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Quiescent operating point

  1. May 22, 2012 #1
    Hi All,

    I was learning a lot of how to calculate the Quiescent operating point for FET's. But suddenly I got this doubt that if i am operating any transistor (BJT, FET) as a switch then I need not calculate the Q - point. Am I correct in this? Only when some AC signal is applied to avoid distortion I need to calculate the Q point?

    Thanks in advance,
  2. jcsd
  3. May 22, 2012 #2
    Calculate dc sweep if switching is not very fast.
  4. May 22, 2012 #3


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    Science Advisor

    Yes, you are right. The FET or BJT used as a switch would normally be biased OFF so the device can be turned ON by the pulses that drive the switch.

    If you used the device as a linear amplfier, you arrange the biasing to avoid distortion in the output, but this is not done when the device is used as a switch.
  5. May 22, 2012 #4
    I am sorry I did not understand what you said, for me when browsed for dc sweep is the calculation of currents and voltages, am i correct in this? how to know if the switching is fast or not ? Could you please explain me?

    Thanks in advance,
  6. May 22, 2012 #5
    If you design the switch you don't need to calculate any Q point.
    When BJT work as a switch this means that hi work on the saturation region.
    So all you need to do is to ensure that BJT is in saturation region.
    And simply if we ensure that
    Ic = β * Ib > Vcc/Rc the the BJT is in saturation region.
    So for a given Rc we need Rb smaller then

    Rb ≤ (Vin - Vbe)/(Vcc - Vce(sat)) *βmin/K * Rc

    K is a overdrive factor, typical from 2 to 5.

    Or even simpler rules
    Ensure that
    Ic/Ib = 50...10
  7. May 22, 2012 #6
    Do you have any idea of dc sweep of CMOS inverters? You BJT/FET switching circuit can be simulated like that. FYI, dc sweep is varying the dc value of input voltage and looking at the voltage and currents in the circuit.

    There are capacitance values provided in the datasheet of the component. From there you can have an idea of range of operating frequency.
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