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

[booboo1]

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!?
again I applied 1.86v and got 10.31 then the gain is 5.54301! holy crap! why isn't the gain stable? because of input offset voltage? How can I stabilize it?

 

[JimB]

Try drawing a graph of Input Volts vs Output Volts, is it linear?

Consider that you may have the wrong expression for the the gain. Gain = 1+ R2/R1 may give better results.

JimB

 

[Tony Stewart]

Try scaling up your values x10 to x1000 and try again

 

[jjw]

The gain is 1+R2/R1 ~ 5.5
Input offset voltage is max. 5mV and it could explain the results.

 

[Tony Stewart]

The gain is 1+R2/R1 ~ 5.5
Input offset voltage is max. 5mV and it could explain the results.

5mV * Av only explains an error of 5.5*5mV= 27.5mV error which on 1V out is only 2.7%

 

[crutschow]

You should increase the resistor values to that their sum is at least 2kΩ.

 

[booboo1]

Try drawing a graph of Input Volts vs Output Volts, is it linear?

Consider that you may have the wrong expression for the the gain. Gain = 1+ R2/R1 may give better results.

JimB

What would happen if that would not be linear?

 

[booboo1]

Your false assumption is the output resistance of Op Amp is 0, which it is not, so that must be considered when choose Rf.

Try scaling up your values x10 to x1000 and try again and observe Vo vs Io to compute ESR or see Imax and plan on using much less.

From your data, you can now compute internal ESR of OA output.
got it?

if not...
Calculated gain is 1 + Rf/Rin = 5.5069124423963133640552995391705
using 1% R values this becomes 5.507 +/-2%

You got applied 0.15v to the input and got 0.94v in the output. then the gain is 6.26667.
which using Rin=217
Av(-) = 5.267 Rf= 5.267 * 217 = 1143 Ω
But your design used 978 Ω,

so the Op Amp ESR must be 1143-978 = 165 Ω but since you used DMM to measure R accuracy is 0.1%

which is typical for an op Amp. Rail to Rail CMOS types are much higher (kΩ)

WoW! Thanks for great post.
How the "output resistance" could influence on choosing Rf?
Totally my big problem is that I don't know how to choose the best resistor for my op-amp and my job (Rin and Rf)?

 

[booboo1]

You should increase the resistor values to that their sum is at least 2kΩ.

Thanks
The most common resistors for an Op-Amp is 100k and 1k. but I don't know how to choose the best resistor for my job.

 

[JimB]

What would happen if that would not be linear?

You thought the gain was varying.
I plotted your values on a graph, it was linear, so the gain was constant, but there was an offset.

Try some higher values of resistors as other have suggested, do some more measurements and see what happens.

JimB

 

[booboo1]

You thought the gain was varying.
I plotted your values on a graph, it was linear, so the gain was constant, but there was an offset.

Try some higher values of resistors as other have suggested, do some more measurements and see what happens.

JimB

I used 145.6kohm as Rf and 2.14kohm as Rin and got acceptable output(gain=69.03). for example 70.1mv as input, got 4.83v

 

[audioguru]

The output resistance (ESR) of the opamp affects the gain only a small amount since the open loop gain of the LM324 opamp is typically 100,000 at DC and low frequencies. The circuit's negative feedback effectively reduces the output resistance to almost zero.

 

[Mosaic]

I guess you can treat it as low impedance once you remain within the spec'd output load current.

BTW, if you're using a high impedance voltage divider on the OPA input and a large gain, you can end up magnifying the input offset current and input bias current voltage (I x R) errors.