Bjt vs mosfet drive

[Tekko]

Im working on a high power amplifier that can offer peak power reaching up to around 500 watts, but im still not sure which kinda drive im gonna use.

I've simulated both BJT drive and MOSFET drive, both offer the same performance as the same power.

The output stage consist of five MJW21195 and five MJE21196, The bjt drive consist of a MJL4281/MJL4302 driven by a MJE340/MJE350.

The mosfet drive would be a IRF540/IRF9540 or a IRFP240/IRFP9240.

Which drive to choose ? the performance difference is a whopping 0.01% between the two, bjt version does 0.02% at 550Wpeak at 20kHz in 4 ohms, mosfet drive version does 0.03% at same power, frequency and load.

Main purpose of this amp would be bass, so low THD at higher freqs is not mandatory.

Schematics:
http://i.imgur.com/EjIDd.png
http://i.imgur.com/Z6ae6.png

[lineup]

I would take BJT here.
There is no compare of how many Bipolar is around.
They are easily found and good price.
You just get more to select in BJT and they are also more 'complementar pairs' than MOSFETS (Hexfet).

[Tekko]

Oki.

Though triple EF may not be needed, it was just my simulation that dident like VAS driving the 4281/4302 directly.

The MJE340/350 drivers are likely capable of driving the outputs on their own anyways without any help.

[Bensen]

My opinion,

- 3EF is difficult to stabilise (for me as a rookie)
- 2EF will load the VAS and IPS to much
- FET's as drivers solves in my opinion the above disadvantages. This when the the VAS current is not to low. To have Ciss as low as possible is not a bad though I think when choosing good FET's for drivers.

In my current design, when I switched drivers from BJT to FET's, the high order harmonics in the FFT decreased a lot, THD1 went from 0.00x to 0.000x in LTSPICE.
I didn't use HEXFET's but small lateral's, see attachment.

PS: BOB216 and BOB79 are improved models for 2SK216 and 2SJ79.


Gr

[ingenieus]

Could you post the LTspice .asc file? I would like to have a go at your circuit.

[Bensen]

Originally Posted by ingenieus

Could you post the LTspice .asc file? I would like to have a go at your circuit.

Thanks for the interest. Any info good and bad is welkom.

[nigelwright7557]

The problem with MOSFETs is the voltage required to turn them on.
The bias voltage is likely to be around 7 volts so that is 3.5 colts from each rail that wont be used.
I suppose you could compensate by having a slightly higher rail.

[ingenieus]

Thanks, I've just downloaded it. I was wondering about the improved SPICE models that you mentioned, but I see you included them as well.

[Bensen]

Originally Posted by nigelwright7557

The problem with MOSFETs is the voltage required to turn them on.
The bias voltage is likely to be around 7 volts so that is 3.5 colts from each rail that wont be used.
I suppose you could compensate by having a slightly higher rail.

You are right when the OPS are MosFET's, those are voltage driven.
When BJT's are used in the OPS, the voltage swing at the output is not effected while these are current driven.
Not?

[richie00boy]

No, the MOSFET driving the BJT still needs to have it's gate swung higher than if it was a BJT.