# A question about common emitter amplifier

1. Feb 19, 2016

### Harrison G

1. The problem statement, all variables and given/known data
Hello my fellow PF mates! Today i received my electronic textbook from Russia. The book is great, however i cant explain myself one single equation, but i hope you will help me:-)

The question is about the blocking capacitor in a common emitter amplifier. R1 and R2 are the voltage divider resistors (sorry, but i still havent learned how to post images) and C is the blocking capacitor.
There is one relationship that i cant figure out.
2. Relevant equations
The textbook says that when choosing C, C>= 1/2пf(R1||R2)
My question is why C should be equal or greater then 1/2пf(R1||R2)?

3. The attempt at a solution

2. Feb 19, 2016

### Staff: Mentor

What are the major contributing factors to the input impedance of the circuit?

3. Feb 19, 2016

### Harrison G

The input impedance is 8kOhms and the frequency is 200Hz, for this is used a capacitor of 0.1uF

4. Feb 19, 2016

### Staff: Mentor

What are the two functions of the input DC blocking capacitor in the CE amplifier circuit?

http://webpages.ursinus.edu/lriley/ref/circuits/img119.gif

5. Feb 19, 2016

### Harrison G

As far as i know, the blocking capacitor have 2 main uses. 1st they block DC and allow only AC to pass and 2nd they incidentaly serve as a high pass filter and must be calculated for the working frequency so that they dont cause any trouble. But as i see now, maybe i didnt formulated correctly my question -- How do we calculate that capacitor and what its value depends upon?

6. Feb 19, 2016

### Staff: Mentor

Correct. So to be a HPF, that input C and the two input base bias resistors have to have a certain relationship. And it looks like that relationship is the equation you posted in your first post...

7. Feb 19, 2016

### Harrison G

Thank you verry much for youre help! :-)