1. Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.
    Dismiss Notice

Why this circuit doesn't work , and how to simulate it?

Discussion in 'Circuit Simulation & PCB Design' started by ashour, Oct 22, 2010.

  1. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0
    Hi

    my professor gave us a circuit to create as an assignment , he wants us just to learn the technical part before we gets into real science , so i don't understand how it works i just have the diagram.

    i connected it on a bread board but it didn't do anything! just nothing i think there is an error in the values of resistors or maybe i got wrong transistors he didn't mention what model to use just npn or pnp so i got help from a pre-junior friend and told me about some models but i'm not sure if they are correct.

    the circuit is supposed to amplify the sound produced from the mic , but i get nothing.

    A second question , how would i simulate such circuit on multisim i can't find the transistor models which is pnp bc160 and npn c945 in its database also i can't find a speaker or a mic?

    note:
    mic is capacitor type (didn't find any other type at the shop)
    resistances are 3Ohms and 2 Watts
    diodes are pn junctions up to one ampere
    speaker is of 8 ohms and 0.5 watts of power

    Thanks
     

    Attached Files:

  2. MikeMl

    MikeMl Well-Known Member Most Helpful Member

    Joined:
    Mar 17, 2009
    Messages:
    11,049
    Likes:
    541
    Location:
    AZ 86334
    Cant work as drawn. No simulator needed. I'll let you discover the screw up.

    To simulate a mic, use a voltage source that makes the same output as the real mic.
    To simulate an audio (non-critical) circuit with a transistor you don't have in the library, look up the data sheet for the transistor, and find a similiar one that is in the library.
    A 1N4148 or 1N4007 will work for almost any Silicon Diode.
    For a first order approximation, use a 8 resistor in place of the speaker. If you can measure some physical parameters of the speaker and its enclosure, you can make a more complex model that accounts for resonances, speaker box tuning, etc, but just get the basic sim working first...
     
  3. RCinFLA

    RCinFLA Well-Known Member

    Joined:
    Jul 23, 2009
    Messages:
    1,065
    Likes:
    31
    Location:
    FLA, USA
    Hint, reverse the C-E connections on PNP.

    You need base bias. Let you figure how to do it.
     
  4. dave

    Dave New Member

    Joined:
    Jan 12, 1997
    Messages:
    -
    Likes:
    0


     
  5. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0

    well i forgot to mention that the other lead of the mic is grounded but still after reversing the pnp as u said its not working i tried simulating it by just putting an 8 ohm resistance and a DC voltage as a mic but it just shows very poor voltages across the resistance (speaker)
     
  6. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0
  7. magnatro

    magnatro New Member

    Joined:
    Mar 23, 2010
    Messages:
    295
    Likes:
    0
    Location:
    India
    I simulated it and got this..!
    [/ATTACH]
     

    Attached Files:

    • test.pdf
      File size:
      14.7 KB
      Views:
      343
  8. magnatro

    magnatro New Member

    Joined:
    Mar 23, 2010
    Messages:
    295
    Likes:
    0
    Location:
    India
    View attachment test2.pdf
    this is what i got when i changed the diodes biases..
    the diode is forward biased to npn transistor on positive half cycle..
    i think that's the error
     
  9. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0
    How do u get such a pdf? (sorry for noobish question) i just put voltmeters in the circuit and double click them to know the results , but i don't detailed graphs like that u use multisim too?

    i'm gonna try the circuit as you said now - but in real - hope it works
     
  10. Grossel

    Grossel Member

    Joined:
    Sep 16, 2008
    Messages:
    918
    Likes:
    22
    Location:
    Norway
    Producer tag of pdf strongly indicate use of pdf printer software :)
     
  11. magnatro

    magnatro New Member

    Joined:
    Mar 23, 2010
    Messages:
    295
    Likes:
    0
    Location:
    India
    hey... it only a pdf printer software.. you can download one from 100s available..
    i used the microsim simulator PSpice.
     
  12. Mosaic

    Mosaic Well-Known Member

    Joined:
    Jun 3, 2010
    Messages:
    2,569
    Likes:
    128
    Location:
    Caribbean
    Just on the surface of it, I'd either tie the collectors together or the emitters as an emitter follower but not collector to emitter.

    Secondly, the diodes are a problem, they will Block the forward biasing of the Transistors. Even if you turn them around to fwd bias the transistors they will attenuate the mic signal by .6V and that's a lot. Replace the diodes with 1K resistors to limit the base current drive to prevent saturation.The nature of a bipolar transistor is it will amplify the waveform that forward biases it. Base Diodes are not a requirement unless you are playing with RF detectors.

    The ground is the common junction between the dual pwr supplies.
     
  13. colin55

    colin55 Well-Known Member

    Joined:
    Feb 14, 2009
    Messages:
    3,534
    Likes:
    82
    Location:
    Melbourne Australia
    There are a number of things wrong with the circuit.
    The supply voltages are wrong and incorrectly drawn.
    The lower transistor is incorrectly drawn and connected incorrectly,
    The 3R resistors will reduce the output to the speaker considerably, "The mic input will not have sufficient drive to drive the speaker via the two transistors in push-pull and the speaker goes to earth - but we don't know how the battery is connected.
    The speaker is directly coupled by we don't know how the battery is connected and this may not be correct - it may need to be AC coupled.
    On top of this the bases are not biased and the purposes of the diodes are not utilized.
    The purpose of the diodes is to reduce cross-over distortion but the circuit does not provide this feature.
     
    Last edited: Oct 28, 2010
  14. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0
    @magnatro i didn't mean how you write the pdf :D i mean how you output those graphs from multisim
     
  15. magnatro

    magnatro New Member

    Joined:
    Mar 23, 2010
    Messages:
    295
    Likes:
    0
    Location:
    India
    its called transient analysis.. it in every p spice program..
     
  16. ashour

    ashour New Member

    Joined:
    Aug 9, 2010
    Messages:
    22
    Likes:
    0
    I think that there is a fatal problem with this circuit it amplifies signals but it doesn't have any pre-amplifier part .

    so someone have a sugesstion about some other circuit even using ICs that takes input from mic and outputs it to a speaker?
     
  17. Mosaic

    Mosaic Well-Known Member

    Joined:
    Jun 3, 2010
    Messages:
    2,569
    Likes:
    128
    Location:
    Caribbean
    Use an LM324 Opamp as a preamp and an LM386 (1.25W) or LM380 (2.5 - 5W) to drive your speakers. Google for circuits...lots of those around.
     
  18. audioguru

    audioguru Well-Known Member Most Helpful Member

    Joined:
    Mar 16, 2004
    Messages:
    32,445
    Likes:
    937
    Location:
    Canada, of course!
    ONLINE
    The two transistors including the PNP that is connected backwards are turned off all the time so they will not do anything.
    A mic has an output level of only 0.005V. It needs to have a preamp with a voltage gain of 100 to 200.
     

Share This Page