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OP-AMP Minimum Gain

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didimus

New Member
Hello Guys.

I´be built an differnce op-amp. Vrey plain and simple out of the text book. I've planned to have a gain of 33 V/V (using two matched 33K and 1K resistor).

I am trying to read the voltage drop over a 0.03R shunt so i can see the current. The gain of 33 will give around the real value of the current.

The problem is when I have no current running on my shunt, I keep getting some bias ouput voltage. I though it was due to the input ffset voltage, but i set and 10k pot on my null terminals but the ouput voltage did not even move.

I am using a 741C.

Does anyone knows what that is? And how to get rid of it.?

Thanks in advance.

Did
 

audioguru

Well-Known Member
Most Helpful Member
A 741 opamp is 41 years old. It has such poor performance that it should be buried.

Many newer opamps have much less input offset voltage and input current.

But since you did not post your circuit then we are just guessing.
 

crutschow

Well-Known Member
Most Helpful Member
If you are using a difference amp to do high side current monitoring than the tolerance difference in the two sets of 33k and 1k resistors will give you an offset proportional to the high side voltage, the tolerance, and the amp gain. You said the resistors are matched. How matched is that?
 

didimus

New Member
okay.... I am using 1% resitors... Not that matched after all... However, I don't belive that to be the problem. I have played iwth the resistoe and that value seems to be there all the time. How can i check that if i am aplifying the input offset voltage.???
 

MikeMl

Well-Known Member
Most Helpful Member
The problem likely is that you are exceeding the allowed input common-mode range of the crummy 741. You would have to be using a positive supply at least 10v higher than the voltage at the shunt resistor, and likewise the negative supply would have to be several volts lower than the voltage at the shunt resistor.

Go get a modern CMOS rail-to-rail inputs, rail-to-rail output amplifier.
 

fernando_g

New Member
A 741 opamp is 41 years old. It has such poor performance that it should be buried.

Many newer opamps have much less input offset voltage and input current.

But since you did not post your circuit then we are just guessing.

I second that motion.
The 741 was an innovative device many moons ago. But it had several drawbacks including a terrible offset voltage. As a matter of fact, the device has two terminals, labeled "offset null" in an attempt to zero out the large intial offset these devices had.
 

didimus

New Member
The problem likely is that you are exceeding the allowed input common-mode range of the crummy 741. You would have to be using a positive supply at least 10v higher than the voltage at the shunt resistor, and likewise the negative supply would have to be several volts lower than the voltage at the shunt resistor.

Go get a modern CMOS rail-to-rail inputs, rail-to-rail output amplifier.

I totally agree. However, I am having a hard time selecting one for three reasons. Operating temp. (Up to 85Celsius), 16V supply and, Cheap.
Suggestions?
 
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