role of capacitor in ac circuits

[Rohitesh]

why do we use capacitor in transistor circuit with input ac signal?
how capacitor block dc current and allow to pass ac current?

 

[devilwar]

XC = 1/(2pi(f)C) ------ > capacitive reactance.

so you can see that XC is inversely proportional to frequency f. Now for DC f=0. Hence XC is infinite(theoritically). Hence it blocks DC

---------- Post added at 02:04 ---------- Previous post was at 02:02 ----------

When we pass any DC value to the input of transistor, the biasing of the transistor changes hence the q-point may shift and you may not get proper amplification. Hence we use a capacitor to filter out dc....