BJTs power dissipation

Discussion in 'Homework Help' started by bonjing, Sep 4, 2009.

  1. bonjing

    Thread Starter Member

    Dec 7, 2008
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    hi,

    am thinking deeply.. and really confused about this,

    Does BJTs dissipate the same amount of power at 4ohm and 8 ohm load impedance?

    say for example 180watt at 4ohm load, does it mean that it also 180 watt at 8 ohm load?
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
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    Well, 180 watts is 180 watts no matter the load resistance. Watts are simply volts times current. To go back to the 4 and 8 ohm loads, use some voltage, like 100 for an even number to work with. 180 watts and 4 ohms at 100 volts means 25 amps current. 180 watts and 8 ohms means 12.5 amps.

    If the BJT in question is in the ohmic region (linear operation), then dissipation may be higher with the 4 ohm load, as the BJT will be passing more current. The dissipation depends on the collector resistance of the BJT. If it is very close to saturation, the dissipation will be less. Generally speaking, a BJT's dissipation is greatest when its resistance is equal to that of the load.
     
  3. Ratch

    New Member

    Mar 20, 2007
    1,068
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    beenthere,

    I don't think so. When the load is small or zero, the current is at its highest, and all the supply voltage is across the transistor. Ouch! It gets hot. I am thinking of the pass transistor of a linear power supply.

    Ratch
     
  4. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    But in "normal" condition (not short-circuit) BJT dissipation is greatest when Vce=0.5Vcc
    Ptot=(Vcc^2)/(4*RL)
     
  5. Ratch

    New Member

    Mar 20, 2007
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    Jony130,

    I don't think so. When the load is very small, the current will be at the highest for a given voltage, and the voltage across the transistor will be at the highest. That is because the transistor has to drop the voltage that the load does not, and the small resistance value of the load cannot drop a lot of voltage So high current and small load voltage in relation to Vcc stresses the transistor the most. The ultimate stress to a transistor is a zero load or short circuit.

    Ratch
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Are you talking about the output transistors in an audio power amplifier?
    If the amplifier is class-AB then they heat with the amount of output power. A 4 ohm speaker will try to draw up to double the power than an 8 ohm speaker so the transistors will heat with up to double the dissipation. But if you turn down the volume control so that the output power into 4 ohms is the same as the output power into 8 ohms then the transistors' heat is the same.
     
  7. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    If you say so.
    Since you don't understand those basic principles.
    Try to calculate the max dissipate power in BJT for this case:
    1. Emitter follower with Re=8Ω; Vcc=12V and inputs signal change from 0 to 12V
    2. CE amplifier with Rc=8Ω; Vcc=12V and input signal from 0V to voltage that saturate the BJT.

    When you finish you calculation, then you will know that
    Ptot_max=Vcc^2/(4*RL) is correct answer.

    What is interesting this equation is correct for class B amplifier for peak power dissipation. The average power is equal Ptot_avr≈0.1*(Vcc^2)/RL
    http://sound.westhost.com/soa.htm
     
    Last edited: Sep 5, 2009
  8. Ratch

    New Member

    Mar 20, 2007
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    Jony130,

    I think I do. After all, the same current is going through both the transistor and the load. It is just a matter of voltage distribution between the two.

    You are assuming a constant load amplifier. I made it clear in post #3 that I was referring to the pass transistor in a linear power supply. The load can change in that situation.

    For a constant load amplifier, the max power dissipation of the transistor occurs when the resistance of the transistor equals the resistance of the load. That is from the max power transfer theorem. For a power supply, the load resistance can be anything. If the power supply is called to supply a high current to a small resistance load, that pass transistor is going to get hot is a hurry. That is because most of the supply voltage is dropped across the transistor along with a high current.

    Ratch
     
  9. kdillinger

    Active Member

    Jul 26, 2009
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    So was the OP's question: 4Ω or 8Ω.
     
  10. Ratch

    New Member

    Mar 20, 2007
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    kdillinger,

    A 100% increase in load resistance tells me that the load is variable.

    Ratch
     
  11. kdillinger

    Active Member

    Jul 26, 2009
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    Do you do this often?
     
  12. Ratch

    New Member

    Mar 20, 2007
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    kdillinger,

    Please elucidate.

    Ratch
     
  13. bonjing

    Thread Starter Member

    Dec 7, 2008
    12
    0
    yes, it is all about output transistor power dissipation...
    because the amp is pure design to drive a 8 ohm load as I look into the transformer it is rated 250VA with the secondary of 46vac-0-46vac.

    am wondering if I can connect a 2x200 8 ohm load impedance so that I can get a 4 ohm load?
     
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