Voltage follower capable of up to 150mA delivery - what do I need?

Discussion in 'General Electronics Chat' started by peskywinnets, Apr 1, 2012.

  1. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    I hope to order the parts & trial this soon...ron re your circuit in post #56 could you explain what is happening on the latter part of the circuit ie to the right side of R2 onwards.

    I'm curious why you are feeding back to the opamp +ve pin etc.
     
  2. Audioguru

    Expert

    Dec 20, 2007
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    A complimentary pair of emitter-followers have almost the same output voltage as the input voltage and have a fairly high output current.
    They can be included in the negative feedback loop of an opamp.
     
  3. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    Thank you .....such an arrangement was my first thought ....but this supply quite low voltage in the first place (3 x 1.5V batteries) & the leds are being run at 3.3V ...I'm not sure what the voltage loss is across such an arrangement (0.7V?), but it's likely going to mean that the batteries can only fade away a small amount before that circuit won't be able to follow the input voltage any longer.
     
  4. Audioguru

    Expert

    Dec 20, 2007
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    The two series complimentary emitter-followers I showed have almost no voltage loss.
    The output voltage is almost the same as the input voltage.
    But the output of the opamp that drives them must be at least 0.7V higher than 3.3V.
    Most opamps also have a voltage drop.
     
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    The MOSFET is connected as common source, so it inverts. It and the op amp form a voltage follower that can only source current, but can get to 3.3V, while supplying hundreds of milliamps (if the LED will allow it) if the input is 3.3V (after filtering). The feedback is connected to the +input because the MOSFET inverts.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    This is true when the input is not near the rails.
    If the input goes to 0V, the output can go no lower than ≈+0.7V. If the input goes to +3.3V, the output will go to ≈+3.3V.
     
  7. peskywinnets

    Thread Starter Active Member

    Jan 19, 2009
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    That OPA376 ...what are the main characteristics I should look for if I have difficulty sourcing one (I always try & avoid any opamp starting "OPA"...as theyre normally 3 times the price of similar variants!)
     
    Last edited: Apr 14, 2012
  8. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I chose the OPA376 for the voltage-controlled current source circuit. The critical parameters were low input offset voltage, and rail-to-rail I/O. For the voltage output circuit, you shouldn't need low offset, which should open up your options considerably. I can't guarantee that your eventual choice won't oscillate in the circuit I posted. If you pick another one, we can try to simulate it, but if it doesn't oscillate, that's still no guarantee.:rolleyes:
     
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