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carpenter 21st July 2009 05:18 PM

mosfet gate voltage question
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For my continuing education, can anyone tell me why the voltage regulator in the picture doesn't blow the mosfet gate when the zeners value exceeds 20 volts.

Also, are there conditions where the mosfet gate is allowed to exceed 20 volts in an amplifier circuit?

peranders 21st July 2009 05:39 PM

When the voltage is more than 20 volts across gate and source you will blow the mosfet but now the voltage is only 3-4 volts. A good way of learning is to use LTSpice which is free to download.

The gate must always stay under 20 volts because excessive voltage is distructable. The mosfet will die if you apply gate voltages around 25-30 volts. I have tested this myself.

carpenter 21st July 2009 05:50 PM

I use Simetrix, and understand that it works. I just wondered why it works.

I've even exceeded 20 volts to the mosfet gate in my sims and the simulation ran. I didn't have the gates referenced to ground (2nd gain stage) and it operated at 25 volts.

lineup 21st July 2009 05:59 PM

The voltage at GATE can be 100 Volt or more .. even 1.000 Volt
as long as the voltage at source is not too low.

In your circuit, if the voltage at GATE is 40 V
then at the SOURCE will be at something like 36 V. Only 4 Volts lower.
And those remaining 36 Volts will be across the load.

The absolute Voltage related to GROUND, is something the MOSFET never can see.
MOSFET will see the difference between Gate-Source,
and in most cases Vgs is like 3-6 V in a normal circuit.

carpenter 21st July 2009 06:18 PM

Brilliant answer, lineup. This reminds me of the LM317 regulator: as long as the in and out pins don't exceed 35 volts, or so, you can run it where ever you wish.

Thanks a bunch! :)

Nelson Pass 21st July 2009 06:38 PM

Gate protection is the reason for the ordinary diode in the
circuit also. Think about it.


carpenter 22nd July 2009 02:34 AM


Originally posted by Nelson Pass
Gate protection is the reason for the ordinary diode in the
circuit also. Think about it.


Thanks for that! Makes perfect sense.

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