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Op-Amp problems with the gain and the output

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sushidhar

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OP-AMP help

I hv designed a differential amplifier circuit using two OP07. This was designed to amplify a minimum of 0.01 mV.

The desired gain was 21*30(21 on one,30 on the other)
For the first Op-amp i chose MFR resistances of R1= 68 K-ohm, R2= (1000+470) K-ohm in series
For the second Op-amp i chose MFR resistances of R1= 33 K-ohm, R2= 1000 K-ohm

On circuit completion i found the gain to be 30*30 (the gain of first increased from 21 to 30 though gainof second remained same)
Secondly, when the circuit was used for amplification of the signal, i found that the output of the amplifier took a really long time to come back to its initial level (0 volts) after i had turned off the input.

For example an input of 0.01 mV gave us an output of around 9 mV but it took 10
minutes to come back to its initial level (0.00 V) after switching off the input.

PS: circuit img. is shown below 4 a single op-amp. We have just used two op-amps in series
.
img755.gif
 

VVV

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Re: OP-AMP help

When you say switched off the input, do you mean disconncted it?
If so, is there any resistance from the input to ground?
Without one, the input will be left floating, so the opamp may saturate.
 

sunking

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OP-AMP help

R is too large.
 

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