# Electronic – Common emitter with low voltage input

I have to do a project involving a common emitter amplifier. The only given data is: the amplitude of the input signal is 100u, and offset voltage is 0.

I need that signal to be amplified. But my circuit won't amplify the signal, instead it will add offset voltage or even attenuate it.

What have i done wrong? Any ideas how to dimensionate the elements ( if that's the problem ) ?

Start with large signal calculations for the bias, eg. collector current. I suggest something in the 100uA to 1mA rang for starters if you have no idea.

Yo want there to be some room for the collector voltage to swing around so your collector voltage should be somewhere near the middle of the emitter voltage and 5V.

You want the voltage across the emitter voltage to be large compared to Vbe for stability with temperature etc so maybe pick 1V, which means the collector voltage should be around 3V.

Since you know the current and you can assume Ic~=Ie you can can calculate R2 and R4.

For R1, R3 you want Ib * (R1||R3) to be << 1V so the beta has little effect but you also want them to be as high value as possible so as not to load the input. The ratio should be (5-1.7)/1.7 to give you 1.7V for the base (and thus 1V at the emitter).

Check that Xc << R1||R3 so the coupling cap is high enough.

You will find that the gain varies with your choice of Ic and the load resistance. Gain = gm * R2||R5