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| {{More footnotes|date=April 2009}}
| | Marvella is what you can call her but it's not the most female title out there. One of the issues she loves most is to study comics and she'll be starting some thing else along with it. I am a meter reader but I strategy on altering it. Years in the past we moved to North Dakota.<br><br>Look at my page [http://we.mahshar.com/index.php?do=/profile-3192/info/ over the counter std test] |
| In [[electronics]], a '''common-drain''' [[electronic amplifier|amplifier]], also known as a '''source follower''', is one of three basic single-stage [[field effect transistor]] (FET) amplifier topologies, typically used as a [[voltage]] [[buffer amplifier|buffer]]. In this circuit (NMOS) the gate terminal of the transistor serves as the input, the source is the output, and the drain is common to both (input and output), hence its name. The analogous [[bipolar junction transistor]] circuit is the [[common collector|common-collector amplifier]]. This circuit is also commonly called a "stabilizer."
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| In addition, this circuit is used to transform [[Electrical impedance|impedance]]s. For example, the [[Thevenin's theorem|Thévenin resistance]] of a combination of a voltage follower driven by a voltage source with high Thévenin resistance is reduced to only the output resistance of the voltage follower (a small resistance). That resistance reduction makes the combination a more ideal voltage source. Conversely, a voltage follower inserted between a driving stage and a high load (i.e. a low resistance) presents an infinite resistance (low load) to the driving stage—an advantage in coupling a voltage signal to a large load.
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| == Characteristics ==
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| [[Image:N-channel JFET source follower.svg|thumb|Basic N-channel JFET source follower circuit (neglecting [[biasing]] details).]]
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| At low frequencies, the source follower pictured at right has the following [[small signal]] characteristics.<ref name="circuit analysis">[http://webpages.eng.wayne.edu/cadence/ECE7570/doc/cdrain3.pdf Common Drain Amplifier or Source Follower]—Circuit analysis, low frequency, high frequency, and impedance calculations.</ref>
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| {| class="wikitable" style="background:white;text-align:left"
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| ! !! Definition !! Expression !! Approximate expression !! Conditions
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| ! '''[[gain#Current gain|Current gain]]'''
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| |<math> {A_\mathrm{i}} = {i_\mathrm{out} \over i_\mathrm{in}} </math>
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| |<math> \infty\ </math>
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| |<math> \infty\ </math>
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| |-
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| ! '''[[gain#Voltage gain|Voltage gain]]'''
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| |<math> {A_\mathrm{v}} = {v_\mathrm{out} \over v_\mathrm{in}} </math>
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| |<math> \frac{g_m R_{\text{S}}}{g_m R_{\text{S}} + 1} </math>
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| |<math> \approx 1 </math>
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| |<math> (g_m R_{\text{S}} \gg 1)</math>
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| |-
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| ! '''[[Input resistance]]'''
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| |<math> r_\mathrm{in} = \frac{v_{in}}{i_{in}}</math>
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| |<math> \infty\ </math>
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| |<math> \infty\ </math>
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| |-
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| ! '''[[Output resistance]]'''
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| |<math> r_\mathrm{out} = \frac{v_{out}}{i_{out}}</math>
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| |<math> R_{\text{S}} \| \frac{1}{g_m} = \frac{ R_{\text{S}} }{ g_m R_{\text{S}} + 1 }</math>
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| |<math> \approx \frac{1}{g_m}</math>
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| |<math> (g_m R_S \gg 1)</math>
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| |}
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| 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]]).
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| ==See also==
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| {{Portal|Electronics}}
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| {{colbegin|3}}
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| *[[Negative feedback amplifier]]
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| *[[Buffer amplifier]]
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| *[[Two-port network]]s
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| *[[Hybrid-pi model]]
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| *[[Common collector]]
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| *[[Common emitter]]
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| *[[Common base]]
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| *[[Common source]]
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| *[[Common gate]]
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| *[[Common emitter#Emitter degeneration|Emitter degeneration]]
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| {{colend}}
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| == References ==
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| {{reflist}}
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| {{Transistor amplifiers}}
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| [[Category:Single-stage transistor amplifiers]]
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| {{electronics-stub}}
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