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I am trying to understand the Source Follower amplifier as a primitive buffer, and am running some simulations with a SPICE simulator. I have the amplifier hooked up as shown below with a current source acting as the bias for the NMOS transistor M1.

enter image description here

I am able to see a relatively good copy of the square-wave input on the the source leg of M1 with a slightly smaller amplitude and no phase change, which I understand is the case with the source follower (Gain less than 1.0).

However, the amplifier shifts the output bias point by one volt. For example, if the square wave Vin is biased around 0.5V, the output on the source leg is biased around -0.5 V. Changing the magnitude of the Ibias current source only affects the shape of the square wave output waveform, it does not shift the bias point of the output.

In order to keep the input and output roughly centered around the same bias point, what modifications would you suggest to the amplifier? Do I need to look at resistors at the drain? Or perhaps a bias network on the gate?

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  • \$\begingroup\$ Further experimentation shows the output bias is largely dependent on the transistor model itself. Modifying the NMOS length and width reduces the amount of the shift of the output, but at the cost of deforming the output waveform shape. \$\endgroup\$ – Dr. Watson Mar 14 '15 at 15:08
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It's called a source follower because source voltage follows the gate voltage but, because the device needs (simplistically) the gate biased slightly higher than the source you get an "error". Depending on what you are trying to achieve you can do nothing, rebias the output signal or apply some feedback system to equalize dc voltages.

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  • \$\begingroup\$ Is the offset between input and output = the threshold voltage of the FET? \$\endgroup\$ – endolith Feb 15 '16 at 20:50
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    \$\begingroup\$ Depending on how much source current is used the "difference" in voltage may be the stated threshold voltage or it may be a bit higher. Threshold voltages are usually at a few hundred micro amps thru the source so be aware of that @endolith \$\endgroup\$ – Andy aka Feb 16 '16 at 8:06

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