Audio frequency amplifier and the purpose of the capacitor?

Question

Electronics

I read the article - habrahabr.ru/post/142012 about ultrasound.
But I don’t understand in any way, the properties of the capacitor under No. 2: why does it pass two half-waves of a sinusoid?
After all, it weighs simultaneously in the DC circuit ...
And why during positive half-cycles, when the positive input Uin and the voltage of the power supply GB are summed up at the base, the circuit currents increase. How do they sum up if he does not miss it ...

Answer 1

why does it skip two half-waves of a sine wave?

Because it's a capacitor - it passes alternating current (starting from a certain frequency) and does not pass direct current.
the power supply voltage enters the base through the resistor R2, the input voltage (its variable component) through the capacitor C2.