Need name of two transistor switch circuit that allows for sharp threshold

Discussion in 'The Projects Forum' started by Mrdouble, Jul 25, 2015.

  1. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    I'm using a sense resistor to measure current. With the Vbe across resistor I want the transistor to go high and drive another transistor that drives an led. My circuit works great but if the current very slow in transition the LED lights up gradually. I have found the exact circuit I'm looking for and it works great but now I want to do a little further research on the circuit but I don't know what it's called. I know that it's often linked to a transistorized hysterisis circuit, Only without the hysterisis
    Thank you in advance
    Spike
    image.jpg
     
  2. ScottWang

    Moderator

    Aug 23, 2012
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    The circuit you attached is a non inverting circuit and what you asking the circuit that it could be a schmitt trigger circuit, and the circuit without hysterisis is an inverting circuit, it transferring the status directly.
     
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  3. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    So it's a Schmitt trigger without hysterisis?
     
  4. ScottWang

    Moderator

    Aug 23, 2012
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    Actually it didn't call that way, as the circuit you attached that it called non inverting circuit, and adding or reducing a stage of npn bjt that we call it inverting circuit, and a inverting circuit adding hysteresis function then we call it schmitt trigger circuit.
     
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  5. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Umm, I really not understanding you. I'm very sorry but what I'm getting is that a "Schmitt trigger" is only classified when it's inverting and that this circuit I posted is called "non inverting circuit". I can't google "non inverting circuit" and expect any relatable links. Again I'm very sorry for my misunderstanding
     
  6. bertus

    Administrator

    Apr 5, 2008
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  7. ScottWang

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    Aug 23, 2012
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  8. OBW0549

    Well-Known Member

    Mar 2, 2015
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    No.

    The circuit you showed is not a Schmitt trigger; it is merely a cascade of two inverting common-emitter stages, and there is no hysteresis involved; and the book's description of Fig. 3-47 as a circuit with a "sharp threshold" is wrong. "Sharper threshold" (i.e., sharper than the threshold of a single common-emitter stage) would be more accurate.

    The classic Schmitt Trigger circuit, as shown in the article at https://en.wikipedia.org/wiki/Schmitt_trigger , uses two transistors arranged with positive feedback via their emitters which creates hysteresis and a sharp "snap action" switching characteristic. The circuit you showed does not have any feedback or hysteresis, only a switching action which is sharper than what you would obtain from one transistor alone.

    If you want to make your circuit turn on the LED more abruptly, you can add a bit of hysteresis by simply connecting a resistor between the base of Q1 and the collector of Q2. A 10 megohm resistor will give you about 12 millivolts of hysteresis referred to the input; for more hysteresis, use a smaller resistor.

    (BTW, the classic Schmitt Trigger circuit is non-inverting, not inverting.)
     
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  9. DickCappels

    Moderator

    Aug 21, 2008
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    The net threshold of two cascaded common emitter stages will almost certainly be sharper than that of a single stage.
     
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  10. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Thank you guys, I know what a Schmitt trigger is and its function. Some how this question got blew away from original question.
    So the answer is there is no particular name for the circuit.
    Fair enough :)
     
  11. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    True, DC, but because it is running wide open, it still can have a rail-to-rail noise burst on the output for slow input transitions. I have run into this in surprising places, where the circuit designer basically thought a single transistor was about the same thing as a single stage of a 74ACT14. That digital goop is ruining our children!

    ak
     
  12. Mrdouble

    Thread Starter Member

    Aug 13, 2012
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    Orders of magnitude better :)
     
  13. ScottWang

    Moderator

    Aug 23, 2012
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    Bjt schmitt trigger circuit.
    Op amp schmitt trigger circuit.
     
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