performance of the Rx chain.

Why low noise amplifier?

In RF circuits noise is characterized by noise figure or noise factor.

Noise factor at the input of the Rx chain is given by

(1)   \begin{equation*} F_{Rx} = F_{\tiny LNA} + { F_{sb}-1 \over G_{\tiny LNA}} \end{equation*}

F_{\tiny LNA} \rightarrow Noise factor of LNA
F_{sb} \rightarrow Cascaded noise factor of subsequent block of Rx except LNA
G_{\tiny LNA} \rightarrow Power gain of LNA

Eq-(1) suggests higher gain of LNA is required to suppress the input referred noise coming for subsequent stages after LNA in Rx chain. This improves NF and hence sensitivity of Rx front-end. Therefore high gain and low noise figure are very important LNA design specifications for best noise performance of any receiver.

LNA Design Specifications

Following are the important performance specifications for the design of LNA

  • Noise figure – directly impacts the sensitivity of a receiver
  • Gain – reduce the impact of noise of subsequent circuits after LNA, and thus the sensitivity of the receiver
  • Linearity(IIP3/IIP2 and P1dB) – decides the blocking performance
  • Input/Output impedance matching – minimize the reflections
  • Operating frequency and Bandwidth
  • Stability
  • Power consumption – should be a low as possible to improve battery life.

LNA Topologies

In the following analysis, only thermal noise of MOS transistor and resistors are considered. In the real design, gate induced noise, and other noise sources need to be considered for accurate results. Flicker noise, most of times is shaped out by passive networks for narrow band circuits at radio frequencies and can be ignored.

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