Editing Common drain (section)

== Characteristics ==
[[Image:N-channel JFET source follower.svg|thumb|Basic N-channel JFET source follower circuit (neglecting [[biasing]] details)]]
At low frequencies, the source follower pictured at right has the following [[small-signal]] characteristics.<ref name="circuit analysis">{{cite web |title=Common Drain Amplifier or Source Follower—Circuit analysis, low frequency, high frequency, and impedance calculations |url=http://webpages.eng.wayne.edu/cadence/ECE7570/doc/cdrain3.pdf |access-date=2008-04-21 |archive-url=https://web.archive.org/web/20131228034917/http://webpages.eng.wayne.edu/cadence/ECE7570/doc/cdrain3.pdf |archive-date=2013-12-28}}</ref>

{| class="wikitable" style="text-align:left"
!  !! Definition !! Expression !! Approximate<br/> expression !! Conditions
|-
! [[gain (electronics)#Current gain|Current gain]]
| <math>A_\text{i} = \frac{i_\text{out}}{i_\text{in}}</math>
| <math>\infty</math>
| <math>\infty</math>
| 
|-
! [[gain (electronics)#Voltage gain|Voltage gain]]
| <math>A_\text{v} = \frac{v_\text{out}}{v_\text{in}}</math>
| <math>\frac{g_m R_\text{S}}{g_m R_\text{S} + 1}</math>
| <math>\approx 1</math>
| <math>g_m R_\text{S} \gg 1</math>
|-
! [[Input impedance]]
| <math>r_\text{in} = \frac{v_\text{in}}{i_\text{in}}</math>
| <math>\infty</math>
| <math>\infty</math>
| 
|-
! [[Output impedance]]
| <math>r_\text{out} = \frac{v_\text{out}}{i_\text{out}}</math>
| <math>R_{\text{S}} \parallel \frac{1}{g_m} = \frac{R_\text{S}}{g_m R_\text{S} + 1}</math>
| <math>\approx \frac{1}{g_m}</math>
| <math>g_m R_S \gg 1</math>
|}

The variable ''g<sub>m</sub>'' that is not listed in the schematic is the [[transconductance]] of the device (usually given in units of [[siemens (unit)|siemens]]).