Class D Amplifier question

Discussion in 'General Electronics Chat' started by Kkein, Jun 22, 2014.

  1. Kkein

    Thread Starter Member

    Jun 10, 2011
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    Folks,
    I have implemented a class D amplifier using 555 timers, couple inverters, mosfet driver and a half bridge Mosfet. The original input is 2.7V pk-2-pk.

    From simulation:the amplified modulated digital signal is from 0- 8.6V Pk and amplified filtered or demodulated signal is 0.24V pk-2-pk as shown on the diagram. I was expecting a much bigger output signal - see the attachment for more info..
    Why is the filtered output of my class D amplifier (attached) less than the amplified input signal?
    Thanks for your help in advance.
    Kkein.
     
  2. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    Your zip-file is empty.

    If the change in the PWM signal is small, the output signal will be small.

    The cases I know of the class D amplifiers, they use a triangular oscillator and a comparator.

    You could have a look at the following thread, started by Bill_Marsden:
    Class D and triangle wave linearity

    Bertus
     
  3. Kkein

    Thread Starter Member

    Jun 10, 2011
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    Thanks, Bertus. Here is the file again.
     
  4. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    The file in the Zip is in a format that I can not use.
    It is a project file of some kind of circuit simulation software.

    Bertus
     
  5. Kkein

    Thread Starter Member

    Jun 10, 2011
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    0
    Thanks, Bertus.
    It's proteus version 8.1. You should be able to open it using earlier version by selecting whatever version you have to open the file.
     
  6. crutschow

    Expert

    Mar 14, 2008
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    The filtered output signal is proportional to the modulation level (PWM duty-cycle variation). For full output you need a duty-cycle change of near 0% to 100%. The modulation level is determined by the input signal level and the modulation gain (relation between input signal level and PWM duty-cycle level). It would appear that the modulation gain of your circuit is low.

    Try increasing the input signal to increase the modulation level. If that works to increase the signal level then you could add an opamp amplifier at the signal input if you want more sensitivity.

    Also is the filter frequency correct for the PWM carrier frequency?
     
    Last edited: Jun 22, 2014
  7. Wendy

    Moderator

    Mar 24, 2008
    20,765
    2,536
    Personally I don't use simulators at all, which is why my files are in .gif format. You can always take a screen shot using <PrtScn> and display that.

    If you look at my design I use a simple variable gain op amp circuit to get the gain to the circuit where it needs to be. I suspect this is the norm.

    I have a PoP (Pile of Parts) on this project started (what I call kitting), I have set it aside for a power supply project and an electronic load for the moment.
     
  8. bertus

    Administrator

    Apr 5, 2008
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    Hello,

    You are probably using the PWM circuit from the attached datasheet.
    In figure 8 the schematic is shown:

    [​IMG]

    In figure 9 the waveforms are shown:

    [​IMG]

    As you can see the PWM signal will not reach the 0 or 100 %.

    Bertus
     
  9. crutschow

    Expert

    Mar 14, 2008
    13,002
    3,232
    Below is the simulation of a reasonably simple Triangle Generator/PWM circuit that can be modulated to 100% with about a 1Vpk signal, which may be of interest to you.

    PWM Ckt.gif
     
  10. Kkein

    Thread Starter Member

    Jun 10, 2011
    46
    0
    Folks,
    I appreciate your comments/responses very much. Indeed this forum has highly experience knowledgeable people.
    @Crutschow: thanks for the circuit. I will try it out today.
    @Bertus: here is the .png file for circuit attached. It's possible that the 555 timer is not reaching 100% duty cycle.

    I was hoping we could use cheaply available components such as 555 to make it work.
     
  11. Kkein

    Thread Starter Member

    Jun 10, 2011
    46
    0
    Hi Crutschow,

    Not getting any square and triangle output from the circuit you provided.

    Not sure why. Output is constant 5V (instead of sawtooth) from LT1312 and 2.25v(instead of square wave) from LM339.
     
  12. crutschow

    Expert

    Mar 14, 2008
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    The 10uF capacitor across the pseudo ground voltage generated by R1 and R2 has a long startup time constant. That's why I used the .IC command to set that voltage at the start. You might try that.

    Also does R1 really make a connection to R2? There's a gap in the schematic and the voltage at the node appears to be 4.6409V which would be incorrect.
     
  13. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Pins 3 and 12 of the 339 should be +V and Gnd respectively. You have them the wrong way round. You also have pins 4 and 8 of the LT1213 the wrong way round.
     
  14. Kkein

    Thread Starter Member

    Jun 10, 2011
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    Thanks for folks but it still doesn't work. Here is a snapshot with pins connected correctly and 2.27v supply connected to +ve pins (7) as reference.
     
  15. Kkein

    Thread Starter Member

    Jun 10, 2011
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    0
    Gentlemen,

    I have no success generating a square wave let alone a sawtooth one. I've used the same circuit provided in the datasheet for lm358 to see if there would be some luck to no avail. I'm attaching the project file (Proteus V7.) for anyone to run the simulation to see if the problem is with my compiler.
    A snapshot is also as shown only showing 2.5V DC constant at the output without square wave. Looking forward to your support.
    Thanks.
     
  16. crutschow

    Expert

    Mar 14, 2008
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    What are the voltages at the outputs of each of the amps? I can't tell from your posted simulation.
     
  17. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    Hi, Kkein don't forget that all simulation programs have a big problem with all kind of oscillators.
    I open your SWG.zip file and I change the capacitor value to 1μF.
    After that change the circuit start working. So this give me a clue that the problem lies in simulation time step.

    Also here you have some classic self oscillating PMW modulators.
    http://images.elektroda.net/23_1232916064.gif
    http://images.elektroda.net/92_1233100008.png

    I even build this one and its work quite good
    http://images.elektroda.net/41_1233182187.png
     
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  18. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Your scope trigger is set for 'one-shot'. Try 'auto' instead.
    Channel A is set for DC, but channel B is set for AC. Why different?
    Try increasing R2 to 33k or more, and increasing the U1 voltage to 4.5V.
     
  19. Kkein

    Thread Starter Member

    Jun 10, 2011
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    You're spot on, Jony130. In fact the software can simulate successfully for opamp frequency of up to 125Hz. Alec_t, thanks for your input.

    Much appreciate it.
     
  20. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    The circuit will work in "Graph mode" but only if you turn off "initial DC solution".
     
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