OP07 circuit help

Discussion in 'General Electronics Chat' started by picstudent, Dec 19, 2014.

  1. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Hai,
    I have a OP07 non -inverting amp circuit working from dual supply +12V -12V. The gain is around 12.
    The signal to be amplified is DC in the range of 0 -250mV. (input) amplified to somewhere around 0 to 2.5Volts.
    I have a variable preset resistor in the feed back loop of OP07 to Adjust the gain. But I cannot get a lenier gain in this case. Suppose I adjust gain to 12 by providing 50mv and measuring the output and then apply 200mv and check the output, the gain is not 12.
    Why this is happening and how I can overcome the issue?
    Thanks
     
  2. #12

    Expert

    Nov 30, 2010
    16,250
    6,746
    You are going to have to post a schematic of your circuit for us to find the error.
     
  3. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Here I am attaching the schematic.
    On Simulation it seems Ok, But in the actual I think some other factors like component or noise is influencing in different signal levels.
    Thanks
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Although not likely the root cause of your problem, what are R6 and R4 supposed to do?

    In the real circuit, do you have any bypass capacitors? How stable are the supplies? Is the center-tap between the supplies tied to circuit common?

    Have you looked at the output with an O'Scope?
     
  5. ScottWang

    Moderator

    Aug 23, 2012
    4,850
    767
    Do you want to in series with a voltage follower before the R6?
     
  6. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Thanks for the response.
    Yes now I understand R6 and R4 are redundant. Removed.

    The power supply is two 12V SMPS connected back to back to create + and - volts. center grounded.
    They may not be very good, I believe. So I will try by changing to normal step down transformer power supply and try.
    But since SMPS will be expected as the source ultimately how can I provide smooth filtering?
    I have bypass capacitors in the Power supply pins of OP07 to GND (.1uF). Any where else needed?
    Not yet checked with a Oscilloscope. What should I check with a scope?
    Noise or ripples in the output?
    Thanks
     
  7. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Actually I am using this circuit to measure the current through R5. The output of this op-amp goes to ADC pin of a Micro-controller.
     
  8. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Actually R6 is needed, but it should be about 5K. It serves two purposes. It provides a modicum of protection to the opamp input from transients on the current sense line, and it balances the input offset voltage, which for this opamp is almost negligible.

    If you had a 'scope, you would see if it is noise or an oscillation.

    And you dont need a follower; the circuit should work. I think you will find that the switchers are crap. I hate them around analog circuits.
     
  9. picstudent

    Thread Starter Active Member

    Feb 3, 2009
    76
    0
    Got it!
    Thanks, will be around!
     
Loading...