Need 50mV to 5V amp circuit

Discussion in 'General Electronics Chat' started by dougal, Feb 26, 2009.

  1. dougal

    Thread Starter New Member

    Feb 26, 2009
    1
    0
    Being a newbee to this with some electrical experience but, not that much electronics as such like amplifiers... I need to amplify a 50mV 1 microamp signal to 5 V with 50 milliamp max current (10 milliamp better).
    I would like to use a single 5 volt d.c. supply but, this might not be practical?

    I am using a linear potentiometer to measure distance travelled and the pot has a maximum working current of 1 microamp and a variable resistance of 50kohm and I am using a supply voltage of 5 volts d.c. (This could be changed to something higher).

    Any schematics with components and values greatly appreciated.

    Thanks.
     
  2. mik3

    Senior Member

    Feb 4, 2008
    4,846
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  3. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The link Mik3 provided should be helpful.
    Also look at our online E-book:
    http://www.allaboutcircuits.com/vol_3/chpt_8/1.html
    Output current is determined by the capability of the opamp and the load. If you want 10mA output current, use a 500 Ohm load resistor; but typical output loads are more like 2k to 10k, or even higher.

    You will need to use an opamp that has rail-to-rail outputs, and an input capable of sensing down to ground. An LM8272 dual opamp is capable of such input/output (see National Semiconductor's website for a datasheet). Most (if not all) opamps will have very limited output current capability near Vcc/Vee. A 741/1458 opamp won't even get close and would be woefully inadequate.

    So, the maximum allowable voltage across the pot is 50mV. Input to the opamp must be FET/JFET or CMOS for low input current.

    One problem you'll run into is the opamp's offset voltage. An LM8272 has an input offset voltage of 5mV to 7mV, which is amplified along with the input signal.

    You can get opamps with much lower offsets, and you can also get opamps that have adjustments for offsets. There are tens of thousands of different opamps out there.

    But, we don't have a really good idea of what your expected output is; what are you planning on doing?
     
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