# Actual gain Vs. Bode plot gain?

Discussion in 'General Electronics Chat' started by Alshatri, Nov 9, 2013.

1. ### Alshatri Thread Starter New Member

May 14, 2013
4
0
Dear all,

I have the gain @100Khz in bode plot almost=1dB, while the actual gain= 20 log(Vout/Vin)=20 log(3/0.005)= 55.56dB (from the transient values)

Could someone please explain why I have this weird bode plot gain reading?

Regards

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2. ### ronv AAC Fanatic!

Nov 12, 2008
3,404
1,492
I think it is because the bode plot is expecting an input of 1 volt so the scale is wrong. I didn't draw it up and try it but I think that's it.

3. ### Alshatri Thread Starter New Member

May 14, 2013
4
0
Could you tell me please what is the 1V that are you talking about?

4. ### WBahn Moderator

Mar 31, 2012
18,093
4,918
I think the problem might be related to having a resistor whose value is dependent on the signal. I think the AC analysis linearizes things and so I don't know how it would deal with a voltage-dependent resistance.

5. ### LvW Active Member

Jun 13, 2013
674
100
Alshatri, when you like to see the gain (Vout/Vin) you must display the ratio Vout/Vin.
However, your Bode plot shows only Vout.
Only in case your AC input is 1Volt it is sufficient to display Vout (because it is identical to the gain).
Remark: I cannot see which resistor value is "dependent on the signal".

EDIT: Sorry, a sharper look on the drawing has revealed that R3 depends on the input.

Last edited: Nov 10, 2013