Aerospace & Defense Technology - February 2021 - 24

RF & Microwave Technology

A Comprehensive Way to Use Bonding to
Improve RF Performance of

LOW NOISE AMPLIFIERS
Introduction
Is RF engineering a form of black magic? This article
analyzes the challenges faced in the development of the
ADH519S, an 18-GHz to 31-GHz low noise amplifier (LNA)
for the aerospace market. The die used in the product development for space was originally released to the commercial
industry in the LC4 package. To release this product for the
space and high-reliability market and comply with MIL-PRF38535 standards, this part was assembled using the most
suitable and available hermetically sealed ceramic package.
This article presents a unique solution and process education
to improve RF performance via bonding. This product development process presented the following challenges:
* The most suitable and available space-qualified hermetically sealed ceramic package had a large cavity in relation
to the die in the originally released package. The larger
cavity drove the need to double the length of bond wires,
which, when combined with the parasitics of the new
package, had the potential to cause device instability.
* Even if instability did not occur, the parasitics of the
long bond wires could degrade the S-parameters.
This article reviews the different methods used to overcome these challenges and how to achieve the best stability and noise figure performance from the new hermetically sealed ceramic package.

The four LNA engineering types are:
Eng1 - The LNA die was simply positioned in the center
of the package and wire-bonded with double round bond
wires. As expected, due to package parasitics and bond
wires, the μ stability factor across the specified operational
frequency range was less than 1 and even close to 1 at some
frequencies. To achieve stability across the frequency
range, improvement was needed for the input return loss
(S11). This would require reducing the parasitics to the
input of the LNA. This resulted in the development of
Eng2.
Eng2 - To improve the stability, a 0-dB attenuator was
added to the input of the LNA. Adding the attenuator at
the LNA input improves the input matching, resulting in
improved input return loss (S11). As a result, bond wires
were also shortened, which contributed to reduced parasitics. Input return loss was improved but as a result of the
current and thermal noise of the attenuator passive components, the noise figure did not meet specifications for
this device. To improve the noise figure, configurations
Eng3 and Eng4 were designed and evaluated.
Eng3 and Eng4 - In both circuit types, the attenuator
was placed at the output of the LNA die to improve noise
S11: Probe Data
1
Probe Eng2-Unit#1

Project Description and Design

=

| s *11

1 | s11 |2
>1
s22 | + | s12 s21 |

24

Equation 1

Probe Eng2-Unit#2
Probe Eng3-Unit#1

Input Return Loss (db)

In an effort to achieve improved stability and noise figure across the specified 18-GHz to 31-GHz frequency
range, a 0-dB passive attenuator was integrated into the
package to shorten the RF in/out bond wire length.
Four different types of circuit configurations were built
during the engineering phase and compared in terms of
critical parameters of an LNA, which includes stability, Sparameters, and noise figure. The Mu (μ) stability factor
was used to measure and compare stability, as shown in
Equation 1. The magnitude of μ is a measure of stability.
The larger the μ factor, the more stable the device will be.

-2
-4

Probe Eng3-Unit#2
Probe Eng4-Unit#1

-6

Probe Eng4-Unit#2

-8

-10
-12
-14
-16
18

20

21

22

23

24

25

26

27

28

29

30

31

32

Frequency (GHz)

Figure 1. Input return loss - probe data.

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