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a question about rail to rail op

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zxasqw123

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hi everyone
i have designed a rail to rail op as a unit gain buffer stage ,then i find my output voltage is a little larger than my input voltage ,i do not know why?
my gain is about 60DB-70DB,and the device works well
Bcz I think Vout=Vin*A/1+A,then my output voltage should be a little smaller than my input voltage ,and my devition is about 1/A*Vin.
so can anyone tell me what is wrong?
thanks in advance
 

A.Anand Srinivasan

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how much is the difference that you find,... what is the opamp you are using...
 

zxasqw123

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thanks,0.5%--1%larger than Vin,my OP is a rail to rail op,it can works in common voltage about 1V---12.5V,and my supple voltage is 13.5V,but the question is in my common votage range ,my input voltage is smaller than my output voltage,i am really confuse ,i can not understand what is devison depend on?thanks
 

Humungus

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If your input signal is close to the amplifiers GBW, as you have it in follower configuration you could be suffering from a peaking effect. In that way, the gain could be higher than 1. This peaking effect could also influence the frequency response to quite low freqsuencies.

It could also happen that you are experiencing a very low frequency oscillation. This could be due to your test setup. I observed in one of my past test setups some nulling amplifiers I used make the test loop to oscillate to some mHz. If you are entering in this case a DC voltage, the by chance you could have read the output at a positive peak.
 

DenisMark

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Finite gain can cause the difference between input and output of unit gain buffer, but there are another reason. Do u consider offset? It looks like ur opamp has systematic offset.
 

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