# Voltage less than one threshold voltage

1. ## Voltage less by one threshold voltage

I want a circuit that inputs V volts and outputs V-Vth volts, where Vth is the threshold voltage of a pMOS or nMOS.
Is a circuit possible? I tried with pass-transistors, its not happening.

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2. ## Re: Voltage less than one threshold voltage

The simple way to drop a fixed amount of volts, is with a series zener diode.

If Vth is just a few volts, then you can do it by stringing together a sufficient number of diodes or led's.

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3. ## Re: Voltage less than one threshold voltage

Diode voltage drop will be different from MOS threshold voltage. The circuit is made up of MOS. So I want is just one Vth drop.

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4. ## Re: Voltage less than one threshold voltage

In IC it is easy - simple class A source follower with mosfet working with inversion coefficient=1. Of course it is only a DC drop, signal amplitude is still not changing (almost)

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5. ## Re: Voltage less than one threshold voltage

I don't recognize a clear question. What do you mean with "its not happening"? Very clearly, a diode connected MOSFET can be used to generate a Vth voltage offset, but of course with non-zero drain current. You could have easily avoided to make others guessing by showing a reference circuit.

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6. ## Re: Voltage less than one threshold voltage

Originally Posted by Dominik Przyborowski
In IC it is easy - simple class A source follower with mosfet working with inversion coefficient=1. Of course it is only a DC drop, signal amplitude is still not changing (almost)
Thanks.
I understand, a source follower will have an output voltage very close to Vin-Vth if its tail current is close to zero. Am I right?
Can you explain what exactly is inversion coefficient in mosfet?

7. ## Re: Voltage less than one threshold voltage

Source follower shifting DC voltage by it gate-to-source voltage, which could be choose equal to threshold voltage by biasing mosfet in center of moderate inversion - its drain current is equal to technology current, which is nonzero of course. The inversion coefficient is a ratio between actual drain current and technology (sometimes called specific) current.

More info You can find in articles describing EKV or ACM mosfet model.

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