How to amplify from a positive ground?

Discussion in 'General Electronics Chat' started by brucester, Jan 20, 2013.

  1. brucester

    Thread Starter New Member

    Dec 24, 2012
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    Hi,

    I have a simple circuit set up that generates a clippped voltages +/- 0.8 volts using 2 diodes on a positive 9 volt signal.

    For example, 9 volts is ground, signal varies between 9.8 and 8.2 volts.

    I want to amplify the millivolt part. If ground was zero i could just use a standard op amp with 100 gain and end up with a signal +/-9 volts.

    But if i do it here, it multiplies the original 9 volts, plus the extra. So 9.1 volts become 910 volts.

    Is there a way to do this?

    Thanks
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,645
    2,344
    Hello,

    You could use a summing amplifier.
    Put the signal to be measured on one input and a fixed reference signal on the other input.
    Have a look at this page of the eBook for more info:
    http://www.allaboutcircuits.com/vol_3/chpt_8/8.html
    If the measurement signal is + 9 Volts and the reference is - 9 Volts (and the gain of both inputs is the same) the output will be 0 Volts.

    Bertus
     
  3. MrChips

    Moderator

    Oct 2, 2009
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    Is the millivolt part a changing signal at some reasonable frequency, say greater that 10Hz?
    Then you can AC couple the signal to your amplifier.
     
  4. brucester

    Thread Starter New Member

    Dec 24, 2012
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    Basically, i am trying to achieve the N-Mosfet version of this setup:

    Circuit

    It works great here when using a P mosfet because the load is under the fet and runs to ground.

    But when i do it with an N mosfet, the load is above and ground is positive.

    This circuit amplifies the last 800 millivolts of an inductors flyback.

    I want to do the exact same thing but with an n mosfet.
     
  5. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you post the schematic in a graphics format like .png?
    The falstad simulator will hang my browser.

    Bertus
     
  6. brucester

    Thread Starter New Member

    Dec 24, 2012
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  7. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    What is the goal of your circuit?

    What op amp are you using?

    I am not sure you (want to / need to) tie your inputs to the + and - rails.

    Capacitor coupling your signal (AC) can be used to remove a 9 volt DC offset. Capacitor value will be determined by the AC frequency of your signal (if it is AC).
    Or,
    Like Bertus said, you can adjust the dc level going into your non-inverting input to null the 9vdc offset.
     
  8. brucester

    Thread Starter New Member

    Dec 24, 2012
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    It pulses a coil and amplifies the final stages of the flyback. Beginning sage of a pulse induction metal detector.

    I'll read up on adjusting dc levels.
     
  9. brucester

    Thread Starter New Member

    Dec 24, 2012
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  10. THE_RB

    AAC Fanatic!

    Feb 11, 2008
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    Since this an AC signal you just want to amplify (and not measure) it is standard practice to "capacitor couple" it to the amplifier. ie; all you need is a capactor to run the signal through. That allows the AC voltage signal through but will ignore the DC level.
     
  11. brucester

    Thread Starter New Member

    Dec 24, 2012
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    Thanks a lot RB. That explains the capacitor on the schematic :)

    I tried it out on my sim here: http://www.falstad.com/circuit/#%24+1+5.0E-6+3.7524723159601+50+5.0+43%0Av+368+240+368+176+0+0+40.0+9.0+0.0+0.0+0.5%0Av+368+352+368+288+0+0+40.0+5.0+0.0+0.0+0.5%0Ar+384+128+432+128+0+2.0%0Ar+528+160+528+208+0+220.0%0Ar+560+256+592+256+0+390.0%0Al+480+160+480+208+0+1.0+3.0896079289548306E-6%0Af+448+304+480+304+0+1.5%0As+384+288+416+288+0+1+false%0Aw+448+304+432+304+0%0Aw+432+304+432+288+0%0Aw+368+176+368+128+0%0Aw+480+320+480+400+0%0Aw+368+400+368+352+0%0Aw+368+240+320+240+0%0Aw+320+240+320+352+0%0Aw+320+352+368+352+0%0Aw+480+128+480+160+0%0Aw+480+128+528+128+0%0Aw+528+128+528+160+0%0Aw+480+256+480+288+0%0Aw+560+256+528+256+0%0Aw+528+256+480+256+0%0Aw+528+208+528+224+0%0Aw+528+224+480+224+0%0Aw+480+224+480+208+0%0Aw+480+224+480+256+0%0Aw+592+256+624+256+0%0Ad+624+128+624+208+1+0.805904783%0Ad+656+208+656+128+1+0.805904783%0Aw+528+128+624+128+0%0Aw+624+128+656+128+0%0Aw+624+208+624+256+0%0Aw+656+208+656+256+0%0Aw+656+256+624+256+0%0Aw+480+416+480+400+0%0Aw+384+128+368+128+0%0Aw+432+128+480+128+0%0Aw+368+288+384+288+0%0Aw+432+288+416+288+0%0Aw+368+400+368+416+0%0Aw+368+416+480+416+0%0Aw+656+256+656+288+0%0Aw+656+288+624+288+0%0Aw+624+288+624+416+0%0Ar+624+416+480+416+0+3000000.0%0Ac+672+256+688+256+0+1.0E-6+-7.425171588693047E-13%0Aw+672+256+656+256+0%0AO+688+256+752+256+0%0Ao+5+64+0+35+7.62939453125E-5+9.765625E-5+0+-1%0Ao+41+64+0+35+10.0+9.765625E-5+1+-1%0Ao+47+64+0+34+10.0+9.765625E-5+2+-1%0A

    But it still outputs 9+ volts.

    Is this a bug in the sim?

    Or am i doing it wrong?
     
  12. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Do you have an image of the schematic? It's not loading for me.
     
  13. brucester

    Thread Starter New Member

    Dec 24, 2012
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  14. brucester

    Thread Starter New Member

    Dec 24, 2012
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    I get how the cap only lets AC through, but my signal is the voltage, so 9+ volts still happens so i'm not sure if this is what i need.
     
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