'biasing' op amps??

Discussion in 'Homework Help' started by hasie, Oct 13, 2009.

  1. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    Hi guys, Ive really been thinking about something.

    The answer might be straight forward, but went through a few books, and cant seem to find a solution ...

    I want to set up a amplifier, with two different gains per voltage(If that makes any sense)(Or just an way to 'bias' op amps if its possible.

    Anyways, this is kinda what im trying to say.

    I have an input pin to a amplifier. This pin will be recieving from +\-250mV --> +\-1V DC.


    The voltage on the input pin to the amplifier will always have between those voltages on them.


    Now, when it gets a min voltage of 250mV, it must have a gain of 16 (so output pin should have minimum 4V DC)

    and then when the input to the amplifier get a max input voltage of 1V, it should have a gain of 8(So output pin should have a maximum of 8V DC)

    Then in between gain should be as linear as possible between the 2 limits..

    Is there a way to do this with op amps? Or how is the correct way in doing this??

    Dont know maybe the aswer is straight forward but cant seem to sort it out.

    Any help appreciated
    Many thanks.
     
  2. steinar96

    Active Member

    Apr 18, 2009
    239
    4
    Op amps are linear devices, so i can't imagine there being any "simple" way to do this. Why do you need this kind of behaviour ?
     
  3. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    I want to take a voltage produced by a LM35 and feed it to a pulse width modulation circuit for a triple5 timer. The circuit I am using the pulse width is 0% in 4V, and 100 on 8V.

    And the temp range is 25degrees celsius to 100degrees celcius max.

    so that equates to min 250mV(lm35 is 10mV per degrees celsius) min and 1V max...

    Any other ideas how I can achieve this?

    Many thx
     
  4. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    Anybody maybe have a solution or even info to how to solve my problem maybe?

    much appreciated
    Many thx
     
  5. k7elp60

    Senior Member

    Nov 4, 2008
    478
    69
    You could use an analog switch to switch in a different resistor to change the gain of the op amp. Use a comparator to sense the high output and control the analog switch, then it would be automatic.
     
  6. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
    864
    40
    Why don't you try setting up your circuit so your temperatures between 25 C and 100 C yield an output between 0V and 4V and simply add 4V to the output of that circuit? That way you only need to have a single gain in the circuit. That will be much easier than trying to make something with a gain that varies with the input voltage.

    I hope that is enough of a hint...
     
  7. hasie

    Thread Starter Member

    Sep 28, 2009
    35
    0
    OK I think I can make out what your trying to advise.

    Will try it and let you kxnow many thanx
     
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