OK..
I found a lot of different answers here on this question:
Do we need to have Vbs in amplifier with IRFP240/9240 or not(trim pot is just OK)?
Do we need to have drivers like in bipolar amps to drive these MOSFET's?
Can somebody give me answers from his working amplifier?
I have a plan to build amp with pair of these.How much power I can get?
Thanks
I found a lot of different answers here on this question:
Do we need to have Vbs in amplifier with IRFP240/9240 or not(trim pot is just OK)?
Do we need to have drivers like in bipolar amps to drive these MOSFET's?
Can somebody give me answers from his working amplifier?
I have a plan to build amp with pair of these.How much power I can get?
Thanks
You DEFINITELY need a Vbe multiplier for the IRF(P) parts. They have a positive temp. coefficient up to about 50-75% of maximum current - and I'm sure you don't want your idle current THAT high That being said, they have a lower temp. coefficient than BJTs so your Vbe multiplier needs to be 'degenerated' by inserting a small resistor in the emitter of the compensating BJT or it will overcompensate. You can also ose a resistor in series with the Vbe multiplier but that will make it difficult to adjust (effective temp coefficient will change with idle current adjusment). Finally, you could use a Vgs multiplier (MOSFET version of the Vbe multiplier).
As for drivers, depends on what you want to do. If you are not looking for extreme speeds, you may get away with driving them directly from a relatively high current VAS (10-20mA, more than that may get impractical due to power dissipation on the VAS which needs larger and usually slower transistors). For absolutely top speed, you need a driver stage. Also, a driver stage will sometimes reduce distortion as the MOSFET capacitances depend on voltages between it's electrodes, which means your VAS sees a nonlinear capacitance, which is a source of distortion. A driver stage will reduce the apparent capacitance seen by the VAS.
That being said, they have a lower temp. coefficient than BJTs so your Vbe multiplier needs to be 'degenerated' by inserting a small resistor in the emitter of the compensating BJT or it will overcompensate. You can also ose a resistor in series with the Vbe multiplier but that will make it difficult to adjust (effective temp coefficient will change with idle current adjusment). Finally, you could use a Vgs multiplier (MOSFET version of the Vbe multiplier).As for drivers, depends on what you want to do. If you are not looking for extreme speeds, you may get away with driving them directly from a relatively high current VAS (10-20mA, more than that may get impractical due to power dissipation on the VAS which needs larger and usually slower transistors). For absolutely top speed, you need a driver stage. Also, a driver stage will sometimes reduce distortion as the MOSFET capacitances depend on voltages between it's electrodes, which means your VAS sees a nonlinear capacitance, which is a source of distortion. A driver stage will reduce the apparent capacitance seen by the VAS.
Hi buddy
Hi ZOX,
As you have access to complementary mosfets, you may use the following combination:
Input stage = dual differential 5mA per device equals to 10mA per pair with current sources.
VAS = cascoded loading for high speed with idle current around 40 to 60 mA depending upon the parallelling of mosfets.
no additional driver stage is necessary.
But the supply rails of the differential and VAS must be atleast 12 volts higher than output mosfet rails[better if regulated also].
use a symmetric VGS multiplier .
regards,
kanwar
Hi ZOX,
As you have access to complementary mosfets, you may use the following combination:
Input stage = dual differential 5mA per device equals to 10mA per pair with current sources.
VAS = cascoded loading for high speed with idle current around 40 to 60 mA depending upon the parallelling of mosfets.
no additional driver stage is necessary.
But the supply rails of the differential and VAS must be atleast 12 volts higher than output mosfet rails[better if regulated also].
use a symmetric VGS multiplier .
regards,
kanwar
Hi buddy
Hi ritchie rich,
I have once constructed VAS using mosfets. Driving the mosfets in VAS through gate relatively confers to increased high frequency distortion and phase-shift.
Either use a folded cascode or full cascoded loading when using mosfets in VAS.
regards,
kanwar
richie00boy said:
Hi ritchie rich,
I have once constructed VAS using mosfets. Driving the mosfets in VAS through gate relatively confers to increased high frequency distortion and phase-shift.
Either use a folded cascode or full cascoded loading when using mosfets in VAS.
regards,
kanwar
You will get better thermal tracking and a more appropriate
voltage with a Vgs multiplier.
Depending on the Source resistance, the regulation through
the VAS, the amount of sinking, size of the output stage and
the bias current, you might be able to use a resistor for the bias -
increasing any or all of these increases bias stability.
voltage with a Vgs multiplier.
Depending on the Source resistance, the regulation through
the VAS, the amount of sinking, size of the output stage and
the bias current, you might be able to use a resistor for the bias -
increasing any or all of these increases bias stability.
(Obviously) I would have to concur with Nelson, he is THE guru after all
In fact, chosing the correct MOSFET in the Vgs multiplier can even enable the use of a single pair MOSFET output entirely without source resistances (Better Gm, but read on), plus for best results you want to carefully look at the (measured) data on your MOSFETs. Once again, IRFPXXX/9XXX (XXX being the same number for both N and P) are not by far the ideal complements, but if you take the unorthodox path, you can find ones that are very well matched.
In fact, chosing the correct MOSFET in the Vgs multiplier can even enable the use of a single pair MOSFET output entirely without source resistances (Better Gm, but read on), plus for best results you want to carefully look at the (measured) data on your MOSFETs. Once again, IRFPXXX/9XXX (XXX being the same number for both N and P) are not by far the ideal complements, but if you take the unorthodox path, you can find ones that are very well matched.
Heh, I would not go as far as using a Vgs multiplier and multiple parallel pairs without degeneration. But I have used a Vgs multiplier and no degeneration with single pair outputs so many times I've lost count
Choosing a small MOSFET (2N7000, BS107, BS170) with this approach will even overcompensate a pair of IRFP outputs...
Choosing a small MOSFET (2N7000, BS107, BS170) with this approach will even overcompensate a pair of IRFP outputs...
Re: HIgher voltage rails???
Why not add a small transformer in series with your 42v power supply.
This costs very little, takes up little space, with regulation it works as good as independantly from the main PS.
I've used that on a few amplifiers, currently using this approach for the Leach amplifier with discrete +/- 75 vdc regulators.
zox2003 said:
Why not add a small transformer in series with your 42v power supply.
This costs very little, takes up little space, with regulation it works as good as independantly from the main PS.
I've used that on a few amplifiers, currently using this approach for the Leach amplifier with discrete +/- 75 vdc regulators.
It's not too difficult to get high voltage rails at lower current, enough for the front end + driver of an amp, using a charge pump off the main transformer. In theory, the extra rails are 2x original rails, but using appropriate regulation and capacitors you can tailor that as you need it.
Look for the A75 under amplifiers -> legacy projects at passdiy.com for an example how to do this.
Look for the A75 under amplifiers -> legacy projects at passdiy.com for an example how to do this.
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