Hi. The book i'm reading is just going over the basics of a BJT amplifier using the circuit below. It says the ac voltage is superimposed onto the dc bias voltage by capacitive coupling, but it doesn't seem correct to me. I've read some bits on the internet about what capacitive coupling is and I think I understand it. I also read the AAC info on biasing techniques http://www.allaboutcircuits.com/vol_3/chpt_4/9.html which has a similar circuit to the one below, but which makes sense to me. For this circuit below, I don't get how can be the ac voltage shown. Using superposition is just isn't it? Also, I don't really get the purpose of capacitive coupling here. Why not just put the ac and dc sources in series with one another and , and leave out the capacitor altogether? Could someone explain it to me please? Thanks!