AD Vs BD Modulation in Class-D

[Workhorse]

Originally Posted by phase_accurate

It is actually common mode input that I meant when I wrote about offset ... Think it is time for a holiday.....


Regards

Charles

I think tying down both the input nodes with a resistor to ground is needed to keep VCM in range of opamp.

regards,
Kanwar

[phase_accurate]

I was once thinking about a different compensation method using an OP-AMP in inverting setup. The input is going to the Supply rail and the output is going to the two input nodes via a resistor in order to compensate for the common mode voltage. I can make a drawing if it is unclear wha I mean.

Regards

Charles

[Workhorse]

Charles, a drawing would surely help alot.

regards,
Kanwar

[phase_accurate]

Here you are

[Workhorse]

As always a picture speaks 1000 words, thanks once again Charles.
Can you elaborate a bit more on its working.


regards,
Kanwar

[phase_accurate]

Due to the lack of an op-amp symbol with differential outputs I took a normal one - but I think this shouldn't give too much problems for the interpretation of the circuit's workings.
On the left we have the differential audio inputs. We can also see the two NFB inputs with the lead compensation.

The inverting input of the auxiliary op-amp at the lower part of the diagram is fed with the rail voltage which is then inverted and fed to the two inputs. The circuit does nothing else than subtract a voltage - proportional to the rail voltage - from the two inputs (to be more correct it does actually subtract current). It shouldn't be too difficult to find out how it should be dimensioned (we don't have to hint the copiers from china to everything - do we ?). In the drawn form it can be used for the situations where the negative rail of the power stage is at the same potential as the ground of the input and modulator circuitry. If the input circuitry's ground is above that (if for instance its negative rail = negative rail of the output stage) it has to be completed with an additional resistor.

Regards

Charles

[Workhorse]

Thanxz Charles,

Nice explanation.
Got its working now, sure we don't want to attract copycats from far-east lands. The subtraction should be done in such a way that the input voltage range should be within input common mode voltage range of the opamp, in other words it looks like string puller

The fully differential opamps chips we have in market have a dedicated pin called VCM for setting up the offset at their output, Is this of any help here.

http://www.national.com/ds/LM/LME49724.pdf

regards,
Kanwar