Biasing MOSFET amplifier

[HighTower]

Im am currently doing some tests on a commercially available mosfet (car) audio amplifier, and I'm having some doubts as to the correct bias setting procedure . It's a classical Class (A)B amplifier with FQP22P10 and FQP33N10 mosfets and 0.22 Ohm source resistors.

I am using a HP8903B distortion analyser, measuring distortion at a 4 ohm load. But what is a good compromise between THD and power consumption, besides the effect of thermal runaway? And what would be a good output level for this measurement? 1 Watt? And what frequency, 1 Khz?

Without any load attached I can make crossover distortion dissapear with modest biasing currents, but with a 4 ohm load this is not possible so I have to find some compromise.

I tried searching the web for your average mosfet biasing basics but failed in finding any. Offcourse I am still interested to receive any interesting links..!

[Osvaldo de Banfield]

For a right class A working, the quiscient drain current must be equal or greater than the maximum expected load current, i.e VDD/Rload. Did you check this?

[dungdochi1978]

Right...the temp on heatsink will too hot if amp run at class A mode.all most class AB.

[Osvaldo de Banfield]

Originally Posted by HighTower

It's a classical Class (A)B amplifier with FQP22P10 and FQP33N10 mosfets and 0.22 Ohm source resistors.

This phrase confuses me. Class A or AB?

[richie00boy]

1 watt, 1 kHz, 0.01% THD. What bias does that need? Has it just one pair of output devices? Then 50-100mA would be acceptable.

[AudioSan]

its a class AB. all AB amps run in class A up to bias current.

[Itsmee]

Originally Posted by HighTower

...average mosfet biasing...

Most commercial mosfet A/B amps are biased at 100mA.

[HighTower]

I'm sorry, I must be expressing myself wrong. I don't want to run the amp in class A, I want to apply just enough quiescent current to make the amplifier run in class B (or AB) without excessive crossover distortion.

The reason I posted "class (A)B" is that I thought that this was called class AB, though Douglas Self clearly states in one of his books that an amplifier that has just enough quisecent current to avoid crossover distortion is called "class B".

Anyway, in my opinion, the default quiescent current setting for this amp was off causing high levels of crossover distortion. So the question is, how to go about when setting the quiescent current for this mosfet amp?

Originally Posted by richie00boy

1 watt, 1 kHz, 0.01% THD. What bias does that need?

I wish
I could run it in to class A and the analyser still wouldn't read 0.01%.
Have to say that I don't use an "A weigthed" filter, still looking for one. I will check tomorrow with the 30 KHz filter enabled.

Originally Posted by richie00boy

Has it just one pair of output devices? Then 50-100mA would be acceptable.

The amp uses two pairs of output devices for each channel at 48 volt railvoltage (96 rail-to-rail). At 100 mA this would be 4,8 watt for each output device. 4 mosfets per channel, 4 channels, means 76,8 watts of power consumption. Add the SMPS and protection circuitry and the total current draw at 14.4 volt exceeds 6A. I don't think this is possible in terms of thermal stability.

Fresh out of the box the current draw for this amp is around 1,5A.

[Tekko]

I believe most if not all car amplifiers run very low or even zero bias as they are made for output power rather than sound quality as most car audio is just *BOOM**BOOM**BOOM**BOOM* anyways.

[Osvaldo de Banfield]

Originally Posted by Tekko

I believe most if not all car amplifiers run very low or even zero bias as they are made for output power rather than sound quality as most car audio is just *BOOM**BOOM**BOOM**BOOM* anyways.

Unfortunately you are right. I hate such kind of "audio".