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Common mode input range of opamp

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but the law of opamps says that with negative feedback, the + and - inputs will be at the same voltage.....the gain of the opamp causes this.......the opamp still has gain even if an input is above the common mode input range level.?
 
but the law of opamps says that with negative feedback, the + and - inputs will be at the same voltage.....the gain of the opamp causes this.......the opamp still has gain even if an input is above the common mode input range level.?


Look here.:D
 
Look here.

...the only mention in terms of buffers is with regard to the small input bias current......i cannot see anything there that says that a simple buffer wouldnt work even if the input was well above the common mode input voltage range.
 
i cannot see anything there that says that a simple buffer wouldnt work even if the input was well above the common mode input voltage range.
Well why do you think it SHOULD work outside the specified range? If the manufacturer says it is Vdd-1.2V then that is what you can rely on. If you use it outside this range, you will either get consistent latch up problems, or you will get random latchup to etiher of the rails, or some other weird behavior.
If you want your life easy just rely on the manufacturer specs. If you want to fiddle with it, test different batches of chips and find out their behavior across the temperature and power supply range and other key parameters, and then if you got the balls to trust your products to these tests then go ahead and use it like that, but keep in mind that the chip might change and still be in the original specs but your assumptions might not work anymore.
Or just get an opamp that really does what you expect from it.
 
Common Mode

Like I said... The input stage cant work above the common mode range so your output will be Vcc - the common mode voltage. Something like this: Not your op amp, but you get the idea. You need a rail to rail input device.
 

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so will i get latch up to one of the rails , or just clamping of the output to the common mode range limit voltage?
 
"latch up" sounds pretty evil?......you could have a temporary spike up in the voltage going into the opamp input, and then you'd be latched up forever after (or at least until the supply is cycled).
 
"latch up" sounds pretty evil?......you could have a temporary spike up in the voltage going into the opamp input, and then you'd be latched up forever after (or at least until the supply is cycled).

hi flyback,

I guess you know that most manufacturers, of any type of equipment, are 'optimistic' in their datasheet specification claims for their devices.

At the end of the day their intention is sell as many devices as possible, to a wider market as possible.

Which generally means that the performance of some devices will fall below the claimed specification.

As we 'old timers' keep telling you on this thread and your other threads regarding operating devices at the 'limits' of the claimed specification, don't do it, it will cause you grief.:rolleyes:

E
 
Some opamps have The Opamp Phase Inversion Problem where the output suddenly goes high when an input goes lower than the input negative common mode voltage limit.
 

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