Many errors in this simple circuit?

Discussion in 'Homework Help' started by Yabba Rays, Oct 12, 2016.

  1. Yabba Rays

    Thread Starter New Member

    Oct 5, 2016
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    [​IMG]

    This is supposed to be a basic audio amp. How many errors here? No battery decoupling, DC going through the speaker and what's going on with Q2, Q3? Will that even work - or am I missing something?

    Sorry, title should read 'errors' not 'effors' but can't find a way to correct that.

    (Mods, please change heading if possible. Thanks)

    Janet (Yabba Rays)
     
    Last edited: Oct 12, 2016
  2. crutschow

    Expert

    Mar 14, 2008
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    Yes, it has DC current going through the speaker which is generally a no-no.

    You are correct, Q2 and Q3 won't work as shown.
    You either need a resistor from the collector of Q2 to V+, or Q3 needs to be a PNP.
     
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  3. WBahn

    Moderator

    Mar 31, 2012
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    MOD NOTE: Title changed as requested by TS. Moved to Homework Help.
     
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  4. ci139

    Member

    Jul 11, 2016
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    are most likely the NPN darlington drawn . . . has it happens -- coz the replacement of upper one with PNP enables a current path through the base of then PNP Q3 whitch eighter saturates the least or emtys the battery fast or reduces the terminal voltage or ruins the Q3 or/and Q2

    i considered the collector feedback for the input stage Q1 - because the application runs on a battery - say 2x 1.5V that changes it's terminal votage from 2.9V down to say 2.3V during battery discharge-/service- time
    i considered using different polarity transistor for Q1or reversing rails×polarity for Q23 darlington (both emitters go to speaker)

    the circuit looks like drawn by instructions at bad quality phone call ???
     
  5. JoeJester

    AAC Fanatic!

    Apr 26, 2005
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    Which book did you get that from? Can you supply the "text" as well?
     
  6. ci139

    Member

    Jul 11, 2016
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    on a second thought i'm not so sure about what i just said
    ► it's the 3× bjt gain of 200³ = 8000000 -- eight million

    i attempted something like this - an audio power amp -
    and for a reason - i had to negative feedback - in principle -
    Q2 and also Q3 e.g. both stages - to get un-distorted
    near 1:1 voltage amplified signal ??? while not varying
    the supply keeping it steady 3V
     
  7. ci139

    Member

    Jul 11, 2016
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    ok, i can't design transistor wide bands (fq. × feed voltage), but since we don't have anything here , here what i attempted
    Random_B2A-fp.png Random_B2Ab.png
    Random_B2A.png Random_B2Ab.png
    (just a blind shot not that i'd had anything speciffic in mind - and i didn't yest compute anything)
    optimizing . . .
    Random_B2Ac-fp.png
    Random_B2Ac.png
    still hasn't computed anything . . .
     
    Last edited: Oct 13, 2016
  8. Yabba Rays

    Thread Starter New Member

    Oct 5, 2016
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  9. MrChips

    Moderator

    Oct 2, 2009
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    There is not a lot wrong with the circuit, only the drawing of Q3 is incorrect.

    BC558 is a PNP transistor. The emitter should be on the +ve side of the battery. The collector is connected to the loudspeaker.

    You can replace Q1 and Q2 (BC548) with 2N3904.
    Q3 (BC558) may be replaced with 2N3906.
    I would choose an 8Ω speaker (or higher), not a 4Ω speaker.

    I would decrease R1 from 1M5 to about 560k-1M for less distortion.
    Otherwise, the circuit works fine on 3V supply.
     
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