Author Topic: Operational Amplifiers: relationships between parameters  (Read 285 times)

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Offline raff5184

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Operational Amplifiers: relationships between parameters
« on: December 02, 2017, 06:24:11 am »
Hi everybody,
I'm studying opamps and I understood the theory, how they work and the parameters to describe them. But when it comes to the practice it gets a little hard especially when I need to get these information from a datasheet.

What I don't understand are the relationships between the following:
Max input signal
closed loop gain
max output swing
voltage supply

So, for example, assuming an inverting op amp, what happens if I increase the inverting gain (using appropriate resistors)? Does it "take" more voltage from the supply reducing the max input voltage I can feed, or is it independent?
Or, how does the value of max input signal that I can feed to the op amp changes with the supply voltage?
 

Offline suicidaleggroll

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Re: Operational Amplifiers: relationships between parameters
« Reply #1 on: December 02, 2017, 06:29:28 am »
The datasheet will tell you the allowable input and output voltage ranges relative to the supply rails, eg: min might be Vss+1.0V and max might be Vcc-2.0V.

If you know your supply voltage, then you can find the allowable output voltage range, then use the gain to calculate the allowable input voltage range.  Or, if you know your input/output voltage requirements, you can then calculate the required supply rails.
 
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Offline Brutte

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Re: Operational Amplifiers: relationships between parameters
« Reply #2 on: December 02, 2017, 07:52:24 am »
Quote
Or, how does the value of max input signal that I can feed to the op amp changes with the supply voltage?
I hope you understand this is opamp-specific question.

First distinction we must make is the chapter named:
a) Absolute maximum rating and
b) Operating conditions

Chapter (a) describes how to not disable the device permanently. Once you violate those rules - there is no need to read chapter (b).

Conditions from chapter (b) describe where the device behavior is qualified.

So if chapter (a) starts from: "Thou shalt not put more than Vdd on input pins." then just don't. But that sentence does not say that if you apply Vdd-1V then the opamp would amplify at all.
Such details are described in chapter (b) where it is written that: "At 25degC, Vdd-Vss=5V, common mode input voltage range that we tested is 0V up to Vdd-1.5V" for example. Then you can expect input stage to operate as intended.
Of course that sentence does not say that the inverting amplifier arrangement with +1V input would output -1V. The output stage operation is characterized with the output swing parameter (and also some plots usually). Most likely it won't output anything below Vss.



 

Offline rstofer

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Re: Operational Amplifiers: relationships between parameters
« Reply #3 on: December 02, 2017, 08:40:17 am »
When you think about it, every number in a datasheet is telling you what you can and can't do with the device.  The graphs are particularly telling.  It is up to the user to read the datasheet (completely) and understand the features and limitations.  App Notes are particularly helpful if they are available.
 


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