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[SOLVED] The output of LM324 Op-Amp has error as non-inverting Amp

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booboo1

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Hi everybody
I'm a newbie and working on LM324 to learn Op-Amp. this is my circuit:

**broken link removed**

One of the resistors is 217ohm and another 978ohm. then the gain should be 4.50691 as you can see. so far so good. I know this is for ideal Op-Amp. I applied 0.15v to the input and got 0.94v in the output. then the gain is 6.26667. again I applied 0.35v to the input and got 2.03v then gain should be 5.8!?o_O
again I applied 1.86v and got 10.31 then the gain is 5.54301! :shock: holy crap! why isn't the gain stable? because of input offset voltage? How can I stabilize it?
 

Your gain formula is wrong. Gain of non-inverting amplifier is R2/R1 + 1.
 
Thanks, yes. but still the output isn't acceptable IMO.
 

Your formula may be wrong but the gain value which you need to get is almost correct. When considering your problem is your input voltage is stable?
And also increase the operating supply voltage and make sure your output value should not be nearer to the operating voltage range. So try maintain some level between those two.
 

Your resistor values are very low for opamp feedback. If the input voltage is higher than maybe 10V then the opamp will have trouble driving the resistors which are overloading the output.
 

Booboo1,
The gain formula is wrong. The correct one is (1+Rfeed/Rin) as FvM told you.
Try with bigger values, for instance, if Rfeedback is 12K and Rin is 3.3K the gain will be:
Gain = 1 + (12K/3.3K) = 4.6 (Similar to the one you calculated).
 

THe problem with gain stability is you neglected the assumption that Vo is not zero ESR.
If you use the calculated gain xRin and compare actual gain. you can measure your exact driver ESR.

Check Op Amp nominal current and voltage drop to get Zout or ESR assume it will be around 150 Ohms and use Rf values 100x bigger or more.

Rail To Rail CMOS , assume is around 5k ESR.
 

booboo1, why don't you adjust the results to your need, like we all do?:)
Repeat the test with 1K resistor to the input and 1uF from the input to ground.
 
Last edited:

LM324 can source 10 mA minimum (20 mA typically), so although the feedback resistor values are a bit low, they would still avoid output current limiting. Input offset voltage explains part of the observed apparent gain error, but the 0.15V Vin / 0.94 V measurement correponds to 20 mV offset, outside the specified +/ 7mV limits. So there must be an additional source of error.

It could be superimposed input AC voltage, also self oscillation of the amplifier with inappropriate load or lack of supply bypassing.
 

Right, it's behaving as if there is a fixed 20mv offset.

What's your setup look like? Check the ground connection between your voltage source and the input in particular. Stray currents through a long wire could create that offset.
 

Some LM324's cannot pull down below 20mV (see data sheet 20mV max), so 20mV is within spec for this device, depending on the quality and exact maker of LM324A
 
Either choose an Op Amp with suitable low Input offset when using high gain or choose an offset null method

upload_2015-4-27_20-34-31.png
 

By using low input offset opamp only will solve your problem
 

LM324 or LM324A? The datasheet shows that a more expensive LM324A has a maximum input offset voltage of 3mV and the ordinary cheaper LM324 has 7mV.
 

A better input offset will only help if the opamp has a negative supply, if only 0v and Vcc you need an opamp that can (guaranteed) pull down on its OUTPUT to less than 20mV, or less than 5mV, etc depending on your requirement, actually the LM324 beats out a lot of much more expensive op-amps in this respect. But if no neg supply, then low input offset will not help... read the data sheets properly people...!
 
Hi,

If you are talking about DC input voltage, then output ESR should give no significant error (because it is a feedback circuit).

Usually the OPAMPs are better than the specifications in the datasheet.
Therefore:
- either the ground potential you refer to is not a single point (DVM connection, Input voltage source connection, feedback resistor connection should refer to a single point)
- or the DVM gives wrong values
- or you see wrong voltage readings because of DVM input resistance (for example with high ohmic voltage source to input of OPAMP)

If you have two voltmeters, then check if both show (almost) the same reading with 0V and 5V input). Then connect one to input, the other to output.

Klaus
 

KlausST said:
Hi,

If you are talking about DC input voltage, then output ESR should give no significant error (because it is a feedback circuit).

Klaus
Click to expand...

I agree. contrary to what I indicated before.

ESR loading issues with large swing signals and R values < 1K for BJT and < 10k for CMOS may cause saturation and clipping from IR drop on output thus a DMM would indicate a lower reading than expected.

this as I was trying but failed to point out.

Disregard as this is a calulation and measurement error issue for small signals with low gain and an offset error for high gain.
 

p.s. the input offset voltage x the gain will appear at the output, hence the need for low Vios opamps in some apps.
 
Let's face it guys, you can't solve this mystery. I think you should let one of your clever simulators give the final verdict.
 

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