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Hi everyone I'm studying Op-Amp but I can't understand this point in the AC circuit :

In the first picture it says Vo = Ic * Rc, and in the second picture I have the value of Ic, when I try to apply this in the given formula before the answer is wrong. Why?

This second formula is used instead of the first one?

Is there is a difference ?

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enter image description here

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You're first picture has no component called \$R_c\$, but we can clearly see that \$V_o = I_c R_c\$ does not apply for the second picture, but instead it is $$V_{R_c} = I_c R_c \qquad V_o = V_{cc} - I_c R_c$$

But when doing a AC-Analysis \$V_{cc}\$ is removed, as it is a constant voltage source, and therefor \$R_c\$ is short-circuited to ground. Then you actually have \$V_o = I_c R_c\$.

Attention: AC-Analysis vs. DC-Analysis

We use DC-Analysis to find the bias-/working-point. With the small-signal-model we can use, altough being restricted to a bounded input voltage, a linear model for easier analysis of the circuit, despite the nonlinear behavior of a transistor.

Microelectronic circuits from Sedra&Smith is a very good source of information if you're new to electronics.

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