Electronics > Beginners

What did I miscalculated ??

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GigaJoe:
yes i'm fully understand ,now , OP07 has some behavior when input close to 0,  also output can not be lower then 0.7V ;   when opamp running in a single rail mode.

David Hess:
In the past, typically an OP90 or LT1006 would be used in place of the OP07 for single supply operation.

spec:
 + GigaJoe :D

Just read the above posts. I now see where the confusion comes from.

Just to clarify, an opamp has two supply lines:  a negative supply line and a positive positive supply line.  The negative supply line is always more negative than the positive supply line. This says nothing about the absolute value of the supply line, just the difference between the two.

So, for example an opamp's negative supply line could be -6V and the opamps positive supply line could be 6V, giving the opamp a supply voltage of 12V.

Alternatively, an opamp's negative supply line could be 0V, and the opamp's positive supply line cold be 12V, as in your circuit. Once again the opamps supply voltage would be 12V.

And, in a third arrangement, the opamp's negative supply line could be -12V and the opamp's positive supply line could be 0V. Once again, the opamps supply voltage would be 12V.

To take this to the extreme, an opamp's negative supply line could be -100V and the opamps positive supply line could be -88V. Yet again, as far as the opamp is concerned, it still has a supply voltage of 12V. 

Any opamp can be used with a single supply, as opposed to dual supply, even the classic 741 opamp, but some opamps are more convenient to use with a single supply.

To minimize the many pitfalls of using opamps, I would recommend that anyone experimenting in electronics, especially newbees, should choose a 'perfect' opamp. Of course, there is nothing perfect in electronics but an opamp with the following characteristics comes pretty close in practical terms:

[1] RRIO
[2] Practically zero input offset voltage
[3] Practically zero input bias current
[4] High output sink and source current
[5] Reasonably low power
[6] Reasonable gain/bandwidth, at least 1 MHz
[7] Stable with feedback for a gain of 1
[8] Capable of operating with a supply voltage of 5V, or less, and 32V, or more.
[9] Available in single, dual, and quad versions.
[10] Reasonable cost.

There are probably a number of opamps that meet these requirements, but the best that I have found so far is the Texas Instruments OPA192.

http://www.ti.com/lit/ds/sbos620e/sbos620e.pdf

GigaJoe:
or OPA197 ; it probably whatever didn't fit in 192, but just US$1.13.

spec:
True. The OPA197 is an OPA192 with a wider input offset voltage. Nonetheless the OPA197 would still be a good choice for a 'perfect' opamp.

By the way, this family of opamps in only available in surface mount, whereas experimenters like to have a through hole package for easy handling. The solution is simple: fit a through hole conversion header board to the surface mount package.

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