What is exactly Cob of power transistor?

Discussion in 'DIY' started by Alan0354, Jun 15, 2017.

  1. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    As the tittle, I saw this in every datasheet of power transistor. I really don't know what this really mean. I know Cbe, Ccb and Cce, never really heard of Cob.

    What is the definition of Cob?
     
  2. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Cμ is sometimes referred to as Cob or Cobo in datasheets.

    upload_2017-6-15_18-53-40.png
     
    sregor likes this.
  3. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    Yes, I saw this in Vershey site, But what does it means for emitter follower? Is it Cce+ Ccb?,

    If it is Cce+Ccb, then meaning if you use emitter follower at output pair and each transistor has 500pF Cob as in the datasheet, does it mean the output has 2X500pF=1000pF from the output to ground return? I can assure you not too many amps can withstand 2000pF without bursting into oscillation.

    The reason I ask is because it's time for me to buy more output transistors as I only have about 30 of each left. I picked TTC5200 and TTA1943 because the Cob is less than 200pF compare to 500pF on most other transistors. Now that I have to buy more ( need more to match Vbe), I need to revisit this question again.

    I still have about 40 each of the MJL1302/3281, I can experiment with it, BUT I don't like the idea of try it and see. I want to know it is sound in theory.
     
  4. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    I want to add, the reason it's so important to have a clear definition of Cob are:

    1) If Cob meaning the capacitance from base to emitter (Cbe). Then it's NOT important for emitter follower. The voltage at the base is almost the same as on the emitter. So if you say the gain of the emitter follower is 0.95( from base to emitter), then even if Cbe is 500pF, if you have gain of 0.95, the capacitance from output to ground is only 500pF X 0.05=25pF which is nothing.

    2) BUT if Cob means from output to ground, that means Cob=Cce( remember Cbe is nothing as explained in 1). Then, for emitter follower, it means you literally have 500pF from output to ground. This would be VERY bad if you have multiple transistors output stage.

    You have to know to predict the effect in different cases whether you are using it as CE, CC or CB configuration. I so wish they just specify Cbe, Ccb and Cce and people can calculate according to what configuration they use the transistor.

    That's why the definition is EVERYTHING. Does Conrad know this?
     
    Last edited: Jun 15, 2017
  5. sregor

    sregor AK Subscriber Subscriber

    Messages:
    3,099
    Location:
    Mass
    [​IMG]

    Shouldn't the first term for Cobo in the equation be Ccb?
    Cob is only output to ground for a common base configuration, which an output isn't. It. If the first term is Ccb, then it would be approximately base to collector capacitance, which I believe is correct. My 2 cents
     
  6. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    See, I read this article, this is for common base configuration. Also, Cobo is not Ccb as the formula provided. The transistor is useless if the reverse feedback capacitor is 500pF!!!

    For emitter follower, you need to use this circuit:


    Cob.jpg

    Then still the question is what is Cob?
     
  7. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    I have been searching, problem with the Infineon app note 024 has a typo!!! This is how it should be:

    Cob.jpg

    The Ceb should be Ccb. This make a whole lot more sense now. Not exactly the answer I want, but make sense now.

    Ceb is so much higher than Cce because Ceb is diffusion capacitance of the Forward bias diode compare to Cce which is the junction capacitance of the reverse biased diode, so Ceb + Cce = Cce in most cases. Therefore both Cobo and Cobs is about the same and called Cob.

    I am surprised Ccb(reverse transfer capacitance) is so high for big transistors, it's going to be the major limitation in frequency response. Also, power transistor that has high Cob is like the cap loading the driver transistor in the power amp. So it is critical to choose a power transistor with low Cob for the design.
     
  8. I LIKE MUSIC

    I LIKE MUSIC Super Member

    Sorry I missed the error in that picture. I knew what I wanted to post, but I choose a bad example.
     
  9. Alan0354

    Alan0354 Super Member

    Messages:
    2,302
    Location:
    Bay Area
    Thanks anyway, I got my answer.

    Typo is a killer, you would think they proof read first. One little typo can send you to chase around.
     
  10. FootFungus

    FootFungus Super Member

    Messages:
    1,021
    Location:
    Dallas
    Look up the the Ayre V-3 for an example of this. The main complaint people have with it are the higher frequencies are rolled off. This is actually a byproduct of the design - the industrial MOSFETs outputs used by Ayre have huge capacitances.
     

Share This Page