- #1

chaoseverlasting

- 1,050

- 3

When we set the operating point, we do it using DC voltages which sets the values I

_{c}, I

_{b}and I

_{e}.

In the ac analysis, however, we use the values of AC voltages to calculate the voltage across the output resistor. For the r

_{e}model of the transistor, the value of r

_{e}in the ac domain is calculated by [tex]r_i=\frac{26mv}{I_b}[/tex] where I

_{b}is the ac current.

However, [tex]r_e=\frac{26mv}{I_e}=\frac{26mv}{\beta I_b}=\frac{r_i}{\beta}[/tex] is calculated using the DC value of I

_{e}.

How is that possible? The DC and AC values of currents are bound to be different, but they are used interchangeably!

I thought that the net current would be the ac+dc values of the current (eg. I

_{e net}=I

_{e ac}+ I

_{e dc}).

Sorry for the long post, I couldn't help it.