# CE amplifier gain question

Thread Starter

#### ECE101

Joined Nov 25, 2011
29
Hi,

I'm just looking to verify something

From a mathematical point of view:

Gain = -gm*Rc

gm = Ic1/Vt
Rc = 8k

When Vo = +15

AV = -((15/8*10^3)/25*10-3) * 8*10^3
= -600

When Vo = -10

AV = ((10/8*10^3)/25*10-3) * 8*10^3
= 400

Is this correct?

note to mods:

I posted this thread here: http://forum.allaboutcircuits.com/showthread.php?t=63201 however I think it belongs in Homework help.
If you could delete/merge the other one that would be great.

#### Jony130

Joined Feb 17, 2009
5,211
Your answer is wrong.

The voltage gain is equal to:

$$AV = Rc * \frac{ gm}{ gm * Re||R3 + 1 }\approx \frac{Rc}{Re||R3}$$

#### Adjuster

Joined Dec 26, 2010
2,148
Note also that Vout swings up and down because Ic varies, not Rc. Correcting for this will lead to a completely different answer to your present one.

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