amplifier output transistors

Discussion in 'General Electronics Chat' started by sampinoy, Oct 7, 2004.

  1. sampinoy

    Thread Starter Active Member

    Sep 24, 2004
    31
    0
    hi everybody,

    i hav bin looking at several circuit diagrams of amplifiers and i saw this one that has 2 coupling capacitors connected to the speaker. there are two transistors in the output, PNP & NPN with their emitter joined together. from the point where both emitters are joined a capacitor is connected and the other end is connected to one of the speaker terminal. then from the NPN collector another capacitor is connected and the other end connected to another terminal of the speaker.

    what i usualy see is one capacitor connected from where both emitters are joined and the other end connected to the speaker.

    can someone pls explain? :unsure:

    i hav just bought a battery operated AM radio kit which i will try to assemble this weekend. the circuit diagram also used a single capacitor connected to the speaker. im at a lose. pls help me to understand. :(
     
  2. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    hi sam,

    you should have posted a schem of the circuit you were asking about. anyway i have drawn the circuit base on your description, this could be wrong.

    that output stage is configured as PUSH-PULL COMPLIMENTARY SYMMETRY AMP. its afairly common output stage circuit. but what bugs you is why there are 2 caps connected to the spkr. and not just 1.

    before i explain why there are 2 caps involve, let me give you how a push-pull configuration works. the PNP and NPN conducts alternatively, they do not conduct at the same time.

    C1 is a blocking and at the same time a coupling cap. as a coupling cap it passes the signal to the spkr everytime Q1 conducts. as a blocking cap it prevents the dc negative voltage from the voltage source from entering the emitter of Q1

    C2 is likewise C1. as a coupling cap it couples the signal from Q2 collector to the spkr. as a blocking cap it prevents the dc negative voltage from entering the collector of Q2

    the use of 2 caps connected to each terminal of the speaker is what we call a returned to supply load.

    the values used for C1 & C2 are always equal.

    you mentioned that you have bought an AM radio kit. am not trying to dampen your enthusiasm but i believe you are going too deep for your present understanding of electronics. you might be frustrated with that project in case if it fails to work inspite of the concientious effort to follow instruction. anyway go ahead, i hope the schem includes voltage test points and instructions on how to tune the IFTs. :)

    as a final word of advise for your project, do not use a variable power supply like the ones bought from electronic stores. power it with batteries, if that project has a recommended supply voltage of 9v use 6v. if it is 6v use 4.5v ;)
     
  3. sampinoy

    Thread Starter Active Member

    Sep 24, 2004
    31
    0
    hello,

    sir, thank you for your response.

    you were rite, the circuit diagram of the kit does not have voltage test point but it has instruction on how to tune the IFT and how to align the antena. it is also battery powered. in truth i was planning to use the adaptor of my sister which uses for her yamaha organ. thank you for the advise.

    i have not yet totaly gone thru the 2 books i have bought and already have too many questions going thru my mind. :) yes ur rite i'm goin too fast, maybe coz of my eagernes to learn electonics. it's alrite am willing to go thru the grinder just to learn. :rolleyes:
     
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