Audio Amplifier

Discussion in 'Homework Help' started by huntlk, Feb 28, 2010.

  1. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    I have constructed a discrete audio amplifier with a Class A input stage and a Class AB emitter follower output stage. I have not been supplied with the AC value of Vin. Does anyone know how to determine this value? I must calculate the ac values of vb, ve, and vc.

    Thanks

    The base resistance of Q2 must be adjusted to produce a quiescent voltage of +5 volts at the emitter of Q3.

    adjust the input voltage to yield an ac load voltage (Vout) of 4 Vpp

    adjust base resistance (R7) of Q2 to product a DC voltage of +5 volts at the emitter of Q3.


    I have added a new schematic with 0 volts input
     
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    Last edited: Feb 28, 2010
  2. hgmjr

    Moderator

    Jan 28, 2005
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    Can you attach a diagram of your circuit to aid us in assisting you?

    hgmjr
     
  3. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    Yes, I will try
     
  4. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    I drew this circuit in Multi-Sim. How can I upload it so you can see it? It says it is an invalid file. Help!
     
  5. hgmjr

    Moderator

    Jan 28, 2005
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    What extension does the file have?

    hgmjr
     
  6. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Can you export the file to .png format?
    Or make a screenshot of it to post.

    Greetings,
    Bertus
     
  7. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    I uploaded it as a pdf file. Hope you can view it. Thanks.
     
  8. hgmjr

    Moderator

    Jan 28, 2005
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    Well done. That will do nicely.

    hgmjr
     
  9. hgmjr

    Moderator

    Jan 28, 2005
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    In your diagram it appears that you have a defined AC input. I not clear on what you are asking?

    hgmjr
     
  10. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The C and E pins of the 2N3906 are the wrong way around.
    I mirrored the transistor to show you how.

    [​IMG]

    Sorry for the upside down text.

    Greetings,
    Bertus
     
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  11. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    I'm sorry. That input is wrong. I meant to erase that value.
     
  12. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    Procedures:

    Adjust the base resistance of Q2 to produce a Q voltage of +5 volts at the emitter of Q3

    Input frequency 1k Hz and adjust input voltage to yeild an ac load voltage (Vout) to 4 Vpp.

    Adjust base resistance (R7) of Q2 to product a DC voltage of +5 volts at the emitter of Q3
     
  13. hgmjr

    Moderator

    Jan 28, 2005
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    To get at the value of input voltage that would yield the desired output voltage, you will need to determine the midband gain of the overall amplifier.

    That entails calculating Vout/Vin. Then knowing the desired output voltage you will be able to determine the corresponding input voltage that would yield it.

    hgmjr

    Post edited by a member of the Department of Redundancy Department.
     
  14. hobbyist

    Distinguished Member

    Aug 10, 2008
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    I don't know if you adjusted this value on the fly, or calculated it, but this is a learning assignment, so here is a brief explanation, of how to calculate a ballpark figure for R7.

    VEQ3 = (VCC / 2) = ?
    so VBQ3 = (VEQ3 + VbeQ3) = ?

    VR9 = (VCC - VBQ3) =?

    IR9 = (VR9 / R9) =?

    VEQ2 = (IR9 x R8) =?

    VBQ2 = (VEQ2 + VbeQ2) =?

    VR6 = (VCC - VBQ2) =?

    IR6 = (VR6 / R6) =?

    R7 = (VBQ2 / IR6) =?

    All voltages referenced to ground.
    -----------------------------------

    The Vout sig. of Q1 needs to be lower than the VBQ2 so as to not overdrive Q2 to cutoff or saturation.
     
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  15. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    Can someone please tell me how to calculate the total gain for this circuit? I am totally lost. Thanks in advance!
     
  16. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    Using your method

    VEQ3 = 5V
    VBQ3 = 5.7V
    VR9 = 4.3V
    IR9 = 4.3mA
    VEQ2= 20.21V
    VBQ2= 20.91V (more than the Vin of 10V)
    VR6 = -10.91V
    IR6 = -1.091mA
    R7 = 19.2KΩ

    I'm either calculating something wrong and this is not a correct way to calculate R7.
     
  17. hobbyist

    Distinguished Member

    Aug 10, 2008
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    Ok,

    4.3v. / 4.7K ohms =? = IR9.

    Then recalculate VEQ2 by taking IR9 x 1Kohms. =?

    Add 0.7v. to that to give VBQ2 =?

    hope this helps.




    Now for calculating the gain.

    It needs to be understood that a first order, (ballpark figure) of calculation for the Av. of a CE amp with emitter resistor, is around (RC/RE).

