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Turned on the diode means blocked current? or vise versa? That made me confusing. In here VCC and VEE is +12V, -12V. so, if Vin is 6V then upper OP amp will give +12V. and below OP amp will give voltage -12V. then D1 will be not block and D2 will block. right? so..D1 is turn on? or off? How i have to call that?

enter image description here

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    \$\begingroup\$ You call it "forward-biased" or "conducting". \$\endgroup\$
    – Eugene Sh.
    Dec 10, 2020 at 21:11
  • \$\begingroup\$ @EugeneSh. Thank you to reply me! Umm if forward-biased then turn on? or vise versa? I meant that \$\endgroup\$
    – RNN master
    Dec 10, 2020 at 21:13
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    \$\begingroup\$ Forward-biased = conducting, reverse-biased = not conducting. \$\endgroup\$
    – Eugene Sh.
    Dec 10, 2020 at 21:15
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    \$\begingroup\$ We tend to shy away from on/off for diodes, because on could imply that current could flow both directions. In a diode, current can ONLY flow in the forward direction. Hence, forward-bias or conducting are the preferred terms. \$\endgroup\$
    – Aaron
    Dec 10, 2020 at 21:35
  • \$\begingroup\$ Thank you!~!! i got well. \$\endgroup\$
    – RNN master
    Dec 11, 2020 at 3:42

1 Answer 1

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schematic

simulate this circuit – Schematic created using CircuitLab

Here is a diode. If the anode is at a higher voltage than the cathode it will be forward biased and conducting or "on". The anode voltage has to be about 0.7V higher than the cathode, for a silicon diode, for it to conduct.

If the anode is at a lower voltage than the cathode it will be reverse biased, not conducting and off.

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    \$\begingroup\$ Hi, Regarding: "The anode voltage has to be about 0.7V higher than the cathode, for a silicon diode, for it to conduct." As explained by @Andyaka in this answer, and as shown in figure 1 (If vs. Vf) in this 1N4148 datasheet, the forward voltage drop can be significantly less than 0.7V at very low currents. Perhaps it's worth pointing out that 0.7V is more like a typical voltage drop for small diodes, at single-digit mA current & conduction can occur with <0.7V? Just a thought! \$\endgroup\$
    – SamGibson
    Dec 10, 2020 at 21:42
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    \$\begingroup\$ @SamGibson I absolutely agree with you and am very familiar with diode characteristic curves. However you have to be careful to apply the correct level of detail in a model to match the problem you are trying to answer. I was debating mentioning forward voltage drop at all. \$\endgroup\$
    – RoyC
    Dec 10, 2020 at 21:48
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    \$\begingroup\$ Hi, I realised you would know this (from seeing your previous high-quality answers) and yes, perhaps not mentioning forward voltage drop at all would be better here. I just think if that topic is introduced, it's important to tell readers that diode Vf drop is more complicated than the classic "0.7V drop on a silicon diode" which we (or at least I) was taught in college. Otherwise people are surprised when they later find the places where that simplified statement isn't true! Anyway, now we've had this exchange, I respect it's your choice to write your answer as you want. Thanks :-) \$\endgroup\$
    – SamGibson
    Dec 10, 2020 at 22:02
  • \$\begingroup\$ Really thank you!~!! \$\endgroup\$
    – RNN master
    Dec 11, 2020 at 3:42

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