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Old 1st December 2005, 12:00 PM   #1
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Question IRF540/IRF9540 bias

What is the bias,gate voltage for IRF540/IRF9540?
What is the gate voltage for 250mA or 500mA drain current?
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Old 1st December 2005, 12:10 PM   #2
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What is a datasheet, ever tried reading one ?
Those have really nice graphs you can look at for hours.
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Old 1st December 2005, 07:15 PM   #3
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This will vary between 'same' two mosfets and will change with temperature
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Old 3rd December 2005, 03:17 PM   #4
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i didn`t asked this becose i don`t know how to read datasheet!

In datasheets irf540 and irf9540 doesn`t have same bias voltages and they`re used as a complementary pair.

In some schematics I read that bias voltage should be 2.1V,(for
36V power supply).
I tried it and at the gate voltage 2.1V, the drain current is MUCH less then 500mA as they say it should be.

sorry for the bad english.
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Old 3rd December 2005, 03:20 PM   #5
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if you dont wanna help, dont vaste your time being ironic.
it`s stupid!
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Old 3rd December 2005, 03:42 PM   #6
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Hi Bogdan,
on the very few FETs that I have measured, I found that the Vgs tends towards the upper end of the manufacturer's spec limits.
This is exactly as you have found , a lot less current = higher Vgs.
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Old 3rd December 2005, 03:54 PM   #7
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Quote:
Originally posted by bogdan_borko
In datasheets irf540 and irf9540 doesn`t have same bias voltages and they`re used as a complementary pair.
Do a thread search and you can read a lot on IRFs here on DIYaudio.
Many of them are not complementary pairs, not even the 240 and 9240, as Ilimzn from Croatia took the trouble of explaining several times already.
That is for a major part the trouble with vertical MOSFETs and why the attention has gone to lateral types for so long. Lateral MOSFETs do come/came in decent complementary pairs.

Besides the effect of temperature difference, transconductance between 2 MOSFETs can vary just like Hfe can for 2 BJTs.
Datasheets are guidelines for designing a circuit, getting the right Vgs for a required bias is done by selection or by using a Vgs multiplier and adjusting. Some find the right VGs resistor value with a trimpot and then replace it with a fixed value resistor.
On a hybrid tube/mosfet amplifier i constructed i tried separate Vgs multipliers to compensate for the difference between both IRF output device types.
On advice of Ilimzn i am now trying the 240 and 9140 as complementaries on a pushpull amplifier, as he sees them as a better match for complementarries then the supposed ones.
Till now i saw vertical fets as only good for single ended use.
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Old 4th December 2005, 11:30 AM   #8
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what do you think about this schemtic?

Should I make separate bias voltage sources for each mosfet?

Any idea?
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Old 4th December 2005, 11:39 AM   #9
AndrewT is offline AndrewT  Scotland
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Hi Bogdan,
the Vbe multiplier has variable current as Vrail voltage modulates.
This in turn will modulate the bias currents in the FETs.
One multiplier is correct for the pair of FET Vgs.
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Old 4th December 2005, 10:04 PM   #10
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Quote:
Originally posted by bogdan_borko
I didn`t asked this becose i don`t know how to read datasheet!
In datasheets irf540 and irf9540 doesn`t have same bias voltages and they`re used as a complementary pair.
They are used as a complementary pair only by those who do not know how to read datasheets, more precisely, who do not understand wht is written in tham and what parameters are important for a given application and why.
Looking at the datasheets makes it obviously clear to anyone who has any idea about what complementary means, that these are not complementary. And guess what, they really are not!

Quote:
In some schematics I read that bias voltage should be 2.1V,(for
36V power supply) I tried it and at the gate voltage 2.1V, the drain current is MUCH less then 500mA as they say it should be.
And why is this a mystery if the datasheet clearly says 4V for 1A at 25 deg C? This should make it about 3.3V for 500mA for the IRF540. The IRF9540 has a higher Vgs for the same current and same temperature because it has about half the transconductance of the IRF540.

Quote:
If you dont wanna help, dont vaste your time being ironic.
it`s stupid!
No, it's not - it is a way of politely telling you that you are asking the wrong question. The bias voltage of a MOSFET should be irrelevant in any amplifier, except maybe when you are using the MOSFET as a Vgs multiplier to sense the temperature and compensate the bias CURRENT. A design should provide a means of adjusting the bias CURRENT, accounting for a relatively wide variation in bias voltages found in MOSFETs, and also, there should be a means to compensate for changing temperature. Without this, unless heavily degenerated, vertical MOSFETs WILL go into thermal runaway. In other words, you should really not be asking about a MOSFET bias voltage since this is not the relevant parameter you want to target - it will vary as much as +-50% depending on sample and temperature.

In your case, your Vbe multiplier will overcompensate for temperature, quite badly actually. It would overcompensate even if there were no source resistors (0.22). You need to 'degenerate' the BD139, by putting a small resistor in it's emitter. It will be a matter of experiment to see what it will have to be. Alternatively, you may want to use a Vgs multiplier (replace BD139 with a small MOSFET like the IRF510 or 610). Finally, NEVER EVER implement a bias regulation potentiometer like in your diagram. Should the pot viper have bad contact (which is not uncommon on standard pots, especially while it's being adjusted), you get maximum bias voltage -> short circuit from + to - Vdd through both MOSFETs. Use a fixed resistor from C/D to B/G, and a trim pot from B/G to E/S. Keep in mind the resitor to pot resistance ratio will be different for Vbe and Vgs multipliers because Vbe~~0.6V, and Vgs~~3V.
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