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Quick question about equal input voltages for op amp?

24Volts

Mar 21, 2010
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Hello,

If we have an op amp (LM324) with rail voltages of +/- 15Vdc, and we short both inputs to +5Vdc, is it normal that we get 13.7Vdc as the output???

I read everywhere that if both inputs are equal, the output should be 0 Vdc !!!!
But here on my test bench I get +13.7Vdc ???

confused!
24v
 

duke37

Jan 9, 2011
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The op amp will have a very small offset between its inputs (look up the data sheet) which with the very high gain, will drive the output as high or as low as possible, depending on the offset.

Some op amps have two extra pins so that the offset can be adjusted to very, very small values but this is not often necessary.

In order to use an op amp as an amplifier, it is necessary to have negative feedback from the output to the - input.

Edit
I looked up the data sheet, the LM324 can have a voltage offset of up to 3mV and has a gain of 100V/mV so 0.1mV can drive to the limit.
 
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24Volts

Mar 21, 2010
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Hi Duke37,

So these op amps, internally are not perfect... so, even though I provide the same voltage at the two inputs, there is still some difference internally and therefore the amplifier will amplify this difference.

okay, thanks duke37
 

24Volts

Mar 21, 2010
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Hi Duke37,

The op amp will have a very small offset between its inputs (look up the data sheet) which with the very high gain, will drive the output as high or as low as possible, depending on the offset.

Some op amps have two extra pins so that the offset can be adjusted to very, very small values but this is not often necessary.

In order to use an op amp as an amplifier, it is necessary to have negative feedback from the output to the - input.

I apologize again for bringing up an old post... But just was wondering something about the op amp's offset based on what you said above!

So if I adjust that offset by those two adjustment pins so that my output won't fly off to the rail voltage when I set my two op amp inputs to 5Vdc, and then I use 4 resistors of equal value to accommodate a purely differential amplifier, would it mean that it this case my common mode gain would equal to 0?

Personally, I don't think so, because the common mode gain is dependent on the two gains provided by (r1/r2+r1)(1+ra/rf) for V2 and r1/r2 for V1 right! And therefore doing a careful offset adjustment of your op amp doesn't really mean that it would set the common mode gain to zero right?

Therefore they are two different things.... the offset is one thing and the common mode gain is another right?

Just asking!

Thanks
 
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duke37

Jan 9, 2011
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I think you are right.
I have never had to balance an op-amp in the hobby jobs I do, they have been adequate without balancing.
 

(*steve*)

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So if I adjust that offset by those two adjustment pins so that my output won't fly off to the rail voltage when I set my two op amp inputs to 5Vdc, and then I use 4 resistors of equal value to accommodate a purely differential amplifier, would it mean that it this case my common mode gain would equal to 0?

Yeah, maybe.

But as the temperature changes, or the supply voltage, or the phase of the moon, things will change and the output will be all over the place and eventually saturate to one supply rail or the other.

If you ever see an op-amp being used without negative feedback, you can be sure the designer is expecting the output to be saturated one way or the other.

In fact, you may see positive feedback to ensure it NEVER hangs around anywhere else but at the supply rails.
 
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