- PA: (power amp) amplifies when transmitting.
- LNA: (low noise amp) amplifies when receiving.
- both sit between circuitry and antenna.
- for duplexed signal, passive duplexer shifts between the two on Rx/Tx.
The PA stands for power amplifier, in this case a RF or microwave amplifier used for transmission of a signal. LNA stands for low noise amplifier, normally used for high RF bands or microwave signals as a sensitive signal receiver. PAs and LNAs are not always combined. It depends on the application. I found this article on the web which covers the basic details.
Understanding the Basics of Low-Noise and Power Amplifiers in Wireless Designs By Bill Schweber
Contributed By Electronic
1) 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.
2) 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.
3) 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.
4) 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