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[SOLVED] Opamp vGain lost some certain level of voltage

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thannara123

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Opamp Gain clip the output some certain level of voltage

As a part of one of my project , am using with a differential op-amp LM741,as following circuits
Capture.JPG
I am giving input 0 to 230 volt ac (50Hz) directly .
In this circuit .the output will be voltage clip some certain level (about 500millvolt) see the pic below
ac.JPG
what is the problem ?
I am attaching the proteus simulation file below
https://www.edaboard.com/attachment.php?attachmentid=144320&stc=1&d=1517740192

I need the output if 1 volt at the input the output will be 5millivolt (ignored the level shifting dc).
 

Attachments

  • Ac Voltmeter.rar
    20.5 KB · Views: 128

Hi,

A true differential amplifier circuit needs equal conditions on inverting and non inverting side.
But yours is unsymmetric.

The capacitor at the input side and the 50k feedback...
Make it symmetric before next simulation.

Klaus
 

Hi,

A true differential amplifier circuit needs equal conditions on inverting and non inverting side.
But yours is unsymmetric.

The capacitor at the input side and the 50k feedback...
Make it symmetric before next simulation.

Klaus

unsymmetric mean ? is it the dc level voltage at the non inverting terminal ?
May i get the solution ?
 

Hi,

How to get the solution: Just look at any differential amplifier circuit.
There usually are 4 resistors:
* 2 input resistors are equal. In your case it should be 1M , 1M , C, 1k
* and the other 2 resistors are equal. In your case 50k

But you use 1M, 1M, C, 1k....and the other is 1M, C , 1M, 1K ....this is not equal.
And the other is 50k at one side .... and something totally different at the other side.

****
BTW: your input voltage masurement can't work either, because the source is floating. Neither side is connected to GND.
But the scope measurement is GND referenced

Klaus
 
The design of the 741 opamp you are using is 50 years old! It was one of the first opamps and its input resistance is only 300k ohms minimum. For your circuit to have 1M resistors as passive filters then you need a modern opamp that has extremely high input resistance.

It is too bad that your schematic has a grey background with a crosshatch grid and almost every wire crosses over other wires instead of being straight. Is it Multisim that has <TEXT>, <TEXT>, and <TEXT> and the other problems all over it?
 
The design of the 741 opamp you are using is 50 years old! It was one of the first opamps and its input resistance is only 300k ohms minimum. For your circuit to have 1M resistors as passive filters then you need a modern opamp that has extremely high input resistance.

It is too bad that your schematic has a grey background with a crosshatch grid and almost every wire crosses over other wires instead of being straight. Is it Multisim that has <TEXT>, <TEXT>, and <TEXT> and the other problems all over it?

I am using Proteus ,i plan to use lm358 instead of 741 but unfortunately ,lm358 model is not available in Proteus.

- - - Updated - - -

The solution was already given in one of your last threads. Review post #8 of that thread and see that feedback resistors have the same values.
https://www.edaboard.com/showthread.php?t=374236
Sorry i didnt get you ,the said post saying about burda desisyon ee
 

I notice that the circuit attenuates the high AC input voltage, and does not amplify it. Then the old opamp with the fairly low input resistance will work fine.
I agree that its inputs should be symmetrical and balanced.
 
Hi,

The correct circuit is shown many times in the internet. Also as proteus schematic With explanations.
But obviously in post1 the circuit is modified...and now it doesn't work anymore.
Not sure who modified the circuit and why.

Klaus
 

I notice that the circuit attenuates the high AC input voltage, and does not amplify it. Then the old opamp with the fairly low input resistance will work fine.
I agree that its inputs should be symmetrical and balanced.

If I lower the input impedence it works fine upto some voltage level
But at not my required level [as it is 230 volt]
when the voltage going from a paricular level it clips

if anyway
the problem is
it clips the output after 500-620millivolt (if i changed the desgin according to the previous posts)

- - - Updated - - -

Yes its works The problem is the input resistance ,I changed the Operational amplifier to LM258. Thanks
 

The old 741 opamp was designed to use only a 30V (plus and minus 15V) supply. Your schematic does not show a supply voltage. What is your supply voltage for the opamp?
The LM258 works with a supply voltage as low as 3V.
 

I am using Proteus ,i plan to use lm358 instead of 741 but unfortunately ,lm358 model is not available in Proteus.
Are you sure?
 

Attachments

  • 358.png
    358.png
    36 KB · Views: 111
Hi,

Without symmetric resistor values ... it my work in simulation, because your simulation doesn't care about common mode (AC) voltage.
Don't be surprised if your circuit doesn't work satifactory in real word.

Klaus
 

The old 741 opamp was designed to use only a 30V (plus and minus 15V) supply. Your schematic does not show a supply voltage. What is your supply voltage for the opamp?
The LM258 works with a supply voltage as low as 3V.
My supply voltage is 12 volt

- - - Updated - - -

Are you sure?

I tried with some model but in proteus i didn’t find the MODEL ,I will try when i reach at home thanks

- - - Updated - - -

Hi,

Without symmetric resistor values ... it my work in simulation, because your simulation doesn't care about common mode (AC) voltage.
Don't be surprised if your circuit doesn't work satifactory in real word.

Klaus
What is the meaning symmetry ,In both terminals have same resistor ?
I am referring the circuits follows http://microcontrollerslab.com/ac-voltage-measurement-using-microcontroller/


 

Hi,

I wonder ... you don't know the meaning of "symnetric"?

-->
R1 = R4
R2= R5
R3 = R6
R7= R8
C1= C2
C1 connected beween R2 and R3
C2 connected between R5 and R6
(Naming according schematic of post #15)
Red marked are your mistakes shown in post#1

Both errors lead to bad common mode suppression (CMRR) ... making the circuit to fail in real world.

Klaus

Added:
The article is good. But you need to read through it carefully.
I agree with the author saying "I will recommend you to Google about difference amplifier and its use."

Klaus
 

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