BJT transistor saturation

Discussion in 'Electronics Homework Help' started by Rajinder, Mar 13, 2018.

  1. Rajinder

    Rajinder

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    Hi all
    I have a homework question which i would like someone to verify my results please.
    I have a npn transistor. Rl and Rb are not given. Common emitter configuration.
    Vcc is 18V. Power is 2W.
    So using P=VI, transpose this gives me I = P/V = 2/18= 111mA
    Hfe is IC/IB Note hfe min is 180
    Ib is therefore ic/hfe = 111mA/ 180 = 616uA

    If the input to base is 3V, then to calculate
    Rb is
    3-vbe/Ib, this is then 3-0.7/616uA= 1.8K.

    Dies this seem correct??
     
    Rajinder, Mar 13, 2018
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  2. Rajinder

    Ratch

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    1) What are you trying to find? You never say what.

    2) You should write (3 - 0.7)/616 uA, not 3-0.7/616uA .

    3) Saturation voltage of a BJT is between 0.2 - 0.5 volts.

    4) Does, not Dies.

    5) Personal pronouns are capitalized. I, not i .

    Ratch
     
    Ratch, Mar 13, 2018
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  3. Rajinder

    Rajinder

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    Hi
    Sorry i was typing in a hurry.
    I was trying to find out the collector current, base current and value of base resistor to saturate the transistor.
    Thanks.
     
    Rajinder, Mar 14, 2018
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  4. Rajinder

    Ratch

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    Reduce the 18 volts by the saturation voltage of the BJT to refine your answer. Calculations look OK.

    Ratch
     
    Ratch, Mar 14, 2018
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  5. Rajinder

    Cannonball

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    If you put 3 volts on the base of an npn transistor with the emitter grounded you will let the smoke out.

    .7 volts on the base will drive it into cut off.
     
    Cannonball, Mar 14, 2018
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  6. Rajinder

    Ratch

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    True, but the OP proposes to put a 1k8 base resistor in series with the 3.0 V. According to the calculations.

    Ratch
     
    Ratch, Mar 14, 2018
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  7. Rajinder

    Rajinder

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    Yes i have a base resistor of 1k8 in the circuit.
    Will my calculations saturate the transistor?

    Also if the transistor was a bc847 npn, can someone show me how to use the datasheet to obtain saturation values for Ic and Ib.
    Thanks
     
    Rajinder, Mar 14, 2018
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  8. Rajinder

    Ratch

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    As long as the beta is more than 180, the transistor will be in saturation at the values given for the circuit.

    Saturation depends on the beta and the values of the components and voltages external to the transistor. Anytime the Vec goes near or below Veb, saturation occurs. That can happen for an infinite number of different values of Ib and Ic. You won't find those values on a data sheet.

    Ratch
     
    Ratch, Mar 14, 2018
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  9. Rajinder

    Rajinder

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    Thanks. I have seen on datasheets information like vce(sat)=0.2v, Ic =10mA, ib 0.5mA.
    Are these not saturation conditions too?
     
    Rajinder, Mar 14, 2018
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  10. Rajinder

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In addition to the above post by @Ratch, transistors are frequently specified as having a range of HFE. This means that two transistors picked at random and biased with the same base current could vary significantly in their state. For the same load, one might be in saturation, and the other not.

    I believe we have a thread somewhere where we give some hints on ensuring a transistor is in saturation.

    Basically it boils down to calculating the current required based on the minimum HFE and than applying 3 to 10 times this base current.

    To determine whether a particular transistor is actually saturated, first you need to define saturation. If it is Vce < Vbe (a good starting point) then you can simply measure these values (or even just Vce, assuming Vbe is approx 0.7V).

    Transistors can be forced further into saturation, the datasheet will often give a Vce well under 0.7V when fully saturated (it may be 0.15V).

    For a particular purpose (e.g. reducing dissipation in switching applications) you may want to reduce Vce to a minimum, and here is where you might use a multiplier closer to 10 rather than 3 in the heuristic given above.
     
    (*steve*), Mar 14, 2018
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  11. Rajinder

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Yes your example indicates saturation, but the transistor would not be in saturation for a significantly higher Ic.

    Unless your load corresponds to the example given in the datasheet, you need to do some calculations yourself.

    Out of curiosity, what is the minimum HFE specified for that transistor in the datasheet?
     
    (*steve*), Mar 14, 2018
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  12. Rajinder

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    See here for a discussion of saturation (amongst other things)
     
    (*steve*), Mar 14, 2018
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  13. Rajinder

    Sunnysky

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    Vce(sat) specs are usually Ic/Ib=10~20 and the Vce/Ic=ESR is a calculation to make useful projections of Vce vs Ic. But on the fringe , Saturation is the effective rapid decline in Hfe vs Vce for various Ic levels. In a linea amp operation into a low impedance load, Vce<2V for me is considered going into saturation, due to rapid rise in THD due to rapid decline in hFE, towards 10% of max.
     
    Sunnysky, Mar 15, 2018
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  14. Rajinder

    Rajinder

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    Thanks everyone. Great advice. The information in the link was great.
     
    Rajinder, Mar 18, 2018
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