LNA based on LT AN-159

[Hawaka]

Hello,

As I needed a LNA I decided to go the DIY route and to build the amplifier described in Linear Technology AN-159. I only did a few changes from the original design, but if anyone need it you'll find it here.

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!

[AnalogTodd]

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.

[Hawaka]

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.

[Kleinstein]

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.

[Hawaka]

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.

[AnalogTodd]

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.

[Hawaka]

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