As a short follow-up for the previous post I will show you what is the result of using the right value for R4 in the schematic, which I did wrong in the previous post. I accidentally used a 47k resistor in the collector line of the transistor where it should have been a 4k7 one.
To get it right I worked on the base side of the transistor to improve the bias. It was only at the end I discovered my mistake. Having corrected it today the result can be seen here below. The amplification factor now is 3 (50mV to 150mV).