My N_channel Mosfet Amplifier-opinion

zox2003 · 8th April 2005, 03:51 AM

This is picture of MOSFET-amplifier I just finished.It is based on CITATION 12 schematic with some changes.It works very good.Output MOS are STW60N10.It delivers at least 120W on 4 ohms.

I took picture with my mobile phone(hmmm).

amplifierguru · 8th April 2005, 04:24 AM

Cute - got a schematic?

zox2003 · 8th April 2005, 04:29 AM

Yes i do on paper ...

I have to draw all changes I made....

I will post it soon...

darkfenriz · 8th April 2005, 12:09 PM

I see you've nicely isolated mosfets' dies from each other. How do you isolate the screws?

zox2003 · 8th April 2005, 03:42 PM

They do not have to be isolated.MOSFETS are in TO247 case.No need for screw insolators.

ilimzn · 8th April 2005, 06:33 PM

Did you include the last changes we discussed?

zox2003 · 8th April 2005, 06:52 PM

Yes I did and I am attaching a waveform -square for 10khz.Check it out.
This is on +/- 32V rail ( lab.power supply)-testing purposes.Amplifier will run on +/- 42V.

Does it look good?

ilimzn · 8th April 2005, 08:01 PM

Note how the rising edge of the square wave has an indentation when crossing zero - this is indicative of low current capability of the VAS, but probably more of low bias.
Keep in mind that when you change the rail voltage, the VAS current changes in proportion, and also the bias voltage and hence standing current changes - in your case both get lower. You need to re-adjust bias when you change rail voltages on this design.
Adjust the bias current so this dissapears, just be careful to monitor what it is as on the MF12 design turning the bias pot all the way to the end will result in infinite standing current!
The reason why you get rising edge distortion, but not falling edge is because the VAS current pulling down has a FAR larger limit. In essence, the lower MOSFET can be driven on with a very large current (limited by R19). The upper transistor can be driven up only by the current of the bootstrap VAS (Vrail / (R3_R4)).
If you get oscillation when zero crossing, you may also experiment with the gate damper resistors on the MOSFETs, especially the upper one. This will not make the notch dissapear but may make it less jagged. Then, adjust as best possible with bias current.

darkfenriz · 8th April 2005, 11:10 PM

sorry Ithought they were to220

ilimzn · 9th April 2005, 11:46 PM

I just noticed in your diagram you were running the amp into clipping, in which case most of what I said in my last post may not apply. It is difficult to see anything of the natural response of the amp when driven by a square wave into clipping... the notch on the rising ende may be due to that alone.

8th April 2005, 04:29 AM	#3
zox2003 diyAudio Member Join Date: Nov 2004 Location: usa	yep Yes i do on paper ... I have to draw all changes I made.... I will post it soon...

8th April 2005, 03:42 PM	#5
zox2003 diyAudio Member Join Date: Nov 2004 Location: usa	SCrews??? They do not have to be isolated.MOSFETS are in TO247 case.No need for screw insolators.

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8th April 2005, 04:24 AM	#2
amplifierguru Account Disabled Join Date: Aug 2004 Location: Earth	Cute - got a schematic?

8th April 2005, 12:09 PM	#4
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	I see you've nicely isolated mosfets' dies from each other. How do you isolate the screws?

8th April 2005, 06:33 PM	#6
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	Did you include the last changes we discussed?

8th April 2005, 08:01 PM	#8
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	Note how the rising edge of the square wave has an indentation when crossing zero - this is indicative of low current capability of the VAS, but probably more of low bias. Keep in mind that when you change the rail voltage, the VAS current changes in proportion, and also the bias voltage and hence standing current changes - in your case both get lower. You need to re-adjust bias when you change rail voltages on this design. Adjust the bias current so this dissapears, just be careful to monitor what it is as on the MF12 design turning the bias pot all the way to the end will result in infinite standing current! The reason why you get rising edge distortion, but not falling edge is because the VAS current pulling down has a FAR larger limit. In essence, the lower MOSFET can be driven on with a very large current (limited by R19). The upper transistor can be driven up only by the current of the bootstrap VAS (Vrail / (R3_R4)). If you get oscillation when zero crossing, you may also experiment with the gate damper resistors on the MOSFETs, especially the upper one. This will not make the notch dissapear but may make it less jagged. Then, adjust as best possible with bias current.

8th April 2005, 11:10 PM	#9
darkfenriz diyAudio Member Join Date: Jun 2004 Location: Warsaw	sorry Ithought they were to220

9th April 2005, 11:46 PM	#10
ilimzn diyAudio Member Join Date: Feb 2005 Location: Zagreb	I just noticed in your diagram you were running the amp into clipping, in which case most of what I said in my last post may not apply. It is difficult to see anything of the natural response of the amp when driven by a square wave into clipping... the notch on the rising ende may be due to that alone.

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