# What is a Source Follower

#### person135

Joined Jun 22, 2009
4
Can someone explain to me what a source follower is and how it works?

Thanks

#### Externet

Joined Nov 29, 2005
1,403
Take it as a signal that follows the emitter voltage variations; and as usually there is a low resistance in series with the emitter, it is not far from ground for a common emitter amplifier. But provides a lower source impedance.
Miguel

#### person135

Joined Jun 22, 2009
4
but can someone explain to me how the source follower works? like why the output follows the source from the gate?

#### Externet

Joined Nov 29, 2005
1,403
Sorry, typed the word emitter by habit, not thinking.
Just change it with 'source' :

Take it as a signal that follows the source voltage variations; and as usually there is a low resistance in series with the source, it is not far from ground for a common source amplifier. But provides a lower source impedance.

The output is the source node, will show the same signal.

#### person135

Joined Jun 22, 2009
4
I looked at the wiki page already and did many searches for it already and still can't find much information on them. that's why i'm hoping someone here can tell me more exactly what it is

#### SawabyPlus

Joined Feb 27, 2009
14
For an n-channel FET connected as a Source Follower, you have:

$v_{IN}$ = $v_{OUT}$ + $v_{GS}$

And for small signal operation $v_{GS}$ tends to be constant against incremental variations in Drain Current. so output voltage is almost a DC-shifted version of the input voltage. thus you have:

$v_{in}$ $\approx$ $v_{out}$

which is, actually, what you get from ordinary Small Signal Analysis.

#### GetDeviceInfo

Joined Jun 7, 2009
1,623
'source follower' as I've known it has an output that equals the input in both magnitude and phase (unity gain). What changes is the impedence or drive capacity of the signal.

The simpliest visual form would be an IC opamp with unimpeded feedback.

#### person135

Joined Jun 22, 2009
4
I noticed that there's a page on common drains (aka source followers) on the allaboutcircuits main site. The page says pending. I was wondering if anyone has submitted anything for that and when the page will be ready.

#### steveb

Joined Jul 3, 2008
2,431
Can someone explain to me what a source follower is and how it works?

Thanks
See attached PDF which has 4 relevant pages from
" Integrated-Electronics " by Jacob Millman,and-Christos Allkias. Section 10-7 from this book exactly answers your question. If the explanation is not clear, then you need to do the background study on FETs, and their equivalent AC circuit models.

By the way this book is out of print, but available in PDF form online if you do a search.

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