How to use a Low Noise Amplifier

Low Noise Amplifiers (LNAs) are simply designed to amplify signals such that they rise above the noise floor in radio systems. This is the primary use case of an LNA. However, it is not the only application. For instance, an LNA can be used in transmitter systems or in general, systems where signal amplification is required.

HOW SHOULD I CONNECT MY LNA?

The RFin or RF input terminal of the LNA should always be connected to the source of the signal. In receiver systems, RFin should be connected to the antenna. The unmarked terminal at the other end, should be connected to the RF input of the radio receiver.

LNA use in a Receiver System

In transmitter systems, RFin should be connected to the output of the transmitter. The unmarked terminal at the other end should be connected to the antenna.

LNA used in a Transmitter System

In the case of a transmitter, the primary use of an LNA is to boost the level of the output signal.

What about transceivers or devices that both transmit and receive signals from the same port (such as Wi-Fi access points and Helium hotspots)?

An LNA cannot be used in such situations as it only amplifies signals that are present at the RFin terminal. The LNA does not amplify signals in both directions! In fact, if you use it in a transceiver with a single Tx/Rx port, you run the risk of damaging both the transceiver and the LNA. The only exception to this is if you are able to control the transceiver such that it only operates as a receiver or a transmitter. Typically, this is the case when you use a software-defined radio (SDR). An SDR can be configured as a receive-only device in which case you can connect the LNA as you would with a receiver. 

LNA cannot be used in a system with only one Tx/Rx port

What if your transceiver has separate ports for transmit and receive?

In this situation you can use an LNA on each port. Note that RFin always connects to the source of the signal. In the case of the transmitter, it is connected to the Tx port. For the receiver, it is connected to the antenna.

Using an LNA in a Transceiver with separate Tx and Rx Ports

In this post we have discussed how to use a low noise amplifier to improve the noise figure of a receiver. We’ve also discussed using an LNA in a transmitter system to boost the level of the output signal.

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