Doubling output transistors

Discussion in 'General Electronics Chat' started by GeorgeBuchanan, Apr 12, 2013.

  1. GeorgeBuchanan

    Thread Starter New Member

    Apr 12, 2013
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    Hello! I have a transistor related question, and while I have an idea as to what the correct answer may be, I'd love confirmation / correction.

    If I have the following circuit (simplified, obviously):

    Code ( (Unknown Language)):
    1. (pretty low impedance) signal----transistor-----output 1
    I can clone the output transistor like this:

    Code ( (Unknown Language)):
    1. (pretty low impedance) signal--+-transistor-----output 1
    2.                                |
    3.                                |-transistor-----output 2
    (nb: Unlike this link I am not connecting the two outputs together; they are driving independent loads)

    My question is: Will I still get the same signal out of output 1 for both setups, or does the fact that I'm splitting the current between the two transistors mean that I will get a smaller signal?

    My guess would be that since the transistors are being driven by a (relatively) low impedance signal, the voltage would be the same, and I'd just end up with two outputs instead of one. Am I right?
    Thanks!
     
  2. WBahn

    Moderator

    Mar 31, 2012
    18,079
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    If you have a sufficiently low impedance signal, then if the original circuit works (e.g., does not need a base resistor because of it's configuration) the addition of the second circuit (assuming it would also work if it were the only circuit present) should have little influence.

    This is one area were MOS technology is a clear leader. Even with a high impedance signal you can drive many, many transistor gates. We routinely distributed a high impedance bias voltage to over a million transistors with no problems (well, okay, there are some issues you need to address, but they are pretty tame).
     
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  3. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    I think we need a real schematic. What type of transistors are you talking about - BJTs, or MOSFETs?
    What is the configuration, e.g, common emitter, common collector, common source, common drain, etc.?
     
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  4. GeorgeBuchanan

    Thread Starter New Member

    Apr 12, 2013
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    Thanks WBahn and Ron, I'm away for the weekend but I'll do a proper schematic when I get back. Since it's an audio amplifier there are actually two transistors I want to power, one common emitter, one common collector.

    I must say, mosfets seem to be better at just about everything! However the circuit I have drives BJTs, and I have more of those BJTs already attached to a heatsink, hence me sticking with BJTs.

    Thanks again!
     
  5. GeorgeBuchanan

    Thread Starter New Member

    Apr 12, 2013
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    So I took the covers* off the BJTs and it turns out they are MOSFETs! So this question was actually irrelevant :(
    I'm sorry to waste your time, although I still did learn something, and who knows - one day I might just have to drive multiple BJTs as described!
    Thanks again for your prompt and helpful comments.

    *it had this strange, alarmingly-difficult-to-remove dust cover glued on which obscured the part number.
     
  6. WBahn

    Moderator

    Mar 31, 2012
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    A strange "dust cover" glued on to the transistor? Are you sure it isn't a heat sink?
     
  7. GeorgeBuchanan

    Thread Starter New Member

    Apr 12, 2013
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    Don't worry, it's not the heat sink! It's one of these things.
     
  8. THE_RB

    AAC Fanatic!

    Feb 11, 2008
    5,435
    1,305
    That's not for dust. TO-3 trannies are already dust proof. :)

    It's a TO-3 insulating cover, to give some safety when the TO-3 package is live and the heatsink grounded, against a metal object that might touch both transistor and heatsink.
     
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