There are a couple of things I dont understand about the DA in general and when we use it in the common mode in particular (same input ac voltage to both input terminals). Im using Electronic Circuit and Theory by Louis Nashelsky and Robert Boylstad (p.591 for DA).(adsbygoogle = window.adsbygoogle || []).push({});

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 couldnt help it.

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# Differential Amplifier Common Mode Problem

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