What is a PA/LNA?

• 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 Products
2013-10-24

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 input.

PA and LNA will be at opposite ends of an RF link - and in a duplex link the role switches depending on the direction of the signal. The two components (along with the 2 antennas) do a long way to determining the link-budget, this affects the combination of transmit range and bit rate.

At the receive end, for a given modulation scheme and acceptable error rate, you will need a specific ratio of signal power to noise power. Signal power is determined by transmit power (from the PA), antenna gain, and transmission loss. However, more power is expensive both in components and supply (PA is usually well less than 50% efficient).

LNA amplifies both the wanted signal, and the thermal noise at the LNA input, plus a little more noise. For a good LNA, this will be around 1dB of extra thermal noise. The LNA also needs to be linear to avoid distortion caused by unwanted (often strong) signals that can be filtered out later in the receive chain.

A good LNA is the first thing to invest in, this buys you 1-2 dB fairly easily. Then good antennas, then finally a more powerful PA. There are lots of small details that also contribute - these two components on their own can't rescue a bad design.

When trying to understand PA/LNA, you may also want to understand how they are related to duplexers. However, it was surprising to me to see how difficult it is to find a simple-to-understand diagram of a basic duplexer, that shows both the signal and schematic properties. Of course nowadays you don't even need duplexers as there are different solutions, as often used in mobile phone baseband transceivers.

In this regard, one descriptive picture I found, is this one from a patent.

and from YateBTS site.

Also note that, loosely speaking, a duplexer is a used to TX and RX on the same antenna but (often) not the same frequency, whereas a diplexer is used as either a TX or RX on same antenna, but (often) on the different frequencies.