LNA based on LT AN-159 - Page 1
Forum section: electronics / projects

author: Hawaka
author_rank: Regular Contributor
timestamp: February 10, 2024, 10:28:03 am
content: Hello,As I needed a LNA I decided to go the DIY route and to build the amplifier described inLinear Technology AN-159. I only did a few changes from the original design, but if anyone need it you'll find ithere.I am currently not able to test the full performance (and it would probably need more shielding), but it already has proven helpful into measuring power supply noise.Enjoy!

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 14, 2024, 02:12:38 pm
content: Always good to know people find this useful.Speaking of noise measurement, have you tried shorting the input through something like a 1k resistor and seeing what you get? You'll find that you can actually measure the thermal noise of resistors, I actually saw where someone did it with a 50ohm resistor and was able to get numbers that matched theoretical closely.

author: Hawaka
author_rank: Regular Contributor
timestamp: February 14, 2024, 05:12:03 pm
content: I haven't. I am also pretty limited in term of measuring device, as I only have an Analog Discovery 3, but I might try it next time I take it out.

author: Kleinstein
author_rank: Super Contributor
timestamp: February 14, 2024, 05:52:45 pm
content: A test with a resistor at the input is a good idea. Besides the resistor noise one can also see a contribution of the current noise of the amplifier, especially with a rather large resistor.Given the BJT based front end with quite some current the current noise can be quite large and the impedance for the best noise figure is relatively low. So it may already be with a few 100 ohms that the current noise will dominate. At some 50 ohms all 3 parts (current noise, amplifier voltage noise and the resistors thermal noise) may be comparable with maybe the resistor noise as the largest one.

author: Hawaka
author_rank: Regular Contributor
timestamp: February 15, 2024, 01:06:12 pm
content: I had a try but turns out that it is not so easy to do noise measurement so low. Proper cable and easy way to connect a resistor without creating a big loop is important.I have ordered some cables and connectors, I will report back if I can make some meaningful measurements.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 15, 2024, 02:13:08 pm
content: I know measuring a 50ohm resistor is tough, where I saw the claim of doing that is here:https://github.com/Floyd-Fish/Noise-Amplifier.I do know when I was designing this I took a BNC plug and soldered a 1k resistor across it and was able to measure thermal noise that matched theoretical.

author: Hawaka
author_rank: Regular Contributor
timestamp: March 09, 2024, 01:37:13 pm
content: I got one of those BNC adaptor of aliexpress and tried to measure some thermal noise.Analog Discovery 3 + BNC board + LNA on 100kHz BW. Measure is AC RMS.- short : 0.18uV- 10R : 0.23uV (Johnson 0.12uV)- 51R : 0.37uV (Johnson 0.29uV)- 100R : 0.46uV (Johnson 0.4uV)- 10k : 2.2uV (Johnson 4uV)- 100k : 1.6uV (Johnson 13uV)It's pretty good for low resistance values

author: Smokey
author_rank: Super Contributor
timestamp: March 20, 2025, 09:20:06 pm
content: @AnalogToddDude. That's a killer app note.If you have a minute, I would love to know what went into creating that. Was it your idea or did LT do something like RFP for app notes? How many revisions did it take before you were happy with the results? Stuff like that.Also, it has always struck me that some of the devices like this that are created for app notes are things that people would buy. Was there any talk of using your design as the basis for a thing to sell?

author: AnalogTodd
author_rank: Regular Contributor
timestamp: March 21, 2025, 02:16:21 am
content: Quote from: Smokey on March 20, 2025, 09:20:06 pmThank you for your kind words. That App Note was definitely something I enjoyed doing.The idea for it came about as we were designing our first ultra-low noise LDOs at LT; we needed a way to verify that the noise we quoted was real and there wasn't a solid piece of equipment out there to make the measurement. Add to that the LT mentality that we should be able to verify measurements through multiple methods and we needed a circuit like this which could be plugged into a spectrum analyzer, RMS voltmeter, or oscilloscope and have measurements that reinforced each other. I originally decided to take it on as a side project to make this happen.The circuit design took a couple months in itself on top of my regular projects. I started by copying some of the work Jim Williams had done in another App Note (designers are known for copying useful circuits!) and adjusted as needed. Our focus at LT was to make this something that could be replicated by our customers, so it had to all be commercially available products to create it. We didn't want to sell the App Note design to people, but instead showcase the places where our product performance was stellar and help people understand what was needed to make such difficult measurements. Once I was satisfied with the overall performance of the circuits, then came the period of writing it all up.Before I started writing I actually spent a lot of time reading Jim Williams App Notes. I wanted to get a feeling and understanding as to why everyone found his writing so compelling and try to make mine draw people in as best I could. Writing everything up actually took me close to a year as I did it in tandem with my regular projects. Personally, this worked well as I could write for quite a while, set things down and walk away from it for a good period and come back to review what I had written. When you write non-stop and constantly review it, you often make mistakes by seeing what you expect to have written instead of what you actually wrote. I lost count of how many times I made revisions.I also had the distinct honor of allowing many of my colleagues a chance to review it before publication. Many of these people were titans in the analog design world, I think I was most floored when I sat down after Bob Dobkin had reviewed it to hear his critique and to have him praise what I had created. Their criticisms were honest and often worked into the final product. I did presentations on the App Note when our FAEs came into town and was flattered to have massive turnout for them. I think one of the best feelings I have had since was starting a new job and chatting with an Applications Engineer who had been tasked with doing noise measurements; he said he found this great App Note out there that helped him immensely, went over to his desk, and pulled out my work. I could only smile as I told him I knew it all too well as I had written it.At LT we sent built up copies of the board to some key customers who wanted to do the measurements for themselves. We used it as a sales tool by having a side-by-side comparison of our ultralow noise LDOs next to a previous generation and had a simple switch between the two outputs. The output of the LNA was simultaneously run into an oscilloscope as well as powered speakers and you could both see and hear the difference in noise performance. I know I heard from the people who went out on customer visits about both the part and App Note--they would tell the customer about the great noise performance of our part and would be asked, "How the hell do you measure that?!?" The response was to give them a copy of the App Note and say, "I'm glad you asked..."I still feel a lot of pride about it.

author: Sariel
author_rank: Regular Contributor
timestamp: March 21, 2025, 01:22:09 pm
content: Quote from: AnalogTodd on March 21, 2025, 02:16:21 amTodd, I enjoyed what you wrote tremendously! It is insipring!You should the exact text on Linkedin as well. Many will eagerly want to read it and get inspired.