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Class D power amp confusion

Discussion in 'Electronic Design' started by markp, May 4, 2004.

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  1. markp

    markp Guest

    Hi All,

    I'm a little confused by the specs in the MAX9704 class D amplifier

    Table 2 on page 9 shows that, with a 'gain' setting of 13db with 1.23V rms
    input, 15W output power is delivered into 8 ohm load. I can't make this
    figure add up! (I assume thie gain is defined as 'voltage gain' which is
    actally a power gain figure 6db for doubling of voltage, not 3db). Also
    there is a graph showing output power versus load resistance, and this shows
    a peak at around 8 ohms - this looks like a classic maximum power transfer
    curve, so is there an output impedance to take into account? (it's class D,
    so where's the power going?).

    Any light shone on this appreciated!

  2. Joerg

    Joerg Guest

    Hi Mark,

    A dB is a dB. 6dB is twice the voltage, so your voltage gain would be a little
    over four here. You are right, somehow that doesn't compute.

    Regards, Joerg
  3. markp

    markp Guest

    Well usually 'voltage gain' is described in terms of power, not voltage per

    so it could be 6dB rather than 3dB for every doubling of the actual voltage
    (power depends on square of voltage). Which is it in this datasheet? I agree
    it doesn't compute either way!

  4. Joerg

    Joerg Guest

    Hi Mark,

    Amplifiers are usually not characterized in an "EMC way". The input power is not
    a concern because their input impedance is very high whereas the output
    impedance is just a few ohms. The common terminology is input voltage (without
    regard to impedance) versus ouput voltage into a specified load.

    Regards, Joerg
  5. markp

    markp Guest

    It seems voltage gain expressed in dB as Av = 20log(V1/V2) is only
    technically correct if measured over the same impedance, but nevertheless it
    has become commonplace to refer to voltage gain in this way irrespective of
    impedance. (see example problems)

    also here's an opamp with a gain of 400000, specified as 112dB:

  6. Joerg

    Joerg Guest

    Hi Mark,
    Yes, indeed it has. In the case of an audio application the source driving the
    class D amp is usually much lower in impedance than the amp input impedance. The
    line out of a CD player, for example, is usually around a few kohm. Microphones
    can be even less. So the input of an amp really doesn't matter much for normal
    apps unless it all becomes very noise critical, then it deserves a closer look.

    Still, this all doesn't explain why 13dB don't compute here.

    Regards, Joerg
  7. One might also question why that table shows 9W into 4 ohms, when the specs
    state a max power of 7.5W into 4 ohms.

    One might even question why they specify power of an audio amp at 10% THD.
    Yecch. Oh well, I guess if you're building a cell phone it doesn't matter.
    A stereo cell phone, in this case.

    Indeed, one might question whether the data sheet development process in
    this case was "let's find all the pictures we have lying around from when we
    were developing the chip, and paste them together without bothering to see
    whether they still apply to the final product." Not that I think any modern
    semiconductor manufacturer would do that. Of course not.
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