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OP AMP Basics

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Suraj143

Active Member
Hi guys I’m very new to opamps. I have read many articles & there are some doubts I cannot clarify y myself. So I need your help to sort out those doubts.

A non inverting gain formula is given like this (Vg=1+RA/RB)

Is this formula given to calculate the output volts or just to calculate the gain?
 

kchriste

New Member
Forum Supporter
Both: if you know the input voltage. ;)
 

Suraj143

Active Member
Thanks.

If my input is 20mV I cannot apply to the formula.But I know the gain if it is 5 then output will be 100mV.

Then how do I apply the input voltage to the formula?
 

MikeMl

Well-Known Member
Most Helpful Member
If the gain is 5, then Ra/Rb must be 4, which you didn't ask.

Vout=Vin*Gain = 20mV * (1+Ra/Rb) = 20mV*5 = 100mV

ps, where is South Mald Island?
 
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Suraj143

Active Member
Hi MikeMl now I understood well.Thanks for the explanation.

Now the thing is if I apply 0V what will be the output in a gain 5 of opamp? My opamp is single supply not dual.
 

MikeMl

Well-Known Member
Most Helpful Member
Unless you are using a Rail-to-Rail opamp, with zero volts at the input and on the Vss pin of the opamp, the output will be sitting tens to hundreds of mV. Even a RR opamp cannot pull to zero volts, close but not zero.
In most opamp applications, you bias the input to halfway between Vss and Vdd of the opamp, and call that "zero". Then the opamp output can swing both above and below zero:D
 
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Suraj143

Active Member
Hi MikeMl thanks for be with me. I have more doubts sorry for that.

Rail-to-Rail opamp is a new word to me. I think this a dual supply configuration in opamps.

That means if I have a gain of 5 with a single supply voltage if I give 0V to the non inverting pin then there still few mV in the output.
 
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Suraj143

Active Member
Now I can use single supply & amplify a DC input signal nicely.But the input is nearly "zero" then there is still voltage on the output this is a small error thats ok.

But whats the advantage of using rail to rail opamps? Don;t know whats the meaning either :rolleyes:
 

MikeMl

Well-Known Member
Most Helpful Member
Here is the data sheet for a typical modern CMOS rail-to-rail opamp made by TI. Note the section that talks about how close the output can swing to Vdd- and Vdd+. Which opamp where you planning on using?
 
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kchriste

New Member
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The "rails" are the most positive and most negative supply voltages connected to an OpAmp. To say "rail to rail output" refers to the fact that the output of the OpAmp can get very close (Within millivolts) its power supply rails. A LM358 cannot get that close to the + supply rail. ie: if the supply rail is 12V then the output can only rise to apx 10.5V until it saturates goes no higher.
 
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MikeMl

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An LM358 is half a rail-to-rail op amp:D

It is capable of pulling its output almost all the way to ground, called the negative "rail" (See VOL in the data sheet), but it can only pull its output about 1.5V below the positive rail (see VOH in the data sheet). The LM358 inputs also work near ground, but not when they are near the positive rail (see Common Mode Range in the data sheet).

Compare these specs to the CMOS opamp I gave you the link to above.
 

Suraj143

Active Member
Ok guys now I understood what is "rail to rail".

It means the fact that output of the opamp can get closer to its power supply rails.

MikeMl

LM358 VOL = 20mV that means when single supply mode, the minimum the opamp can pulled its output to ground is 20mV.If 0V is applied to the input then the output will still stay in 20mV.

Even if I use dual supply then it has 20mV gap with its negative power rail.

Am I correct? OK I’m going to compare the datasheets.
 
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MikeMl

Well-Known Member
Most Helpful Member
Ok guys now I understood what is "rail to rail".

It means the fact that output of the opamp can get closer to its power supply rails.

MikeMl

LM358 VOL = 20mV that means when single supply mode, the minimum the opamp can pulled its output to ground is 20mV.If 0V is applied to the input then the output will still stay in 20mV.

Even if I use dual supply then it has 20mV gap with its negative power rail.

Am I correct? OK I’m going to compare the datasheets.
By Jove, I think he's got it :)

Rail to Rail applies to both inputs and outputs of opamps. Some are R-R on input, some are R-R on output and some are R-R on both.
 

Suraj143

Active Member
ok for today I'll go through what I have studied.& begin some new questions tomorrow :D

I don't know why people specially points its a rail to rail opamp,because I see all opamps are designed to work with dual supplies.Then they all have rail to rail capabilities.
 

audioguru

Well-Known Member
Most Helpful Member
I see all opamps are designed to work with dual supplies.Then they all have rail to rail capabilities.
No.
Most opamps have inputs that do not work properly if their voltage is closer than about 3V from a supply rail.
Also, most opamps have darlington emitter-follower outputs that do not get closer than about 1.2V from a supply voltage.
 
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