Problems with class A design

Discussion in 'The Projects Forum' started by cariba, Apr 28, 2009.

  1. cariba

    Thread Starter Member

    Apr 22, 2009
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    :confused:Hi, i'm doing a class A amplifier and a problem turned out that it is clipping very strangely on the negative cycle when a sine wave is applied. please find the attached picture of the wave at the output and the final power stage i am using, the transistor is a tip132

    Regards
    Alistair
     
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
    2,040
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    Do you use an input coupling capacitor? Either the bias is insufficient, or it's being overridden by your signal source.

    With no input signal, the bias should be adjusted so the emitter is at about half the supply voltage.

    Eric
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    If the transistor is very powerful then it can pull the voltage across the speaker up.
    But the 150 ohms resistor value is much too high to pull the voltage across the speaker down.

    Usually a class-A amplifier uses a constant current source at its output instead of your 150 ohm resistor. The current source must provide at least the peak current of the speaker all the time so it wastes a lot of power getting very hot.
    The power transistor supplies double the current used by the speaker.
     
  4. cariba

    Thread Starter Member

    Apr 22, 2009
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    Yes i'm using a coupling capacitor and the circuit is well balanced. What do you mean by insuffiecient bias? base bias or collector bias?

    Regards
    Alistair
     
  5. cariba

    Thread Starter Member

    Apr 22, 2009
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    Hi audio guru , can you please give ma abreif sketch of how I should apply the current source at the final stage please, i am using a 70v supply and a 8 ohm speaker load.

    regards
    Alistair
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    As an example, if you expect your output voltage to swing ±800mV, then on the positive peak the emitter follower must supply a current of 100mA, plus whatever flows through the resistor. However, on the negative peak, 100mA must flow back toward the emitter. Since the emitter can't sink current, that current must be provided by the resistor. If the resistor is too large, the emitter follower will cut off (the base voltage will swing lower than the emitter voltage), which is what you are seeing.
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
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    Please show how your supply voltage is hooked up.
     
  8. Audioguru

    New Member

    Dec 20, 2007
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    A class-a amplifier with a 70V supply will produce a max output of about 65V p-p. then the peak current into 8 ohms is 4.1A. The output devices must always pass about 3.2A so they heat with a total of 224W or more.
    The max continuous output to the speaker is only 66W at clipping.

    Here is an article about a class-A amplifier. The final version has an output of 20W and gets very hot. http://sound.westhost.com/project36.htm
     
  9. SgtWookie

    Expert

    Jul 17, 2007
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    The body diode will cause large problems.

    Choose a power transistor that doesn't have a body diode.
     
  10. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    It's reverse-biased. Please explain.
     
  11. cariba

    Thread Starter Member

    Apr 22, 2009
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    Oh my goid what a power loss:( and so how can I make this continous 3.2a continous current flow without having a resistor, should i use a current source ? or i am ploughing lol
     
  12. cariba

    Thread Starter Member

    Apr 22, 2009
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    the main problem is that I couldnt find a resistor which is capable of handling all that power, maybe I could use some type of configuration which acts the same as a resistor load.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    Didn't you see the class-A amplifier project in my link? It uses a power transistor in its amplifying section and uses another power transistor as its constant current source.

    Why do you want to waste electricity and to heat your entire neighbourhood with a class-A amplifier? Why not use a very good and cool class-AB amplifier?
     
  14. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    SgtWookie must have me on his Ignore list.:rolleyes:
     
  15. PRS

    Well-Known Member

    Aug 24, 2008
    989
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    I made a big 8 ohm power resistor by putting many 10 watt (ceramic-insulator) 100 ohm resistors in parallel. I have it on an insulating plate on my workbench. May I ask, Cariba, what you are using as a supply? And what are you driving this power amp with. What is the output resistance of your driver?
     
  16. cariba

    Thread Starter Member

    Apr 22, 2009
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    Hi paul, I have managed to set up a combination of power resistors to get 8 ohm, altough they heatup they radiate heat very well. and the power stage is perfect now.

    DOnt know if I had the problem before but it seems strange because as i connect the signal generator into the va stages of which i have 2 , the first stage that its input is coupled to the generator is loosing its dc biasing:(

    I have replaced the capacitor and its still loosing biasing even with the lead connected and zero input amplitude:(

    Any ideas to my problem please??

    Regards
     
  17. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Could be the power amp is pulling alot of current from the stage that's driving it. This is common. You might want to include a driver stage between your voltage amplifiers and your power amp. This could be another Darlington set up as an emitter follower. Bias the base between your supply voltages using identical resistors for R1 and R2.

    I'd like to add, Cariba that I have an audio amp project going now which is similar to yours. I think I posted my power amp. It was working just beautifully -- no distortion. Then it went kaput. I think my cat shorted something out and I turned it on and zapped something. Back to work....
     
  18. cariba

    Thread Starter Member

    Apr 22, 2009
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    thats what i did mate, and everythings fine now, i just have the supply riplle which is to loud since its being amplified as noise, now i `ll try to set up an indepeandant suplly for the va stages.

    AHHHHH damm itt, u shall microwave your cat then :mad:
     
  19. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    My cat is too cute to do that to. He's a little black kitten that fits in my boot. I give him slack. As for your ripple, I'm having the same problem. Thing is the fixes all seem to lead to reducing the maximum peak to peak voltage. Any ideas?
     
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