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Simple Integrated Circuit Question

  1. Jan 28, 2015 #1
    I have a simple conceptual question rather than an actual problem so pardon me for not using the provided template.

    There's a question in my Microelectronic Circuits book involving this circuit:

    SS figure p7.51.png

    where it states that
    Part of the solution involves this equation:

    VOV4 = VD4 - VS4 (equation for overdrive voltage of Q4)

    VOV4 = -1.5 - (-1.3)

    My question is: Why is VD4 equal to -1.5 V and why is VS4 equal to -1.3 V? -1.3 V is the voltage at the drain of Q5, how did it become the voltage at the source of Q4?
     
  2. jcsd
  3. Jan 28, 2015 #2

    rude man

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    Unfortunately, what you state is incorrect. The drain of D4 will be more positive than the source.
    The drain of Q4 is > -1.5V. Perhaps as low as -1.3V but certainly no lower.
    The source of Q4 is at -1.5V.
    The drain of Q5 can be as low as -1.3V.
    For an NMOS. the drain is more positive than the source unless it's operated in the "inverse" mode in which case the labels "drain" and "source" become a matter of semantics. I have never encountered a circuit using the inverse mode, except once which turned out to be a design error. So forget the inverse mode.
     
  4. Jan 28, 2015 #3
    Turns out the solution manual had it wrong... it should be VOV4 = -1.3 - (-1.5)
     
  5. Jan 28, 2015 #4
    New question concerning same circuit: Why is R = (VS2 + VGS2)/IREF? Shouldn't it be VD1/IREF?
     
  6. Jan 29, 2015 #5

    NascentOxygen

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    Both seem correct.
     
  7. Jan 29, 2015 #6

    donpacino

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    VGS2=VG2-VS2
    VS2+VGS2=VG2

    VD1=VG2

    due to symmetry VD1=VD2
    therefore R=(VS2+VGS2)/Iref=VD1/IREF=VD2/IREF
     
  8. Mar 21, 2015 #7
    New question related to same circuit in the initial post: the solution manual states that VDS2max = VOV2 = VGS2 - Vth. My question is why is VDS2max = VOV2? Shouldn't VDS2 = VGS2 since the drain and gate of Q2 are connected?
     
  9. Mar 21, 2015 #8

    rude man

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    VDS2max is the max allowed D-S voltage. Yes, obviously, VDS2 = VGS2 so therefore you can also say that VOV2 = VGS2max.
     
  10. Mar 21, 2015 #9
    Isn't VOV = VGS - Vth, so shouldn't VOV2 = VGS2max - Vth? (or VOV2 = VDS2max - Vth) rather than VOV2 = VGS2max?
     
  11. Mar 21, 2015 #10

    rude man

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    VOV in general just means "overvoltage". It could apply to any two transistor terminals.
     
  12. Mar 21, 2015 #11
    I get that but what about the threshold voltage (Vth)? The overvoltage is the voltage across two transistor terminals minus the threshold voltage. Why don't we include it in the equation for VOV = VDS2max?
     
  13. Mar 21, 2015 #12

    rude man

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    No. The overvoltage is the maximum voltage you can apply between any two terminals. It has nothing to do with Vth or any other transistor parameter.
     
  14. Mar 21, 2015 #13
    Ah ok. I was confusing it with the overdrive voltage; I thought they were the same thing.
     
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