Class AB amplifier Issue

Discussion in 'Homework Help' started by Georacer, Apr 11, 2011.

  1. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    The time has come! I 'll ask something in the Homework Help!

    I was asked to design a AB class amplifier for a course in the uni this semester. All was well, schematics and parts given and I went down on building it. Surprise! It didn't work. Here's the schematic built in Multisim, right before final push-pull couple:

    [​IMG]

    Some specification info about the amplifier:

    At first we got a differential amplifier, getting input from the leftmost capacitor. It is powered by the current source built above it so that each branch will receive 0.5mA. Feedback will be driven later on the right branch of the differential amplifier. For now it is grounded.

    The second stage is the driver stage. Q7 and Q8 are supposed to have an Ic current of 10mA and thus we have put Q5 as a constant voltage "module" it is supposed to have 1.5+2*0.6=2.7V at its ends.
    But here is the problem. In order for that to happen, its vertical branch must have current, 1mA as it is given from a current source of Q4. Unfortunately, no current to be found.

    After some messing with the design, I found out that Q6 doesn't conduct and that's my problem. If I put a resistance of 1kΩ in parallel with it for example everything works fine (except that I don't have an amplifier in my hands anymore).

    Is there some design flow with Q6, or am should I look elsewhere? Obviously since it doesn't work in Multisim either it's not a faulty part.

    Thanks in advance.

    Notes:
    There IS a connection to the collector of Q3, the arrow of the probe legend hides it.
    The other 4 legends to the correspond the 4 green probe arrows from top to bottom.
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    Your Multisim schematic is a nightmare of transistors shown backwards. Q1, Q4 and Q8 should have the base on the left side.

    You do not have enough current in Q4 and Q6 to drive the output transistors.

    What is R14 for?
    Where is the output that drives a speaker?
     
  3. bertus

    Administrator

    Apr 5, 2008
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  4. Audioguru

    New Member

    Dec 20, 2007
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    Apparently the BD139 and BD140 transistors are driver transistors for external output transistors.
    I don't know why they are not the output transistors since with a total supply of only 40V then the output into 8 ohms is only 16W to 20W which the BD139 and BD140 can easily handle if they have enough base current.
     
  5. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    I found the Horizontal Flip function. It PNP transistors should be ok now.
    As for the output, I show it here on the full circuit, along with the feedback.

    [​IMG]

    I deleted all the probes and one wire (by accident). After reconnecting it it worked.
    I guess the ghost in the machine played a prank on me...

    If I got this right, R14 is there to stabilise the collector current of Q7 and Q8, when put on a voltage of at about 1.4V.



    There is also the feedback loop, connected on the output, which I left unconnected, after I noticed that the two capacitors grounded my signal, draining all the amplitude of the output. Is that normal? Is the output correctly placed?

    Also, I find that the output has a DC voltage on it. Isn't that undesirable? Shouldn't the output pair be centred around 0?
     
  6. Audioguru

    New Member

    Dec 20, 2007
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    Your C4 and C5 are 220 Farads! They should be a trillion times less at 220pf.
    Actually C4 and C5 add lead frequency compensation that will enhance oscillation at a high frequency when negative feedback is connected.

    Look up Frequency Compensation for Opamps and for audio power amps where a lag capacitor is connected from the collector to the base of Q6 so that the gain is less than 1 at the frequency where the amplifier's phase shift is near 180 degrees.

    Q6 still has a current of nothing so it cannot drive the driver transistors.
     
  7. Georacer

    Thread Starter Moderator

    Nov 25, 2009
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    Duh...
    It's getting too late...

    Thanks for your time Audioguru.
     
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