Triple Emitter Follower for IRFP240


2005-01-01 3:33 pm
With mosfets you dont need a triple EF drive, the gates can be driven directly by VAS if you like, as the loading of the power fets does not traverse down the gate like it does on current driven devices like BJTs.

Mosfets are voltage driven.

However for multiple power fets in parallel, you may still want a driver stage to aid higher frequencies though, as the gate capacitance of each of the fets add up and eventually loads VAS enough at higher frequencies that frequency response goes down and distortion goes up.

But there is no need for a triple EF on a mosfet output, only for higher power output BJT amps working into lower load impedances.
so can I drive 2 ohm load? how many pairs are required at 45v and 80v to drive the 2 ohm load safely?

I would like to know also.

This is how I would do for IRFP240/9240 to drive 2 Ohm speaker.
I would use 4 pairs of MOSFET.
I would let MJE15030/15031 drive current to the MOSFETS.
I would not use too high voltage for the power supply. 45 Volt is enough.


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@rhythmsandy you cannot say emiter follower when talking of mosfet power devices, the correct terminology is Source follower. On the other hand MOSFET ( Metal Oxide Field Effect Transistor ) has it's gate electrically isolated from the DS channel, that intails no DC current drawn by the gate no matter the load, but as @Tekko pointed out there is the need for AC current cus of the Ciss of the power mosfet, IRFP has Ciss of 1,2nF, each added pair adds to that capacitance and so the need for some minimum AC current becomes more and more important with each added pair of mosfets, otherwise that large Ciss would have nasty effects on the performance of the output stage. And for an explanatory example let's look at the upper side of the audio spectrum, let say we have 4 pairs of mosfet's and so 4,8nF combined capacity, at 20Khz a 4,8Nf capacitor would have a reactance of about 1,6K ohms, as you can realise that would not be verry good for the signall. How do you circumvent that? you supply enough gate current as so the Ciss is charged fast enough. For 45V per rail 4 pairs of mosfet for 2 Ohms load would be ok indeed.

@rafjr00 using power mosfet;s as the output devices for an power amp is quite different from usoing BJT's, do not forget that MOSFET's are voltage driven devices, but BJT's are current driven devices, so as an output stage with mosfets could do with less than 100mA of gate current, the BJT's for the same load could need more than 1A base drive, that is quite different.
This is how I would do for IRFP240/9240 to drive 2 Ohm speaker.
I would use 4 pairs of MOSFET.
I would let MJE15030/15031 drive current to the MOSFETS.
I would not use too high voltage for the power supply. 45 Volt is enough.

but can It drive in 2 ohms continuously?

what If im using 80v at psu does it requires more mosfets at the output? I am ready to add 15 pairs of mosfets if required? but need to do a more powerful amp....


2012-12-04 10:04 pm
Used Triple Emiter Follower for MOSFET is not necessary. MOSFET drive by voltage, BJT by current.
About using MOSFET on drivers for BJTs Power output, like IRF610/IRF9610, how value Rg is good choice? I think 100 ohms.
While that is a good simplification, driving a MOSFET can take a good bit of current because of capacitance. But, like you stated, a triple emitter follower is overkill.

The same mistake... there is no Emiter on MOSFET devices, there is the Gate, Drain and Source terminals, so you say Source follower.
Technincally you are correct. Except for one little thing: It's spelled "emitter." There is no "emiter" on a BJT either.

While your point, without the spelling problem, is technically correct, a lot of people, myself included, will refer to any follower as an emitter follower casually. That doesn't mean that we don't know the difference between a source, emitter, or cathode.
Well i'm sorry if i am not an english expert, but i think everyone got my point. Spelling problems are not an issue as long as they are just that, spelling problems, but wrong terminology can cause harm, with a forum so popular world wide like this it's bound to have many newbbyes and they can be misslead by this sort of stuff, so that is the only reason i have stressed out that issue.

I am 100% convinced that you do not have any issue of this cind, but just look at the author of this thread, he did not even know how to describe good enough he's needs, nevermind about the correct terminology ;)
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