velopment of the amplifier for the aerospace market presented the following challenges. Firstly, 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. Secondly, even if instability did not occur, the parasitics of the long bond wires could degrade the S-parameters.
In an effort to achieve improved stability and noise figure across the specified 18GHz to 31GHz frequency range, a 0dB 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, S-parameters and noise figure. The Mu (µ) stability factor was used to measure and compare stability, as shown in the equation below. The magnitude of µ is a measure of stability; the larger the µ factor, the more stable the device will be:
Figure 1. Input return loss—probe data
The LNA die was simply positioned in the centre 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.
To improve the stability, a 0dB 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.
Figure 2. Output return loss—probe data 42
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