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I was planning on using a UA741CP (from TI) as a differential amplifier (to get a current value from a shunt resistor and then administer that value to the ADC of an MCU to avoid noise). However, I noticed that no matter what I did, the output voltage never seemed to go below 1.78 V (when VCC- is grounded).

I do realize that this op-amp was not specifically designed to be used as a differential amplifier, however, the manufacturer did state in the datasheet that the device was a general purpose op-amp and none of the specs mentioned addressed this characteristic. In addition, -correct me if I'm wrong- I assumed that any op-amp would be suitable as a differential amplifier as long as you bias it properly. (1k resistors on each branch would give an output of Vin1 - Vin2, unity differential gain).

Datasheet for the UA741CP:https://pdf1.alldatasheet.com/datasheet-pdf/view/25580/STMICROELECTRONICS/UA741CP.html enter image description here

supply: Vcc- = GND Vcc+ = 3.3 V

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    \$\begingroup\$ I would suggest a more modern opamp than the half-century-old 741. Many opamps these days do not have the limitations of the 741 and are probably cheaper. \$\endgroup\$ Sep 9, 2021 at 21:52
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    \$\begingroup\$ electronics.stackexchange.com/questions/304521/… \$\endgroup\$
    – pipe
    Sep 9, 2021 at 22:17
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    \$\begingroup\$ There's nothing wrong with using a 741 as a differential amp, and it was a perfectly good "general purpose" opamp when it was designed in the last century!!. But what you're looking for is a "single-supply" opamp which the ancient 741 most definitely is not. \$\endgroup\$
    – brhans
    Sep 9, 2021 at 22:46
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    \$\begingroup\$ The old 741 opamp's datasheet specs show a 30V supply but some graphs show some of them doing a little from a 10V supply. Its inputs and outputs do not work when the supply is only 3.3V. \$\endgroup\$
    – Audioguru
    Sep 10, 2021 at 2:28
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    \$\begingroup\$ @A.H.Z The reason I said that is Vcc is never negative and Vcc- is a bad label. It is Vcc and Vss \$\endgroup\$
    – DKNguyen
    Sep 10, 2021 at 14:34

2 Answers 2

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DKNguyen’s answer addresses limitations on the input voltage swing, but I believe you asked about the output swing. That’s generally specified fairly directly in the data sheet as “output voltage swing” or some such, generally qualified with load.

In the case of TI’s LM741 for example:

enter image description here

This tells you that when using +/-15V rails, you can expect the output to be able to swing (typically) to +/-14V if the load is light (10K). Under heavier load, when the opamp is pushing out more current, the swing is reduced. You can see why by looking at the LM741 output stage:

enter image description here

The output current is pushing through the resistors on the emitters and dropping more and more voltage as that current increases. Even if the resistors were zero you'd be limited by the collector-emitter saturation of the transistors.

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    \$\begingroup\$ You cannot buy an opamp with "typical" spec's. You might get an opamp with minimum or maximum specs. I always design circuits so that every part that meets minimum and maximum specs allows every product I make to work perfectly with no failures. \$\endgroup\$
    – Audioguru
    Sep 10, 2021 at 3:05
  • \$\begingroup\$ Oh oops. I totally misread \$\endgroup\$
    – DKNguyen
    Sep 10, 2021 at 14:32
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It appears differently on different datasheets. I've seen two or three ways.

Sometimes it will literally list the input common mode range relative to the voltage rails (i.e. Vcc - 3V and Vss + 3V).

Other times have to infer it because it gives the maximum and minimum input voltage in a table where the voltage rails have been specified. If it does this then you have to subtract the voltages to find how close you are to the rails and then use that value for other voltages and hope it doesn't change too much.

The one you listed is the latter. It lists +/-12V with a supply voltage of +/-15V input voltage which means the input can be within 3V of either voltage rail.

Opamps intended for differential amplification of high-side current shunts are optimized for high CMRR at a high-common mode input.

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  • \$\begingroup\$ I appreciate the tips you've given out. Is it possible for you to recommend any document that might help me understand why this happens? ( I assume that it has a relation to the class of the amplifier but I'm not really sure) . \$\endgroup\$
    – A.H.Z
    Sep 9, 2021 at 22:04
  • \$\begingroup\$ @A.H.Z I mean...that's opamp design. it's about the transistor behaviour inside the opamp. \$\endgroup\$
    – DKNguyen
    Sep 9, 2021 at 22:08

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