building a class AB amp

Discussion in 'The Projects Forum' started by jasnc2107, Apr 29, 2009.

  1. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    hi...i am new to electronics...i am designing a class AB amp...i am posting the design of this amp...i don't know why i am still getting crossover distortion with this setup...?? I also got problems with the calculated stage gains...which do not equal to the gain i got in the simulation..
    Thanks
     
  2. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Without looking at the rest of the circuit, you are getting crossover distortion because your diodes are shorted out.
     
  3. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    ok.. i resolved that problems shortly thanks...i don't knw why i am supposed to get a final stage voltage of 6v and i am getting only 3.8v??...is it possible to achieve a better gain?
     
  4. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    Hi all, For the output stage of the amplifier i am trying to use a darlington configuration. Which transistor do you suggest me?
     
  5. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Try running your C5 output to the center of the two diodes, but leave the lines going to the bases where they are.
     
  6. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    Yes it helped me out.. i am having no more crossover distortion. I haven't understood yet if the output stage, the push pull stage amplifies or not?
     
  7. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    The push pull stage is a power amp, it has no voltage amplification. So is your amplifier working well? Are you dissatisfied with the voltage gain? I believe the concept of your design is very well done.
     
  8. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    I designed the circuit on a bread board...the first two stages worked fine...strangely when i inserted the output stage (i used tip132 and tip137) the dc power supply LED starts fluctuating and the amplifier gives no output...Can anyone tell me what is happening?/
     
  9. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Typically, the output amp sucks up alot of current and so you include a stage called a driver between your power amp and the voltage amp. This could be a Darlington emitter follower.
     
  10. balisong

    Member

    Feb 26, 2008
    27
    0
    Unless you have an ideal power supply (none of us do), you will need decoupling caps for the output stage. Place several caps between 24V and GND as close as possible to the output transistors. You want a mix of capacitors to minimize the distortion over a broad range of frequency. Exact values don't matter, but try something like 1000uF + 100uF + 10uF + 1uF. Use ceramic caps if you can for the smaller values. Don't worry about overkill, more is better.
     
  11. balisong

    Member

    Feb 26, 2008
    27
    0
    An AB amplifier has both the high and low side transistors on at the same time. This is pratically a dead short to the power supply. You need to limit the shoot-through current with some resistors, Re in the attach image. They need to be smaller than you speaker impedance so they don't rob too much power from your output. And they need to be power resistors, your typical 1/4W resistors will self-destruct.
     
  12. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    They result to be active both transistors because of the diodes keeping the NPN on for a little while in negative. So what resistors do i need to place instead of the 4ohm resistors i placed between transistors? PRS i placed an emtter follower in between voltage and power stage..should it be like that? see attached. What is the scope of that exactly?
     
  13. balisong

    Member

    Feb 26, 2008
    27
    0
    I'd try .33 or .47 ohm resistors rated at 2W on the output.

    Does your software allow you set the series resistance of the voltage source?
    This will make the simulation much more accurate when dealing with large current/power.
     
  14. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    balisong so by placing those resistors i will reduce the current that the output stage will suck. don't think the my software got that feature. I placed capacitors as you suggested...on the collector side of the output transistors. Want that kill the dc biasing on the transistor?
     
  15. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    I tried to reduce load resistance to a bigger value...that is supposed to effect the current that it is sucked by the output stage? Still the dc power supply seems like overloaded. The power supply is rated 6A
     
  16. balisong

    Member

    Feb 26, 2008
    27
    0
    The caps go from 24V to GND, in parallel with power source.

    In the simulation (not the actual circuit) raise the voltage on the source to 25V and place a 1 ohm resistor in series with it. This is a simple approximation of a current limit source (like batteries). A power-supply is more complex with voltage regulation and over-current circuit breakers.
     
  17. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Balisong is right about the need for small valued resistors at your emitters. .3 ohms is good. If you don't have them make them using larger value resistors in parallel. Using 2 resistors of 1 ohm you get .5 ohms. I did this for a 7 watt amp and 1/4 watt resistors combined to form a single .5 ohm 1/2 watt resistor.

    As for the emitter follower being used to drive your power amp, its purpose is to match the low impedence of your power amp to the high impedence of your voltage amp. For the sake of maximum output swing bias the base of the emitter follower halfway between you Vcc and ground. You can do this easily by using two similar resistors to bias the base.
     
  18. Ron H

    AAC Fanatic!

    Apr 14, 2005
    7,050
    657
    Jasnc2107, Q3 has no source of base current.
     
  19. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    i inserted an emitter follower stage before the both transistors of the power stage. The dc supply still is not working because i thing is overcoming the 5A limit it has. What can i do to reduce current...?
     
  20. jasnc2107

    Thread Starter Member

    Apr 27, 2009
    23
    0
    please find attached the last version of the circuit. note that i used tip132 and tip137 instead of MJE15032 and MJE15033...
     
Loading...