BJT vs MOSFET - taking the best of both

[Bakmeel]

Originally posted by hugobross
Here's just my opinion:

Have you ever considered IGBT devices?

Actually, I have... and already have sniffed this forum for info about that. I have learned that IGBT's aren't very suitable for the job. In Fact, they suffer from the disadvantages from BJT's and MOSFETs. High input capacitance, and nonlinearity. IGBT's were made for switching applications, and they perform perfectly there. I could use IGBT's in controlling biascurrents, i've heard the perform well in that too....

Reading all opinions posted so far, it seems that MOSFET's have most advantages over BJT's, and can be used best in DIY designs.

Does anyone know a little more about the application of JFETs instead of regular MOSFET's? And how about HEXFET's?

Bouke

[Morrist]

It has been stated previously in this thread but I don't think that there has been enough emphasis placed on this point: MOSFETs are not linear devices.

As mentioned above by traderbam, the relationship between the applied voltage and the resultant drain current is approximately a square: Id = K*Vgs^2 (Where K is a constant). Therefore, theoretically, no matter how much feedback you apply you will be unable to reproduce the signal linearly.

You might now be saying 'Oh, this guy's a MOSFET hater' but that's not true. MOSFETs have their advantages: Little to no input current being one advantage, which would perhaps make them very useful as the input device of a line stage amplifier.

As for BJTs, they aren't linear devices either but they are more linear than MOSFETs. Note that not all BJTs are equal, some are more linear than others. If one takes care in both choosing appropriate BJTs and biasing in class-A, they stand a much better chance of amplifying the input signal linearly.

With that being said I believe that one can biuld a good amplifier using MOSFETs but could just as easily build an exceptional amplifier using BJTs.

Thanks,
Morrist

[subwo1]

One useful feature about JFETs is that the pinchoff voltage is below the source voltage for an N-channel device, and the other way around for P-channel. This characteristic could help in driving them as voltage followers up to the power supply rails. Otherwise, I believe they behave much like mosfets.

[seangoesbonk]

"Isn't that a little like saying a resistor is a transconductance device since its current is related to its voltage?"

A resistor is simply "conductive", because the change in voltage across the resistor results in a change of current through the resisitor. Devices are described as "transconductive" only when the change that occurs happens in a different part of the circuit. A bjt IS a transconductive device because the change in Vbe results in a change in Ic, and Ic does not travel into or out of the base.

Also someone stated an equation in correctly.
It should read Ic = Is*e^(Vbe/Vt) where Vt is the thermal voltage (26mV at room temp for all trans) and Is is the saturation current(typically 10^-12 A to 10^-14 A). Is is device dependent.

Well, that was my first post. I hope I didn't **** anyone off.

[Chucko]

Originally posted by traderbam
I guess Mr. Self is saying that the exponential relationship between Vbe and Ic is more consistent across devices than hfe. Hideous isn't it?

That's certainly the way I read it.

And no, I don't find it hideous. It's a challenge. If anyone ever finds a solid-state device (or electronic amplifying device, period) that is inherently linear, audiophiles everywhere will rejoice, and engineers everywhere will be out of work.

Until then, we have to make the best of what we've got.

There are ways to use the BJT's exponential Vbe->Ic relationship that are inherently linear. They're just not easy to apply unless you're a chip designer.

[P.Lacombe]

"Have you ever considered IGBT devices? "

Hugo,

IGBT are specially designed for switching purposes, at high currents and high frequency, not for linear amplification at audio frequencies. In my opinion it is impossible to obtain a clear sound with such devices, which are more suitable for cooking electrical appliance...

Regards, P.Lacombe.

[sonnya]

To add some salt... What about GT20D101 and D201 from Toshiba? They are made for audio. I have tried them out and the do sound nice to.

And the are IGBT types.



Sonny

[sonnya]

Hint of the day!!

Take one VQ1000J from Vishay.
Use only 2 of the 4 MOSFET's
Set an idle current of 90mA and a VDS of 10V.
Then check the "Transfer Characteristics" in the datasheet.
Also check the values of Ciss, Coss, Crss.
Add some degeneration sourceresistance in the size of 10 - 20 Ohm.

I must say this is better than BJT and even nearly 100% matched.

The perfect one would be a ZVN3310 idling at .2 - .4 Amp and 25 Vds. but it cant handle the heat.

Sonny

[sonnya]

Forgotten!!! Add a heatsink to the VQ1000J

Sonny

[traderbam]

Dear power MOSFET experts, just out of interest (I don't have any datasheets handy), how linear is Id to total gate charge?