Problem Getting TIP120 Transistors To Work

Discussion in 'Analog & Mixed-Signal Design' started by elec_eng_55, Jun 13, 2018.

  1. elec_eng_55

    Thread Starter Member

    May 13, 2018
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    0
    I have an emitter-follower circuit utilizing a TIP120. I have tried half a dozen
    different TIP120s with the same result.

    The dc voltages are:
    Collector 15.00V
    Base 1.96V
    Emitter 1.22V

    Vbe = 0.7V but shouldn't it be 1.4V with a darlington device?

    I have also tried different biasing voltages with no luck.
    New proto board. No loose connections or wiring issues.

    David
     
  2. AlbertHall

    AAC Fanatic!

    Jun 4, 2014
    6,641
    1,552
    The TIP120 has internal resistors which are messing up your bias circuit.
    From the datasheet:
    upload_2018-6-13_20-11-23.png
     
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  3. elec_eng_55

    Thread Starter Member

    May 13, 2018
    129
    0
    Hi Albert:

    I don't know how to deal with this. Any suggestions?

    David
     
  4. Ylli

    Member

    Nov 13, 2015
    401
    103
    Even with those resistors, the base voltage with the resistor divider shown, should be above 10 volts. You have the circuit wired wrong (perhaps you have the pinouts of the TIP120 wrong) or you have a bad TIP120 in the circuit.
     
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  5. ian field

    AAC Fanatic!

    Oct 27, 2012
    6,252
    1,131
    What are you trying to accomplish?

    I'd go for setting the bias with a pot, you can get something approaching high input resistance by throwing in a bootstrap resistor.
     
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  6. elec_eng_55

    Thread Starter Member

    May 13, 2018
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    0
    Hi Ylli:

    I have tried 6 different new devices. If the pinout for the TIP120 is BCE looking a the label
    side of the device then the circuit is wired right.

    David
     
  7. ebp

    Well-Known Member

    Feb 8, 2018
    2,205
    772
    Are all of your transistors from the same batch from the same source? A reliable supplier?

    I know there was a pinout in a previous thread, but just to confirm:
    marked side facing you
    leads pointing downward
    BCE, left to right

    It looks like the voltage across R3 is entirely due to the base current, with no collector current at all. If you omitted R2, the base current would be about (15 - 1.4)V / 11k = 1.24 mA. If you do the arithmetic with your circuit as-is, the emitter current again works out to be equal to what is going into the base.

    You could try putting a small resistor, say 100 ohms, between the collector and the supply so you can actually measure/calculate collector current.

    With the input signal source disconnected, try omitting R2 and changing R1 to 1k. See what you get for voltages. If you have one rated for adequate power, try reducing R3 to 100 to 200 ohms (you could just parallel 5 1k resistors, assuming what you are already using is rated for at least a quarter of a watt.

    Another simple experiment you could try is to make a common emitter circuit with one or two hundred ohms between supply and collector and then see what value of resistor you have to use between the base and the supply to get some reasonable amount of current through the load.
     
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  8. elec_eng_55

    Thread Starter Member

    May 13, 2018
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    0
    Hi Ian:

    I am just trying to get an emitter follower circuit working in
    an attempt to learn how it works. I understand that using
    a darlington opposed to a normal BJT will give you a
    high current gain and a high input impedance.

    David
     
  9. ebp

    Well-Known Member

    Feb 8, 2018
    2,205
    772
    With only 1.3 mA into the base, if there is no collector current, the voltage across the 120 ohm B-E resistor of the output transistor would only be 0.16 V, rather than a normal 0.6-0.7 V, which again agrees pretty closely with what you measured for overall B-E voltage.
     
  10. elec_eng_55

    Thread Starter Member

    May 13, 2018
    129
    0
     
  11. danadak

    Well-Known Member

    Mar 10, 2018
    1,880
    390
    I cannot seem to get a TIP120 merged into LTC lib, any black magic trick I should know about ?
    Tried the web suggestions, no luck.

    I downloaded a ON Semi PSPICE model.lib

    Regards, Dana
     
  12. elec_eng_55

    Thread Starter Member

    May 13, 2018
    129
    0
    Hi Dana:

    It took me forever to get the TIP120 and TIP125 models set up in LTSpice.
    I am not sure how I did it but you need to create two "mylib" folders, one in
    the "sub" folder and one in the "sym" folder. Then copy the 4 attached files
    to the appropriate folders. Hope it works for you.

    David
     
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