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LNA Design

The document discusses Low Noise Amplifier (LNA) design, focusing on the unilateral figure of merit and the implications of S-parameters in amplifier performance. It outlines methods for calculating gain, stability, and noise figure, providing examples of S-parameters and their impact on design decisions. Key concepts include the definition of normalized gain factors and the importance of matching conditions for optimal performance.

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ARUN KUMAR 22116016
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29 views14 pages

LNA Design

The document discusses Low Noise Amplifier (LNA) design, focusing on the unilateral figure of merit and the implications of S-parameters in amplifier performance. It outlines methods for calculating gain, stability, and noise figure, providing examples of S-parameters and their impact on design decisions. Key concepts include the definition of normalized gain factors and the importance of matching conditions for optimal performance.

Uploaded by

ARUN KUMAR 22116016
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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INDIAN INSTITUTE OF TECHNOLOGY ROORKEE

LNA Design

Prof. Darshak Bhatt


unilateral figure of merit

• For many transistors |S12| is small enough to be ignored,


and the device can be assumed to be unilateral.
• The error in the transducer gain caused by approximating
|S12| as zero is given by the ratio GT/GTU.

• U is defined as the unilateral figure of merit,

2
Expression for Gs and Gl

• For Bilateral case:

• For Matched conditions:

• Define normalized gain factors 𝑔𝑠 and 𝑔𝐿 as:

• Then we have that 0 ≤ gS ≤ 1 and 0 ≤ gL ≤ 1.


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• Constant gain circles for input section:

• Constant gain circles for output section:

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Attributes
∗ ∗
• Angle of circles are defined by 𝑆11 and 𝑆22
• when gS (or gL) = 1 (maximum gain)
-> The radius RS (or RL) = 0,
∗ ∗
-> The center reduces to 𝑆11 and 𝑆22
• The 0 dB gain circles where will always pass through the
center of the Smith chart.

• Example: Gain=11 dB, freq= 4.0 GHz,

• Plot constant-gain circles for 𝐺𝑆 = 2 and 3 dB, and 𝐺𝐿 = 0 and


1 dB.

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• Ans: 1) Find stability. K>1, Δ < 1,
• 2) Find Maximum gain

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7
• We choose G S = 2 dB and G L = 1 dB, for an overall
amplifier gain of 11 dB.

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Low noise amplifier design

• NF of two-port amplifier can be expressed as:

• 𝑌𝑆 = 𝐺𝑆 + 𝑗 𝐵𝑆 = source admittance presented to transistor.


• 𝑌𝑜𝑝𝑡 = optimum source admittance that results in minimum
noise figure.
• 𝐹𝑚𝑖𝑛 = minimum noise figure of transistor, attained when YS
= Yopt.
• 𝑅𝑁 = equivalent noise resistance of transistor.
• 𝐺𝑆 = real part of source admittance.

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• Converting admittance to reflection coefficient.

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• For fix value of the NF this equation will represent the circles
in 𝝘𝑆 plane.
• Noise figure parameter N is given by:

• The circle equation is given by:

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Examples

• S-parameters at 4 GHz. 𝑆11 = 0.6∠ − 60 ◦,


𝑆12 = 0.05∠26 ◦,
𝑆21 = 1.9∠81 ◦,
𝑆22 = 0.5∠ − 60 ◦,
𝐹𝑚𝑖𝑛 = 1.6 𝑑𝐵,
𝝘𝑜𝑝𝑡 = 0.62∠100 ◦,
𝑎𝑛𝑑 𝑅𝑁 = 20 .
• Assume device is unilateral, find error in GT.
• Design an amplifier with 2 dB of NF.
• Ans: 1) Find stability. K = 2.78 and Δ = 0.37
• 2) Calculate Unilateral Figure of merit

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