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

Discussion in 'General Electronics Chat' started by booboo, Apr 25, 2015.

1. ### booboo Thread Starter Member

Apr 25, 2015
165
2
Hi everybody
I'm a newbie and working on LM324 to learn Op-Amp. this is my circuit:

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?

2. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
DickCappels and booboo like this.

Apr 5, 2008
15,805
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Hello,

As said the gain formula is wrong.
Try to make the circuit with resistors that are 10 times bigger.

Bertus

DickCappels likes this.
4. ### booboo Thread Starter Member

Apr 25, 2015
165
2
Woops! yes. Thanks
Thanks
What do you mean "you are loading the opamp quite a bit"?
I have already searched whole of the internet but couldn't find a mathematical approach to choose the best resistor for op-amp.

Apr 5, 2008
15,805
2,389
Hello,

You have 978 + 217 = 1195 from the output to ground.
Normaly you would see values from 10K to 1M as feedback resistors in stead of the 978 Ohms that you are using.
The resistor to ground must be adapted to get the correct gain.

Bertus

6. ### Papabravo Expert

Feb 24, 2006
10,340
1,850
If the output of the opamp is at +12V, then by Ohm's Law the current from the opamp output to GND is 12V / 1195 Ω ≈ 10 mA.
Most opamps can source this much current, but are much happier with less.
Without mental gymnastics you could use 9.78 KΩ and 2.17 KΩ and the opamp would now source about 1 mA.

7. ### booboo Thread Starter Member

Apr 25, 2015
165
2
Thanks guys
I used 145.6kohm as Rf and 2.14kohm as Rin and got an acceptable output. for example 70.1mv in the input, I got 4.93v in the output.

Jul 31, 2013
196
25
Single ended supply...I admit to being quite rusty with opamp stuff but thought that in this arrangement the dc conditions should be established at the + input..usually a rail splitting pair of resistors...then the input thru a capacitor. The bottom end of the 217ohm should be via a capacitor..please shoot me down if I'm wrong

9. ### #12 Expert

Nov 30, 2010
16,704
7,351
LM324 "Common mode input range includes ground."
This is a DC amplifier circuit. No need to capacitively couple DC to the input or the ground of the feedback loop.