Low frequency audio amplifier circuit

Discussion in 'General Electronics Chat' started by khanh, Feb 16, 2006.

  1. khanh

    Thread Starter Member

    Jan 6, 2006
    28
    0
    my last post transistor circuit posted (see "Differential DC amplifier circuit" topic) was received very helpful advice from people (like Ron H, chesar1 and the other).Now it comes again:

    I've constructed a standard circuit to perform a similar task: amplify a ac signal.

    The first circuit called "single stage Audio Freqency voltage amplifier", it was tested to have a gain of 60 as I expected. Now the fun part came in, when I setup a "multistage audio frequency voltage amplifier" based on the first part, the 2nd gain returned as I calculated, but the 1st gain which is the gain of the circuit 1 returns to 40 instead of 60?????????

    I know some guys here have a better undertanding of transistor than me (as it is proofed) so please help.

    Attached file: 4 files included (two are in doc format which show you the real picture of the circuits constructed in the case you don't you multisim like me, the others ,of course, are in multisim format 8, so anyone got multisim can test it straghtaway)
     
  2. khanh

    Thread Starter Member

    Jan 6, 2006
    28
    0
    Forget that you need to rename files "562_multi_stage_AF_amplifier.doc", and "558_single_stage_audio_freq_amplifier.doc" to ms8 extension since the forum does not allow to post such extension files? for what reason?
     
  3. JoeJester

    AAC Fanatic!

    Apr 26, 2005
    3,373
    1,157
    khanh,

    If you wanted to post your multisim circuit, I would recommend you export it to a pspice file [.cir extension].

    The last two files of yours were multisim compressed files.

    I am curious why you feel it's necessary to use 1 farad capacitors?
     
  4. jatulm

    New Member

    Feb 13, 2006
    7
    0
    I see a first important thing:
    at the output of the single stage you have a RC divisor with C = 1F and R=2k2 in parallel with R=4k... then you have equivalente resistance Req=1k4

    now at the multistage output you have C=1F and R=2k2 in parallel with R=10k and R=4k and R=beta/gm (input resistance of the base of the second BJT)
    and that makes an equivalent R, Req=800ohm

    that's why you have a smaller gain
     
  5. khanh

    Thread Starter Member

    Jan 6, 2006
    28
    0
    JoeJester: I put 1 fara because I don't want the capacitor to interfere with the resistors since capacitor has its own resistance itself, that way i don't need to care about those capacitors (its main function is to isolate the ac signal and dc one) though you can place smaller and more realistic value but remember to calculate its equivalent resistance.

    Jatulm:
    I don't understand this part, input resistance of the base of the second BJT is 26/(1.8-1.1) = 23.6 ohm
    So how can you get equivalent of 800 ohm. Can you give more specific explaination.
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
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    Well, your arithmetic does't make sense to me (I think you meant 26/(1.8-0.7)), but the emitter resistance is about 23.6 ohms. However, the base current ib=ie/(beta+1), so the input resistance is approximately re*beta, which is the same as beta/gm, as jatulm said (gm=1/re).
     
  7. jatulm

    New Member

    Feb 13, 2006
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    I use gm = Ic/VT and beta = 100...

    it is an approximation...you should see the transistor datasheet... anyhow, my point is that you loose gain because of the Req viewed from the capacitor to ground wich is smaller than the resistance viewd in the single-stage amplifier.

    Remember that in this configuration, you have a current amplification. the voltage you see is due to the resistance viewed to ground at the output (see the small-signal model of your amplifiers)

    regards

     
  8. khanh

    Thread Starter Member

    Jan 6, 2006
    28
    0
    I see your point!!!!
    The beta of 2N3409 varies instead of being a constant : hfe is between 70 to 300 for base current of 1 ma.
    So in practice, do you know any good way to approximate this parameter?
     
  9. jatulm

    New Member

    Feb 13, 2006
    7
    0
    The easiest way is to use a potentiometer (varying resistor) so set the gain, once you have built the circuit.
    Other way is to use a multimeter that messures the hfe of a BJT.
    Here comes creativity :D


     
  10. a_nashed

    New Member

    Jun 13, 2007
    1
    0
    hi
    i have read ur message here, but i can't see any attachments
    could u plz help me
    thanks alot
    khanh
     
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