# Class D power amp confusion

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

1. ### markpGuest

Hi All,

I'm a little confused by the specs in the MAX9704 class D amplifier
datasheet:
http://pdfserv.maxim-ic.com/en/ds/MAX9703-MAX9704.pdf

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!

Mark.

2. ### JoergGuest

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. ### markpGuest

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

http://www.hottconsultants.com/techtips/decibel.html

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!

Mark.

4. ### JoergGuest

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. ### markpGuest

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.

http://www.hottconsultants.com/techtips/decibel.html

http://www.phys.unsw.edu.au/~jw/dB.html (see example problems)

also here's an opamp with a gain of 400000, specified as 112dB:
http://www.onsemi.com/pub/Collateral/MC33077-D.PDF

Mark.

6. ### JoergGuest

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. ### Walter HarleyGuest

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.