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Old 12th December 2014, 10:11 AM   #1
AndrewT is offline AndrewT  Scotland
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Default mosfet driver

I'm thinking about changing to a mosFET driver stage for a BJT output stage.
I have seen this suggested by more than a few Members.

What parameters of a mosFET make them suitable as a Power Amplifier driver stage?
What range of those parameters suit drivers in a power amp?

If it makes much/any difference, I'm looking at the 50W to 150W into 8ohms category.
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Old 12th December 2014, 10:21 AM   #2
sajti is offline sajti  Hungary
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Quote:
Originally Posted by AndrewT View Post
I'm thinking about changing to a mosFET driver stage for a BJT output stage.
I have seen this suggested by more than a few Members.

What parameters of a mosFET make them suitable as a Power Amplifier driver stage?
What range of those parameters suit drivers in a power amp?

If it makes much/any difference, I'm looking at the 50W to 150W into 8ohms category.
Check out the Parasound amplifiers. John Curl uses this solution in many of his desing.

Sajti
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Old 12th December 2014, 11:59 AM   #3
Jay is offline Jay  Indonesia
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Not many mosfets are suitable for that job. High transconductance and and at the same time low capacitance. Just like bipolars where Cob is important for VAS, Cis is also important with mosfet. The closer to the output stage (driver, high current) the more important the transconductance.

But because there aren't many (complementary) mosfets around, you can see a trend on what mosfets are used by designers, starting back from JLH. Only the Toshiba are the new products but I haven't seen it used often (e.g. in a dual sizklai with BJT, if it is considered a driver also). I guess because it is new/rare and expensive.
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Old 12th December 2014, 06:40 PM   #4
Bensen is offline Bensen  Belgium
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Here is a detailed answer and table from Jay:

Low power mosfet's - complementary pairs


Have also a look at the Classe amplifiers. Most of them are using lateral mosfets.
At the moment I have a fet driver, bjt OPS on my workbench. The more I am working, measuring and changing on it, the more I tend towards lateral mosfets running on there ideal bias current to eliminate bias changing of the OPS due to temperature fluctuations.
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Old 13th December 2014, 12:36 PM   #5
AndrewT is offline AndrewT  Scotland
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Thanks.

I had forgotten about the mosFET VAS Thread that I contributed to, in a very small way.
I had also forgotten that same Thread discussed Driver parameters.

It seems from Jay's work that 510/9520 as well as 710/9620 might make good driver transistors. The 510/9520 being limited to <80W into 8ohms.

Can a few of you identify the range of the parameters that make good drivers?
first impressions are that Cob of 100pF to 600pF, gm of 0.7S to 2S, Rds on of 0r5 to 3r are at least three of the necessary parameters. Does any one disagree?
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Old 13th December 2014, 12:38 PM   #6
AndrewT is offline AndrewT  Scotland
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How hard can one push a sot223 with a 2W rating as a driver?
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Old 13th December 2014, 03:22 PM   #7
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by AndrewT View Post
I'm thinking about changing to a mosFET driver stage for a BJT output stage.
I have seen this suggested by more than a few Members.

What parameters of a mosFET make them suitable as a Power Amplifier driver stage?
What range of those parameters suit drivers in a power amp?

If it makes much/any difference, I'm looking at the 50W to 150W into 8ohms category.
I see an advantage and a disadvantage.

Good: a MosFet by definition has "infinite Hfe" so it makes it easier for the Vas stage to drive it.
Remember it will see the load impedance multiplied by the combined Hfe of output transistors and the drivers in the chain.
That's why on very high power amplifiers (multi output devices) often 3 stages are needed, instead of the usual 2 (output + driver) .

In fact, in very high power industrial electronics, think Metro train electric motor switching, soldering machines, etc. , IgBT (insulated gate bipolar transistor) are very common: a die integrated Mos driver and very high current/power bipolar output
Insulated-gate bipolar transistor - Wikipedia, the free encyclopedia

Bad: you'll lose extra 3 or 4 Volts across the MosFet compared to roughly 1V adding an extra bipolar and you'll have to deal with Mosfet high gate capacitance.

EDIT:
Quote:
How hard can one push a sot223 with a 2W rating as a driver?
Mosfet or not, I'd feel very uneasy using a Sot223 cased transistor to drive an output power transistor.

Heat is heat and it must be dissipated.

Those 2W must certainly refer to impossible-in-practice "leads at constant 25 DegC " Lab specs.

I'm quite certain a little Sot223 dissipating 2W will burn a hole in my finger if I touch it.

Last edited by JMFahey; 13th December 2014 at 03:29 PM.
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Old 13th December 2014, 03:43 PM   #8
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I'm exploring using MOSFET drivers in a triple. So far the simulations indicate good performance with this arrangement. I'm looking at using a BJT EF stage to drive the gate capacitances, and the MOSFETs to drive the BJT output transistors.
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Old 13th December 2014, 06:48 PM   #9
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I'm using MOSFET in driver stage.
The good point is: the sounding is awesome! Like when you run EF2. But light load for VAS/TIS.

The MOSFET had been using is 2SK2013/2SJ313.

But, these MOSFET very hard to find. I don't have any experience with IRF710/IRF9610. Are them is good?

Last edited by Walkalone; 13th December 2014 at 06:51 PM.
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Old 14th December 2014, 10:07 AM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by JMFahey View Post
.....................
Mosfet or not, I'd feel very uneasy using a Sot223 cased transistor to drive an output power transistor.

Heat is heat and it must be dissipated.

Those 2W must certainly refer to impossible-in-practice "leads at constant 25 DegC " Lab specs.

I'm quite certain a little Sot223 dissipating 2W will burn a hole in my finger if I touch it.
I am not planning to run a 2W sot223 mosFET device @ 2W
I currently use 15W to 30W BJT devices @ between 200mW and 1.5W with added heatsink.
A 2W device cannot work at 1.5W even with massive PCB pad cooling.
But could it survive 100mW, or 200mW, or 300mW?
That's the question:
Quote:
How hard can one push a sot223 with a 2W rating as a driver?
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