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I learned that MOSFET works by controlling the voltage on the Gate terminal. For NPN MOSFET as an example, when a voltage on the gate is high enough, it lets the current to flow through the MOSFET, and when a voltage is low, the MOSFET is off.

But what happens when a voltage changes on the source or drain terminal? I am studying the voltage level shifter and could not understand the logic behind it. When voltage on the source is high or low, what happens and why? What about when voltage on the drain is high or low?

Thanks!

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  • \$\begingroup\$ As Neil indicates- Voltage is ALWAYS a relative measure. Voltage IS the electrical potential difference between two points. You cannot meaningfully express a voltage without a stated or implied or assumed relative reference point. \$\endgroup\$ – Russell McMahon Jan 21 '17 at 7:53
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The gate voltage here is with respect to source and not ground. So if your gate voltage is 5v wrt ground and source voltage also 5v wrt ground your n-channel MOSFET(nmos) would still be in off state. In such case you must either increase gate voltage to >5 Volts(above forward bias voltage) or reduce the source voltage below 5v to make it work. Next, when proper bias voltage, I.e Vgs is applied, the Vds (drain to source voltage ) can be used to control the current through this MOSFET.

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The controlling factor is not 'the voltage on the gate', but 'the voltage on the gate with respect to the source terminal, Vgs'. As the source voltage changes, so will Vgs.

That type of FET is an N-channel FET, NPN is only meaningful for bipolar transistors.

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Voltage is allways between two points. "Gate voltage" is nothing until someone tells what is the other point. Voltmeter has 2 measuring probes for that reason.

N-channel (not NPN) mosfet gets its external control voltage between gate and source. Mosfet acts as controllable current valve. The controlled valve inside the mosfet is between drain and source.

The load is connected typically in series with the mosfet, connection at drain. The free load terminal is connected to power supply + terminal. Mosfet's source is connected to the minus terminal of power supply. Mosfet controls how much current is allowed to flow through the load.

When one applies a control voltage, typically a few volts between gate and source (+ at the gate), the mosfet opens ad effectively the load is now connected to the power supply. Load current grows as big as the power supply voltage and load recistance allows. Load current drops gradually to zero if one reduces the control voltage gradually.

In many applications it's wanted to break the load current soon. In that case the G-S voltage should be reduced to zero as fast as possible.

Voltage between drain and source is not a way to control the mosfet. I's a cosequence "how much is left" when mosfet lets a certain current to flow thru D->S and the load takes its part due Ohm's law. Together load voltage and Vds are the supply voltage.

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