3 watts BJT Amplifier

Discussion in 'The Projects Forum' started by Merk Morondos, Sep 20, 2016.

  1. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    I would like to ask if what is the possible model of transistor can i used to substitute the 2 output transistor on schematic because it is too hard to find the 2sar542p and 2scr542p in the Philippines. Thank you
     
  2. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,875
    370
    absf likes this.
  3. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    Last edited: Sep 22, 2016
  4. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,875
    370
    You should check the specification on any alternative that you choose as those lists were produced by a website auotmagically. I have not personally checked that they are all good replacements. For instance, they come in different packages. That may not matter if you are building this from scratch but it will matter if you have a PCB designed around those original transistors.
     
  5. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    We tried to test it in breadboard and we observed that the transistors on our output(replacement) gets hot easily and make some smell.And one of our resistor overheats (resistor 0.5) .what we will do to solve that kind of problem on our circuits

    We used 2sc3420 and 2sb1165
     
  6. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,875
    370
    The current through the two 0.5Ω resistors should be equal and so they should get equally hot. If only one of them is getting hot then something is wrong. Which resistor is getting hot?

    Check the transistor connections and if it is R8 which is getting hot check C2 polarity.
     
  7. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    Resistor 7 is the one getting hot. I think our capacitor 2 is on the right polarity. The output transistor have different specs with each other is that ok ?
     
  8. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    Our original transistor have equal specs but the only difference is npn and pnp but on our replacement they have different Ic Vce Power rating but same current gain
     
  9. Colin55

    Member

    Aug 27, 2015
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    The voltage at the join of the two 0.5R should be 6v
     
  10. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    Its 6.6 Vrms and its average is 5.9494
     
  11. Colin55

    Member

    Aug 27, 2015
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    It should be 5.949387435621
     
  12. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,875
    370
    You have 6.6V rms at the output? Is this with no input?

    I can't see where the current is coming from that is making R7 hot if it isn't through R8, but then they would be equally hot.
    Check the component connections very carefully and as it is on a breadboard, make sure that none of the component leads are shorting together.
     
  13. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    Ok I will try again tomorrow. Thanks again
     
  14. Merk Morondos

    Thread Starter New Member

    Sep 20, 2016
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    AlbertHall I found out that our Vin in the function generator is 1.41 when we tested in our circuit but in our schematic it is just 0.5.
    Can you help me to edit my circuit having the following specs :
    Gain of 8
    1 Vrms
    3 Watts
    Any value of load
    12 Vcc
    no clipping

    thank you in advance
     
  15. AlbertHall

    Well-Known Member

    Jun 4, 2014
    1,875
    370
    The short answer to that is no.
    With Vcc = 12V the maximum output you can get is around 4V rms so an input of 1V rms and a gain of 8 will drive the amplifier well into clipping.
    The output power will depend on the load impedance. You will get 3W if the load is 5.3Ω. If the load is greater than that you will get proportionally lower power and if the load is less than this you will get more power delivered to the load.

    Did you fix the overheating? What caused it?
     
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