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

    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


    Mar 14, 2008
    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.
    Yabba Rays likes this.
  3. WBahn


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


    Jul 11, 2016
    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
    Which book did you get that from? Can you supply the "text" as well?
  6. ci139


    Jul 11, 2016
    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


    Jul 11, 2016
    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 . . .
    still hasn't computed anything . . .
    Last edited: Oct 13, 2016
  8. Yabba Rays

    Thread Starter New Member

    Oct 5, 2016
  9. MrChips


    Oct 2, 2009
    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.