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AAOAA New Member Offline Posts: 9	Actual gain Vs. Bodeplot gain ? Nov 09^th, 2013, 7:03am Dear all, I have the gain @100Khz in bode plot almost=0dB, while the actual gain= 20 log(Vout/Vin)=20 log(3/0.005)= �dB55.56 (from transient) Could someone please explain why I have this weird bode plot gain reading? Regards
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AAOAA New Member Offline Posts: 9	Re: Actual gain Vs. Bodeplot gain ? Reply #1 - Nov 9^th, 2013, 7:05am Input transient
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AAOAA New Member Offline Posts: 9	Re: Actual gain Vs. Bodeplot gain ? Reply #2 - Nov 9^th, 2013, 7:05am Bode plot
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AAOAA New Member Offline Posts: 9	Re: Actual gain Vs. Bodeplot gain ? Reply #3 - Nov 9^th, 2013, 7:06am Schematic
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aaron_do

Senior Fellow

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Posts: 1398

Re: Actual gain Vs. Bodeplot gain ?
Reply #4 - Nov 10^th, 2013, 5:51pm

Hi,

it might be because your design is non-linear. You have driven your circuit in an unconventional way. I assume you are trying to model some kind of sensor. Your transfer function is proportional to

(100Vin + 100k)/(100Vin + 200k), whose derivative is,

10M/(100Vin + 200k)²

assuming my maths is right. From the equation, the nonlinearity shouldn't really show up, but that's the only thing I noticed that might be causing issues.

Why don't you just model your sensor in a more conventional manner (unless you are trying to see the nonlinearity)? For instance a Norton or Thevenin equivalent circuit? Also, R1 and R4 don't seem to serve any purpose.

Aaron

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tm123

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Posts: 67
Chicago, IL

Re: Actual gain Vs. Bodeplot gain ?
Reply #5 - Nov 11^th, 2013, 7:02am

I think you are incorrectly calculating the gain from the transient plot. Can you explain why you calculate the transient gain as 20*log(3/0.005)? Just trying to read some approximate values off of the plot I calculate the transient gain as ~1.5dB which matches the Bode plot pretty well. Remember the input to your circuit is the voltage source V2 not the voltage at the 'impedance' node. At least that is how the Bode plot is generated. If you are calculating the transient gain as V(out)/V(impedance) then that is not the same as the gain found in the Bode plot which is V(out)/V(CNT).

Hope this helps.

Tim

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AAOAA New Member Offline Posts: 9	Re: Actual gain Vs. Bodeplot gain ? Reply #6 - Nov 11^th, 2013, 8:37am Dear aaron_do and tm123, Both your answers fixed the problem, by putting an equivalent source to the opamp it gave the right gain now. Thanks, A.A
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