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Old 6th October 2004, 03:26 PM   #1
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Default resistor ok to replace VGS multiplier?

Is it ok to replace the VGS multiplier circuit with a resistor for biasing a p-p mosfet amp? This is for a balanced amp with matched output mosfets, so a little offset voltage wouldn't show up at the speaker terminals. The stage in question is folded cascode with a constant current source.
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Old 6th October 2004, 05:08 PM   #2
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Default Re: resistor ok to replace VGS multiplier?

Quote:
Originally posted by nobody special
Is it ok to replace the VGS multiplier circuit with a resistor for biasing a p-p mosfet amp?
Hi Steve

Yes it is ok....but only if you are using lateral mosfets (like the Hitachis 2SK135/SJ50) or equivalents.

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Old 6th October 2004, 05:12 PM   #3
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Thanks, Jorge. Why not with the other mosfets?
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Old 6th October 2004, 05:25 PM   #4
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Quote:
Originally posted by nobody special
Why not with the other mosfets?
Because the vertical Mosfets have a positive temperature coefficient ,in the "normal" (100...200mA) bias current of power amps.
And ,they only have negatif temperature coefficient ,with much higher currents, in the case of a IRF 530 above 12 Amps of bias current...
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Old 6th October 2004, 06:29 PM   #5
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You can replace the Vgs multiplier with a resistor as long as

a) the DC current through the resistor will be constant. Really
constant.

b) You have enough Source Resistance on your output stage and
enough heat sinking.
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Old 6th October 2004, 07:25 PM   #6
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Thanks for the answers, Jorge and Nelson. 1 more question:
Do I need some resistance to ground on the output mosfet's gates? If so, what value is typical?
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Old 6th October 2004, 11:41 PM   #7
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Quote:
Originally posted by nobody special
1 more question:...Do I need some resistance to ground on the output mosfet's gates? If so, what value is typical?
If the output mosfets are used in the usual source follower
configuration...you don't need any resistor to ground.

Only a series resistor connected directly to the gate pin.

This resistor is for parasitic oscillation protection and the value depends of the input capacity capacity of the mosfets.
Typical values for lateral Hitachi mosfets 220 Ohms...and for vertical IRF mosfets 47-100 Ohms.
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Old 7th October 2004, 11:47 AM   #8
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Thanks, Jorge. I had the gate resistors... just didn't know if it needed to be ground-referenced for some reason. By the way, what kind of motorcycle is that in your avatar? Looks like a Honda or Yamaha? I ride a Honda ST-1100. I wonder how many other DIY audio guys ride.
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Old 7th October 2004, 12:14 PM   #9
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Quote:
Originally posted by nobody special
By the way, what kind of motorcycle is that in your avatar? Looks like a Honda or Yamaha?
No...is my new "baby" a Suzuki GSX 1400...

See it at ...and click in the photos to enlarge

http://www.lerepairedesmotards.com/a...tergsx1400.htm



Quote:
I ride a Honda ST-1100.
Good bike

Before the Suzuki I have a Yamaha GTS 1000....good technology but too heavy...
Quote:
I wonder how many other DIY audio guys ride.
I think that people that ride a motorcycle and the people that like DIY audio have something in common....they like "strong emotions"...

Cheers...

PS: Take care in the road...
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Old 8th October 2004, 02:48 PM   #10
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Nice bike! I don't think that's available in the US. You have to like big displacement motors! You take care too. I was recently run off the road and into a field by someone in a car... no injuries or bike problems, thankfully.
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