Design and Operation of Low Noise Amplifiers (LNAs)

One of the first things that comes to mind about the communication system is an amplifier. So what does an amplifier do? In a clear perspective, an amplifier is responsible for making a weak communication signal somewhat stronger at the cost of either of the factors - dynamic range, loss in return, stable nature, or noise figure. Amplifiers have a huge contribution to the fields of earth science radiometry, transceivers, along with radio astronomy.

But as a counteractive measure for the noise factor, there exists something called Low Noise Amplifier. Let’s know more about it in a broad view.

What is a Low Noise Amplifier (LNA)?

A Low-Noise Amplifier can be said to be a specific kind of electronic amplifier that makes sure to amplify weak communication signals while maintaining proper voltages for the conversion of analog signal to digital signal. These amplifiers are used in certain applications for which the sources are of low frequency and may consist of some kind of antennae or transducers.

Why do we need it?

This particular type of amplifier is the only amplifier that can amplify a weak signal while maintaining the sound-to-noise ratio quite effectively. It means that the signal will be converted to a strong one while also keeping the noise in control. This capability is crucial for equipment used in medical or radio communication systems.

Understanding Noise Figure and its Analysis

Noise Figure is one of the integral terms to be studied in the case of understanding low noise amplifiers with ease. Here is everything you need to know about it.

What is a Noise Figure?

The noise Figure, also termed as Noise Factor, describes itself by a number that says everything about the performance and efficiency of the components included in the signal transition. If the number is low, then it means the low-noise amplifier is performing well.

Noise Figure Analysis

It's important to have a low noise figure which will further determine whether the DUT is performing as expected. The following terms will explain the part of the analysis.

Noise Figure Design Process

LNAs are always assessed for functional requirements and devices are tested to select the one that provides a better ratio for each factor. The process includes the consideration steps of:

• Software modeling
• Functional requirements
• Device selection
• Raw device modeling
• Matching device
• Finishing of design

Noise Figure Design Verification

This is the verification stage where:

• the LNA performance is simulated.
• layout design and prototype model are checked properly.
• design is fine-tuned.
• the values of noise figures in narrow band and microwave frequencies are measured.
• the values of noise figures in devices like mixers and receivers are measured.

After the verification of these points, the verification stage is complete.

Noise Figure Measurement Uncertainty

After all the measurements are made, the checking has to be done again so that any kind of extraneous signals, nonlinearities, uncertainty(instrumentation, ENR, mismatch, or instrument architecture), and unnecessary in-band power are not found in any case.

Things to consider while designing an LNA

While designing an LNA, there must be some considerations to go through so that something does not backfire in the general run. The considerations are hereby listed below.

• Gain

The gain for LNA has to be in the range of 10 dB to 30 dB for optimal usage.

• Power Consumption

The lower the power consumption is, the greater the efficiency of that LNA making the transition of weak signals.

• Noise Figure

The noise figure will determine the ratio of the total output noise to the output noise from the input. These are measured by signal or noise generators. If it gives a lower value, the RF is doing great for sure.

• Stability

The design of LNA would require certain elements to be kept in mind so that stability is not compromised while transmitting weak signals and converting them to strong signals.

• Linearity

Few linearization techniques must be used with great optimization so that one LNA can handle weak signals having either from the majority of frequencies of the frequency band giving them greater opportunity of usage in unspecific devices as well.

• Distortion

This has to be a great consideration as it would help improve the linearity of an LNA further improving usage of the desired devices.

• Biasing

Biasing is another factor to consider. The LNA has to bias at 3 mA for the operation of signal transition with low noise.

How many types of LNA are there?

3 types of LNA exist. They are listed below.

• General Purpose LNA
• GLONASS LNA
• 4G/5G LNA

General Purpose LNA

These type of LNAs mainly consists of SiGe LNA or Wideband LNA. The SiGe is used when there are varieties in supply currents and offers good noise figures and power gains at high frequencies due to better cell arrangement. The Wideband has a design that is used in performance-oriented BFP products and is formed with Darlington configuration.

GLONASS LNA

These LNAs require low noise along with good linearity to obtain better sensitivity for the devices for any circumstances. Smartphones are such devices that require high linearity and low power consumption as a priority. The voltage supply for these amplifiers can be in a wide range.

4G/5G LNA

These amplifiers have the utmost importance in communication systems at present and will also be critical for future improvements. These are used in devices like antennae where signal range and low noise are a top priority.

Understanding S-parameters and Stability Analysis

S-parameters or Scattering parameters are the connections between the connectivity points in the circuit of a LNA. These parameters help determine the communication signals in terms of waves other than the values in current or voltage. There are 4 S-parameters.

• S11 - input voltage reflecting coefficient
• S12 - reverse voltage gain in the circuit
• S21 - forward voltage gain in the circuit
• S22 - output voltage reflecting coefficient

Stability Analysis

In this part, the stability of a low-noise amplifier has to be measured. The measurement would be done in realistic operating conditions. The analysis follows the following steps.

1. Transmission-Line Background Check
2. Addressing the connectivity between two points of the signal network using s-parameters
3. Measuring the gain between the connectivity and comparing it with the gain needed
4. Calculating the maximum power available
5. Measuring gain in output power of the amplifier design circuit

Technology Considerations

There can be certain considerations regarding technological advancements that may comprise certain points listed below.

• Matching Networks

Networks in the connectivity of signal points are an important part of the circuit. Suppose the circuit network does not have matched impedance. In that case, there are only two ways to solve it - either modify one of the components used in the circuit or use excessive or alternative circuit parts to remove the mismatch.

• Bandwidth Optimization

A wide bandwidth or a wide range of frequencies helps out in the process of designing. Certain elements can be neglected if the range is not specific and priorities are not set for an LNA.

Conclusion

Low Noise Amplifier or LNA does have its way of improving communication systems by providing signal-powering abilities to design circuits. These LNAs must be used in most of the telecommunication or channeling services industries to gain more importance in future technologies.