Designing LNAs based on PGA-103+
Many of the PGA-103+ LNA designs floating on the internet repeat the same shortcomings, so I decided to design and build better ones. The things I wish to achieve are:
- Unconditional stability. PGA-103+ is not unconditionally stable as is, as is stated in Mini-Circuits technical note AN-60-064. The remedy, an LCR series network in the input, is explained in the same application note, but many PCB designers haven't included the pads for the additional components...! I did. Quite easy.
- Proper ESD protection. Many designs seem to use the ubiquitous BAV99 diode. The guys from Texas Instruments have published an application report aptly named Does BAV99 Really Protect My System from ESD?. Can you guess the answer? Hint: No. What to use then? The answer can be found in the Mini-Circuits specification for their unconditional stable test board for PGA-103+, called TB-761-103+. You can check it here! They recommend using ESD7L5.0DT5G+ which has an effective capacitance of 0.25 pF when its two diodes are used in series. I don't know better than the engineers who made the PGA-103+ so I'll stick with their recommendation!
- Input selectivity
- Filter option
- Gain adjustment option
- Transient protection in DC bus
- Optional DC feed through coax without any circuit changes
- PCB conveniently sized to fit a cheap and readily available enclosure.
The writing will continue!
Anyways, you can see the result here, optimized for the 2m amateur band. The performance was as simulated, with a gain of 23 dB and noise figure of 0.6 dB. See the S21 plot here. The PCB was sized to fit inside and 37 × 74 mm tinplate box sold by e.g. Eisch Electronics and UKW-Berichte, but I did not want to "waste" a nice box on the first prototype. The altoids tin was a "cost reduction" option.
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Updated 31.10.2018
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