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Old 20th June 2004, 03:09 AM   #1
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Default Whats happening in mosfet differential?

I like read and re-read and re-read all the articles by Mr.Pass. Every time I re-read them (eventhough all the letters stays the same), I always find something that I don't understand before. It comes to this question.

Mosfets has Vgs about 4V. It means below 4V it will not operate.
In differential pairs, what happens if the signal is below 4V? Will the differential works fine? Or the junction of the sources (that is connected to CCS) will be at -4V, so that the all the signals from 0V will be working fine.

Has anyone measured distortion, or study something about signals that has magnitude below Vgs feeded to mosfet differential.

I raise this question because someone is writing about "First Watt". Maybe very small signal behavior is important.

In ZEN V-7, there is somekind of gate lifting mechanism. The purpose is to adjust bias, but I see them also lifting the 0 point of signal feeded to gate, so maybe in Zen V-7 even signal smaller than Vgs will works fine in the differential, because the gate 0 voltage is lifted. I also noticed this in old audio books, they always use base lifters. But I never saw this in modern schematics.

Is that the differential+CCS has already working perfectly without any gate pull-up, inspite of mosfet has Vgs about 4V? Or it will be better that we use pull-up resistors, so the signal is always applied from 4V-->up?
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Old 20th June 2004, 08:13 PM   #2
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Most of the time when you see diff pairs, the inputs are
operating around ground potential, and that means that
the Sources/Emitters of the diff pairs are operating at a
different DC voltage. In the case of N channel Mosfets with
a Vgs of 4 volts, this would be at - 4 volts. If there is a +/-
supply, this is not a problem as long as the - supply is
significantly greater than -4V, allowing extra voltage to run
a current source or bias resistor.
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Old 21st June 2004, 03:52 AM   #3
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Hi, Mr. Pass,

A mosfet have Vgs about 4V. That means any voltage less than 4V between G and S will not operate the mosfet. (As you said, any transistor in audio CCT dont know whats its position, it just operates based on 3 legs behavior)
If the G is about 0V, so the S will be about -4V, like written in all schematics.

In this case, the determining voltage is G (0V, because there is usually R to ground attached in G), and the S is just floating due to nature of 4V of G-S, because the S are usually attached to D/Collector of CCS, so it can float at any voltage.

Is there any difference in sonics, or distortion, if we SET the S at definite voltage (like -4V exactly) instead of letting them floating at -4V? Maybe exact -4V will give better result than nature's -4V floating.

ps. Mr. Pass, I encountered a problem with an audio power amp, which its output is similiar to your ZEN V-7T. Transformer output solid state Do you encounter the same problem with inductor output stage?

In your paper of ZEN V-7T, you did not write about how big the inductance or number of turns needed for ZEN V7-T. You just use primaries of ordinary power transformer. If I wanted to DIY the CT inductor (or secondary output) what is the number of turns or inductance needed for ZEN V7-T?
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