TDA7294 as a giant opamp ?

[rob77]

Hi Guys,

i'm planning to test the TDA7294 D-mos power amp as an opamp - to use it as a output stage for my lab supply i'm designing (probably it will be useless , but we'll see )

my question is the bootstrap pin #6- when operated as audio power amplifier - the attached capacitor is being "dragged" up & down by the output and therefore it's acting like a charge pump - making enough voltage above the top rail to drive the high side...

but if i'll operate it @ DC only, then it means  no charge pump anymore , and as a result the max positive output voltage will be few more volts below the "possible" output level... one solution could be to use a small DC converter and simply feed the bootstrap pin with a voltage few volts above the top rail, but would that be safe ? 

did anyone here used those audio power amplifier ICs purely @ DC ? any experience sharing would be appreciated

[free_electron]

Not going to work. Internally the signal path is ac coupled. These things are designed for bridge application. if you apply a dc voltage level at their input the output will drift to center point. most likely these things also have a DC speaker protection.

and how are you going to do current regulation ?

besides for this thing to work as a 0 ..30 volts power supply you will need to give it both +30 and -30 volts ... total waste of negative rail.

[calexanian]

What frequencies and distortion figures do you need? Why not just use one of the garden variety high power AB all in one audio chips such as a 3886 or so, and if you need more current just add output current boost transistors? for less than 10 dollars you got a +/- 45 volt several amp op amp that is rather stable. 

[rob77]

thanks for the comments , are you sure about the AC coupling inside ? capacitors for the AC coupling inside the silicon would be huge at audio frequencies !
and the negative rail wouldn't be wasted - the plan is to have supply able to swing from -35 to +35V (absolute max for that chip is +- 50V).
current sensing would be at both supply pins of th echip - sampled by adc+micro and the quiescent current of the chip would be subtracted in software (10mA resolution and 2-3% accuracy would be good enough).

if that TDA chip will not work , then i'll have to build a discrete output stage

[rob77]

What frequencies and distortion figures do you need? Why not just use one of the garden variety high power AB all in one audio chips such as a 3886 or so, and if you need more current just add output current boost transistors? for less than 10 dollars you got a +/- 45 volt several amp op amp that is rather stable.

it's not going to be an audio amplifier...i was thinking of using it as a DC output stage. i'm asking about the TDA7294 because i have a handful of them and never going to use them in audio amp

[LaserSteve]

Many systems I work on use LM3886 as a DC to 5 KHz inverting amp with a gain of 25. We seriously use them down to DC.

Does need to have a minimum gain of 20 as configured or it may/will oscillate.

Steve

[nctnico]

Not going to work. Internally the signal path is ac coupled. These things are designed for bridge application. if you apply a dc voltage level at their input the output will drift to center point. most likely these things also have a DC speaker protection.

The TDA7294 will work from DC! Just look at the datasheet. Internally there is a bootstrap capacitor but that is only for high frequency compensation. Amplifiers for music have to work down to DC otherwise no boom-boom sound. Usually the input offset voltage is what kills these kind of amplifiers for instrumentation purposes.