Designing for Maximum Gain in LNA

Designing an LNA (Low-Noise Amplifier) for maximum gain is a critical aspect of RF and microwave circuit design. Achieving high gain is essential for amplifying weak signals while minimizing noise contributions. In this blog post, we will explore the design procedure for maximizing gain in LNAs, along with its implications and drawbacks.

Understanding Stability

Before optimizing for maximum gain, it’s crucial to ensure the stability of the transistor amplifier. Stability analysis involves determining the stable regions for the reflection coefficients at the input (Γs​) and output (ΓL​) ports using techniques such as stability circles on the Smith chart.

Design Procedure

When designing for maximum gain, the input and output matching sections must be carefully designed to achieve optimal impedance matching and maximize power transfer. The design procedure typically involves the following steps:

1. Check Stability and Stability Circles:

• Determine stability and identify the stable regions on the Smith chart using stability circles or other stability criteria.

2. Obtain S11​ and S22​:

• S11​ and S22​ are readily available from S-parameter measurements or simulations.

3. Calculate Γs​ and ΓL​:

• Using S11​ and S22​, calculate the reflection coefficients at the input and output ports.

4. Design Input and Output Matching: Design matching networks at the input and output ports to achieve maximum gain. For maximum gain, S12​=0, simplifying the design process.

What if S12​≠0?

In cases where S12​ is not zero, additional considerations are required:

1. Check Stability and Stability Circles:

• Repeat stability analysis to ensure amplifier stability and identify stable regions.

2. Calculate Γs​ and ΓL​:

• Calculate reflection coefficients at the input and output ports considering the non-zero S12​.

3. Design Matching: Design input and output matching networks considering the non-ideal S12​ to optimize gain while maintaining stability.

Drawbacks of designing for maximum gain in LNAs:

While maximizing gain is desirable, it comes with certain drawbacks:

1. Narrowband Frequency Response:
   • When designing for maximum gain, the amplifier is typically optimized for a specific frequency or narrow frequency range. As a result, the amplifier’s frequency response becomes narrowband, meaning it provides high gain only within a limited frequency range.
   • This narrowband frequency response can be problematic in applications where signals span a wide frequency spectrum or where frequency agility is required. In such cases, the amplifier may not adequately amplify signals outside its narrowband operating range, leading to reduced performance or signal distortion.
2. Limited Bandwidth:
   • The narrowband frequency response of an LNA designed for maximum gain inherently limits its usable bandwidth. Bandwidth refers to the range of frequencies over which the amplifier can effectively amplify signals while maintaining desired performance metrics such as gain and noise figure.
   • Designing for maximum gain often sacrifices bandwidth in favor of optimizing gain at specific frequencies. This limitation restricts the amplifier’s versatility and applicability in systems requiring wideband operation or signal coverage across multiple frequency bands.
3. Higher Noise Figure (N):
   • In LNA design, maximizing gain typically involves trading off other performance metrics such as noise figure. Noise figure (often denoted as N) quantifies the amount of noise added by the amplifier to the input signal and is a critical parameter in low-noise applications.
   • Maximizing gain may lead to higher noise figure, especially if the amplifier is operated close to its saturation point or if additional gain stages are cascaded to achieve higher overall gain. The higher noise figure compromises the amplifier’s ability to amplify weak signals with low noise, reducing its overall sensitivity and signal-to-noise ratio.

Conclusion

Designing for maximum gain is essential for achieving high-performance LNAs. However, it’s important to balance gain with other factors such as bandwidth and noise figure. In the next section, we will explore how to design for less than maximum gain, with a focus on improving bandwidth and minimizing noise. Stay tuned for more insights into LNA design optimization!

Learn more about this topic by taking the complete course ‘Microwave Amplifier and Low Noise Amplifier (LNA) Design Theory and Principles online course – RAHRF526’. Watch the course videos for more detailed understanding. Also checkout other courses on RF system and IC design on https://rahsoft.com/courses/. Rahsoft also provides a certificate on Radio Frequency. All the courses offer step by step approach.