# Trouble with differential amplifier output

Discussion in 'Homework Help' started by paddy611, Oct 9, 2012.

Oct 9, 2012
25
0
Hello,

I am using LM358 op-amp in my circuit.

one op-amp is used as differential amplifier and next is used as an non inverting amplifier.

But the problem is the output of the differential amplifier (at pin no.7) is varying from 0.7V to 2.3V and i need it from 0 to 2.3V.

I have attached the circuit.

thanks,

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2. ### Audioguru Expert

Dec 20, 2007
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The (+) input pin3 of the first opamp is +1.15V and it has a gain of 2 so if its supply voltage is high enough (at least +4.1V) then its output pin1 is +2.3V.

Then if the (+) input pin5 of the second opamp is +1.15V its output pin7 will try to go close to 0V but the current in the very low value feedback resistor prevents it.

The datasheet of the LM358 has a curve of the output low voltage vs current. Its output voltage is +0.7V with your load current of 1.15mA in the 1k ohms feedback resistor but actually the current and voltage are less because the output never goes near 0V.

If the gain-setting resistors are 100k or 330k each then the output voltage of the opamp will be much lower.

• ###### output voltage of LM358.PNG
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Oct 9, 2012
25
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Hi!!

Thanks man!!!!
As suggested by you I hv replaced gain setting resistors with 320K and now output varies from 12mV to 1.96V at the output of diffrential amplifier.

Now I hv replaced the two bridge resistors with the sensor.
But when i am increasing the gain of the non inverting amplifier it is not coming back to zero.

For example,
Say I hv given input to my sensor, but the output is not that which i am expecting. So to get the expected output i hv increased the gain of my non inverting amplifier and reached to expected output.

Now if i removed the sensor input, i am expecting zero at the output.

But this is not the case and i am getting some value at the output.

what i need to do???

thanks

4. ### Audioguru Expert

Dec 20, 2007
10,969
1,253
You cannot remove the sensor input because it provides the input voltage to the first opamp. If you want the input to be 0V when the sensor is removed then put a resistor from the input to ground. The input bias current for the LM358 opamp causes its input voltage to rise when it is disconnected.

The datasheet for the LM358 shows that its maximum input bias current is 250nA. Then a 100k resistor to ground will allow the input voltage to rise to +25mV. Also don't forget that its input offset voltage is a maximum of plus or minus 7mV.

Oct 9, 2012
25
0
Hello,

Thanks for help!!!!!!!!

I request to you to have a look on attached circuit.

LM 324 is used with 24 V single ended supply.

This ckt is used in final product. But the problem with this circuit is the output at pin no. 14 is not linear.

At pin 14 the voltage is changing from 56mV to 2.4V. But this change is not linear in accordance with pot movement.

Initially there is sudden rise upto 1V and from 1V to 1.5V it rises slowly and again from 1.5 to 2.4 it rises sharply.

So what i need to do to make it linear?

Also with the same circuit i am unable to get amplified output at pin no.7. I have replaced the feedback resistor with various values but every time i get the output from several milivolts to 0.7V.

But when i use Non inverting configuration it seems working fine.

Thanks,

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6. ### Audioguru Expert

Dec 20, 2007
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You sketched the inverting amplifiers wrong.
The opamp with pins 12,13 and 14 is not needed and can be replaced by a piece of wire. Then the LM324 quad opamp can be replaced by a smaller LM358 dual opamp that has exactly the same opamps as an LM324.

I corrected your sketch: My clear PNG file was posted as a fuzzy JPG

• ###### opamps.jpg
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Last edited: Oct 15, 2012