    But when this is feeding another stage or a load, then the gain will be {(RC in parrallel with Rload) / RE}

    With a CC stage there is no voltage gain, in fact a loss of voltage gain.

    Also with high beta transistors, the first APPROXIMATION of Zin, to a CE stage is the value of the bleeder resistor from ground to the base.

    With this premises in mind,

    looking at Q1 stage first. The RC for that stage is R3 and this R3 value is in parrallell with R7, so a first approximation for stage gain would be {R3 parrallel with R7} / (R4 + R5).

    THIS IS DC GAIN. NOT AC signal gain.

    Actual signal gain for this stage would be the parrallell combination, of R3 and R7 divided by APPROXIMATELY R4 when C3 is at it's least reactance under signal conditions.

    WITHOUT C3 then the signal gain would be = to DC gain.

    So C3 bypassing R5 allows signal gain to be higher while keeping DC thermal runaway at it's lowest,

    So with stage Q1 gain approximated, then solving for Q2 gain would be, R9 divided by R8,

    HOWEVER THIS IS GROSSLY SIMPLIFIED due to the fact that Q3 and Q4 emitter is grounded, through the load then there is a low impedance there which would lower the gain calculated.

    But probably for a FIRST APPROXIMATION, you could use the simplified gains calculated.

    Notice that voltage gain was not calculated for Q3 and Q4, do you know why??

    Now the Total gain is the product of each stage gain.

    Like I tell everyone, wait a couple days to make sure there is no corrective posts from more experianced posters before taking what I shared as factual. I'm only a hobbyist in this field, so be cautious of what you learn from me.

    If no one makes any corrections, then this is probably right.
     
    Last edited: Mar 2, 2010
    huntlk likes this.
  18. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    I'm sorry I was not clear. I need to know how to calculate the ac signal gain of the circuit. I have calculated the gain of a 3 stage CE amplifier circuit, but I have no experience with this type of circuit. Please help me if you can.
     
  19. huntlk

    Thread Starter New Member

    Feb 19, 2010
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    0
    hobbyist,

    Thank you so much for the new information for calculating R7. This is right. Now if you can help me to calculate the ac signal gain. I just don't know where to begin!
     
  20. hobbyist

    Distinguished Member

    Aug 10, 2008
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    56
    The Av. for CE amp is (RC / RE). ..... Av = volt.gain.

    When this is feeding another stage or a resistive load, then the Av. will be (RC in parrallel with Rload, divided by RE) written as {RC//Rload / RE}

    Now if RE consists of more than one resistor, with one of them bypassed with a capacitor, then the Av. will be (RC//Rload / RE not bypassed). Known as split emitter resistance.

    The main purpose for the emitter resistor is for 1. temperature stability, 2. to design the stage with a determined gain, 3. increase input impedance, 4.anything else I mightr have forgotten. ...lol...

    The Av. CC amp is less than 1. A loss of gain, by the Vbe of the transistor. But it has large current gain, which is needed to drive a low impedance load.

    Now input resistance for a CE amp, is usually calculated as being the bleeder resistor from base to ground, this is only a first order, (first approximation), to calculate actual impedance would be far more involved beyond the scope of this lesson.

    For all practical purposes with high beta transistors, having emitter resistors, the input imperdance Zin = RB1 ... base to ground resistor...
    Now with this info in hand, lets look at your circuit starting from Q1 and going towards the output.

    Q1 stage:

    RCQ1 = (R3 // R7) =? .... remember " // " means parrallel combination..

    REDC = (R4 + R5)=?

    REAC = REQ1 = R4=?

    AvQ1 = (RCQ1 / REQ1) =?.... don't overlook this symbol, " / " meaning divided....

    Now the reason I included the DC value of REDC is to show you what's involved in choosing what resistors is nessacary to calculate the signal gain of a stage.

    Q2 stage:

    RCQ2 =?

    REQ2=?

    Now if the load was much lower than 100 ohms than the input iompedances to Q3 and Q4, would have to be calculated at (Beta +1) x load RE for a first approximation, however since load is 100 ohms and assuming high beta transistors, then the input impedaces are high enough to have negligable effect on the gain of Q2 stage.

    AvQ2 = (RCQ2 / REQ2)=?

    Now trick question,
    Where is the output of the Q3 and Q4 stage. Which terminal, is common between input and output as far as signal voltages are concerned.

    AvQ3 and AvQ4 =?......
    Now to solve for total voltage gain of the entire amp, you multiply the individual gains, That are 1 and above..

    Avtotal= (AvQ x .....AvQ.......)=?
    This is a first order APPROXIMATION, that is satisfactoryu to getting ballpark figures.

    Hope this helps...
     
    Last edited: Mar 2, 2010
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