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:

LNA PCB

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.

3700kHz

 

1800kHz

 

420kHz

 

160kHz

 

75KHz

 

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