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FET source-followers are generally less predictable than BJT (bipolar junction transistor, like NPN or PNP) emitter-followers.

An emitter follower output is one diode drop below the input.  The voltage across a diode varies little as a function of the current, so this offset remains fairly constant.  The absolute voltage across this diode is also reasonably predictable.

The input to output drop of a FET source-follower is whatever gate voltage is required to pass the source current.  The absolute value of that voltage is less well specified than the drop on a silicon diode.  

Take a look at <i>gate threshold voltage</i> specifications for some common FETs.  I just looked up the IRLML2502 as an example of a low voltage FET meant to be driven with logic level gate voltages.  Even this FET is specified for a gate threshold voltage anywhere from 600 mV to 1.2 V, and that's just at 250 &micro;A.

Take a look at the typical characteristics of what gate voltage is required to sustain what collector current:

![Image](https://electrical.codidact.com/uploads/aYdLCP12jmGw5Z2rQWEwHMhU)

First, notice how little this graph really tells you.  It only shows currents from around 15 mA to a bit over 30 mA.  It's not of much help if your current is only, 2 mA, for example.

Second, notice the rather large change in gate voltage to get different channel currents.  Even at the single fixed temperature of 25 &deg;C, it takes 2.5 V to get 20 mA, and over 3 V to get 30 mA.  At 150 &deg;C it takes over 3.4 V to get the same 20 mA.

Another characteristic to consider is the how much D-S voltage is needed before it not longer matters much:

![Image](https://electrical.codidact.com/uploads/LUEGNuMJRNWQUBJaDzH1XnDF)

It takes about 1 V or so at 15 mA, and over 2 V at higher currents.  Contrast that to a BJT where this is well under 1 V except at quite high currents for the device.

So even for this one example low voltage FET, the G-S voltage varies much more than the B-E voltage of a BJT, and more D-S voltage is required before that stops being much of a dependency.  These characteristics are even worse for higher voltage FETs which are usually intended to operate with a 10 V or so gate range.

All around, This means that the G-S voltage of a FET is less predictable to start with, and will vary more, than the B-E voltage of a BJT in similar conditions.  Since all the G-S or B-E voltage variations show up directly on the output of follower circuits, FETs simply don't make as good followers as BJTs do.