Getting Around 0.65V DROP of An BJT

Discussion in 'The Projects Forum' started by MCU88, Mar 26, 2015.

  1. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Dear Experts...

    I have an schematic here with Q1 configured as an emitter follower. The emitter lead on the transistor follows the base junction voltage less ~0.65V.

    Q: How can I counteract this 0.65V drop and bring the output up to 2.2 volts as per the Vref from the LED?

    [​IMG]
     
  2. #12

    Expert

    Nov 30, 2010
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    Add a diode or transistor in series with the LED.
     
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  3. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Perfect. Easy greasy.
     
  4. MikeML

    AAC Fanatic!

    Oct 2, 2009
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    However, the voltage at Vout will not track the LED voltage very well.
     
  5. MCU88

    Thread Starter Member

    Mar 12, 2015
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    What if I were to bias the emitter with a voltage?
     
  6. #12

    Expert

    Nov 30, 2010
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    It depends on what, "very well" is needed. MCU88 is asking about a circuit with 3 parts. I could make it track deadly well, but it would need several more parts and be a lot more involved in the understanding of the circuit.
     
  7. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Keen to see it. Please post an schema and schematic for us...
     
  8. #12

    Expert

    Nov 30, 2010
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    Consider this:
     
  9. #12

    Expert

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    That turned out better than I expected! Choose the right 5 volt op-amp and you might do this in 4 parts.
     
  10. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Ok so the OPAMP has the transistor in the feedback path and therefore regulates the output to the Vref of the LED - ~0.65V.

    Why the FET?
     
  11. #12

    Expert

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    The op-amp drives the transistor so that the output of the transistor is the same as the voltage at the LED, give or take errors in the op-amp. This can be as low as the microvolt range.

    The fet is a constant current device. In this configuration it allows 5 ma to 15 ma to flow, depending on which transistor your fingers touch when you reach into the parts bin. This constant current ability replaces the resistor and does an excellent job of refusing to respond to the voltage variations of the power supply.
     
  12. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Arh yes of course. The emitter is in the feedback path so there it compensates for the 0.65 volt drop.

    I was sorting of hoping for an transistor only solution though...
     
  13. MikeML

    AAC Fanatic!

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    Here is how the proposed circuit is effected by load current and temperature. My LED has a slightly different Vf. Note the temperature effect is largely a function of LED Vf being effected by temperature... which the opamp circuit does not solve.

    183.gif
     
  14. #12

    Expert

    Nov 30, 2010
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    Here, let me flip that around for you. It's just a current enhanced voltage follower. It will track the voltage of the LED religiously. If you want the variations of the LED voltage that are caused by temperature changes, you got it! If you want a steady reference voltage, the LED is a miserable choice in the first place.
     
  15. #12

    Expert

    Nov 30, 2010
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    You can built a sort of op-amp with a differential pair of transistors.
     
  16. MCU88

    Thread Starter Member

    Mar 12, 2015
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    In general I am looking for ways to get around the 0.65 volt drop of an BJT. That's it. Just interested in methods of getting around this limitation that is sometimes an useful aid.
     
  17. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Keen to see that. Can you post an schematic with explanation(s) and practical uses in dot point form?
     
  18. #12

    Expert

    Nov 30, 2010
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    Boosting the output is often done with just a diode. You can use this trick with an LM7805, too. :)

    Design you a differential amp with transistors at 5 am? I'm getting tired right now.
    Maybe tomorrow.
     
  19. MCU88

    Thread Starter Member

    Mar 12, 2015
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    Sleep is for the weak. Knock off time is 8am. Go grab yourself an cup of coffee.
     
  20. MCU88

    Thread Starter Member

    Mar 12, 2015
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    There is an beer on the table for you via PayPal buddy...
     
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