Adjustable gain LNA

Low noise amplifiers are very useful. I built one with single BFG425W transistor with adjustable bias current. The schematic diagram is very simple:

LNA schematic diagram

and the PCB is 20x24mm:


I first soldered prototype with smallĀ  islets cut from FR4 PCB:

First prototype

Preliminary measurements were promising. Then I etched final design (here is 1:1 artwork for toner transfer) and assembled the module. Here is comparison between prototype and final module.

LNA Final design (left and front)

I added small metal cap to cover the amplifier active part. It was salvaged from an old mobile phone.


Here are measurements:

High gain


Mid gain


Low gain


The amplifier is intended for lower bands including VLF. I tested the lowest usable frequency, which turn out to be 75kHz. Such amplifier is therefore suitable also for amplifiaction of the e.g. DCF atomic clock signal.











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