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Current amplifier

Discussion in 'General Electronics' started by Tyron, Apr 15, 2004.

  1. Tyron

    Tyron Guest

    heya ppl! :)
    ne one out there know how to amplyfy the current comming outta the
    opamp?
    the opamp is hooked up to a 8 bit DAC (DAC0832 ) if i could post a
    picture of my circuit i would.. ne ways the current current that i am
    gettin out of the opamp is about 0.32mA and i need the current to be
    between 4mA and 16mA ive seen a coupla circuits online and and if
    tried a transitor as a buffer and op amp curent amplifier cicuit.. but
    it did not seem to help much..
    any help wpuld be appreciated!
    if necessary email me @
    Thnx in advance!
    Tyron
     
  2. Karl Uppiano

    Karl Uppiano Guest

    According to the spec sheet, a garden variety 741 will source 25mA into a
    short circuit without any external buffering. With an external
    emitter-follower buffer circuit, you could get several amps if you have the
    right opamp gain, supply voltages and load impedance. You didn't say what
    your supply voltage is, nor the gain of the opamp, the frequencies involved,
    or the impedance of whatever you're trying to drive with the opamp, so it's
    kind of difficult to say what you need to do.

    Ohm's law says E = I * R, so if you solve for I, then I = E/R. If the load R
    is not negotiable, then you must adjust the opamp gain to provide enough
    voltage E to bring I up to the desired level. That also means the power
    supply voltage needs to be larger than the maximum voltage you expect the
    opamp to deliver. The power supply also needs to be able to provide at least
    16mA.
     
  3. Robert Baer

    Robert Baer Guest

    ....or use the LH0002 which is made specifically for that purpose.
    It is a push-pull complimentary darlington emitter follower; the
    op-amp output pin drives it, and its output goes to the load and
    feedback.
    Since it is inside the feedback loop, the small crossover distortion
    it makes is reduced by the loop gain, and is not seen by the load.
     
  4. vinay sharma

    vinay sharma Guest

    u can use darlington pair as a buffer, as it has current gain
    corresponding to b^2 which is huge.
     
  5. Robert Baer

    Robert Baer Guest

    Then one has to have a standby current larger than the load current.
    With the LH0002, *NO* standby current is needed.
     
  6. Karl Uppiano

    Karl Uppiano Guest

    I'm not sure I understand your remarks about standby current. I have
    built-out buffers on opamps using discrete parts for years, using various
    circuits, including darlingtons as well as topologies like the LH0002, and
    standby current has never been an issue.

    Incidentally, the OP was only getting 0.32mA into the load. Almost any opamp
    can top that, assuming the opamp's output voltage is high enough and/or the
    load impedance is low enough, which leads me to believe external buffering
    will not solve the problem (although it might be necessary to solve other
    performance problems). I'm assuming the load impedance is a non-negotiable
    characteristic of the load, in which case the only solution would be to
    increase the opamp output voltage. That probably means increasing the opamp
    gain, assuming the necessary output voltage is within the absolute maximum
    Vcc for the opamp, and/or the voltage available from the power supply.
     
  7. Robert Baer

    Robert Baer Guest

    It is a foregone conclusion that an emitter follower (ie:
    single-ended) needs standby current to operate. Beta, current gain,
    darlington, whatever makes no difference to that basic requirement.
    And the greater the load current, the higher the standby current must
    be.
    Take a theoretical NPN emitter follower with a beta of 100,000 or
    more.
    Make the load 10 ohms and the requirement that the output signal be
    +/- 10V peak to peak.
    Use supplies of +/- 15V. That emitter rsistor gotza be small to allow
    1 amp to flow from the -15V supply to the load (5 ohms).
    So with zero volts output (standby), the NPN standby current ain't
    exactly peanuts (3 amps).

    Now a class B push-pull complimentary darlington (like the LH0002) is
    a totally different kettle of fish.
     
  8. Karl Uppiano

    Karl Uppiano Guest

    OK, I was assuming class B or class A/B push-pull the whole time. If the OP
    required only pull-up or only pull-down (DC only), they could go with the
    top or bottom half of a push pull arrangement, with minimal standby current,
    i.e., the load *is* the emitter resistor.
     
  9. Robert Baer

    Robert Baer Guest

    Almost all loads on the output of an op amp aer driven symetrically.
    so that is not a viable option.
     
  10. Karl Uppiano

    Karl Uppiano Guest

    OK, I was assuming class B or class A/B push-pull the whole time. If the
    OP
    I would rephrase that: *If* the load needs to be driven symmetrically,
    *then* it is not a viable option, which is more or less what I said: "*If*
    the OP required only pull-up or pull-down (DC only)...". Since we don't know
    the OP's application, it's impossible to say what's viable or not.
     
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