Inverting Op-Amp saturation problem

Discussion in 'The Projects Forum' started by ssingh, Apr 19, 2012.

  1. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    Dear Friends,

    This is my first post in the forum so please bear with me.

    We are using Op-Amp in inverting mode in our project to get a gain of 1/2 (which is not possible in non-inverting configuration). We are facing a problem that whenever the input to op-amp drops to around 0 volts the output saturates to max. voltage.

    Is there any way to stop this?

    With regards
    Shourya
     
  2. wayneh

    Expert

    Sep 9, 2010
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    It would help if you can post a schematic, and identify the op-amp model.
     
  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    Non-inverting gain of 1/2 can be done with a resistive divider in front of a voltage follower.
     
  4. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    Dear Friends,

    Thank-you all for your reply.

    I am sorry to inform that after doing much testing I have found that saturation problem is not due to Inverting Circuit but due to the Non-Inverting Circuit output of which is fed to the inverting stage. I am attaching the diagram for the same with component values. As you may see in the Required Parameters the Minimum Input Voltage requirement is given to be 0.0025 V. If the voltage goes below that value the output is saturated.

    Now can you please suggest some solution.

    With regards
    Shourya
     
  5. wayneh

    Expert

    Sep 9, 2010
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    And your power supply?
     
  6. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    Single supply voltage is applied to op-amp which is +24 Volts and Ground.
     
  7. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    How accurate does this need be? I ask because if you ADD a 0.0025V signal to the input wave then the Minimum Input Voltage requirement will always be met.
     
  8. Ron H

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    Apr 14, 2005
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    But we don't know what the range of the input signal is, do we?
     
  9. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    The range of input voltage is from 0-10 Volts.

    As suggested a small signal voltage (DC) may be added to the signal but the problem is when the input is open i.e. no input is applied at all, in that case also the output saturates to max. saturation voltage.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is because you have no path to ground for the input bias current. Any op amp will do this. CMOS types might float between the rails, but it will be unpredictable. Some op amps will saturate to the negative rail.
    Why is your input sometimes floating?
    The solution is to add a large value resistor from the input to ground, like 100k or 1Meg.
     
    ssingh likes this.
  11. Audioguru

    New Member

    Dec 20, 2007
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    Your opamp is an LM224 or LM324. It has PNP input transistors that need a resistor to 0V to prevent their input bias current from causing the non-invertring input to float to a positive voltage. Then the output also goes to a positive voltage.
     
  12. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    Dear Friends,

    Thank-you all for your valuable suggestions and inputs.

    I would like to inform that after adding 100k/1Meg. resistance from input to ground the output is no longer saturating.

    I would like to know which of the two register values is better (as there seems to be no apparent change in output signal with any of the values)?

    Also is there any drawback in adding this input resistance to the circuit?

    With regards
    Shourya
     
  13. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The 100k resistor will load the source more than the 1Meg resistor. If you have a high impedance source, the resulting attenuation may be more than you can tolerate.
    The input bias current multiplied by the value of this resistor will generate a voltage which is above ground. Look up the input bias current in the datasheet and calculate this voltage, then decide what you can tolerate.
     
  14. ssingh

    Thread Starter New Member

    Apr 23, 2011
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    Dear Ron H,

    Thanks for your suggestions.
    I will look into the matter as suggested.

    With regards
    Shourya
     
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