Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Common drain
open-in-new
<h2 id="characteristics">Characteristics</h2> <p>At low frequencies, the source follower pictured at right has the following <a href="/facts/Small-signal/P3VYrx4K">small-signal</a> characteristics.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p> <table><tbody><tr><th></th><th>Definition</th><th>Expression</th><th>Approximate expression</th><th>Conditions</th></tr><tr><th><a href="/facts/Gain_(electronics)/CWbehg27">Current gain</a></th><td> A i = i out i in {\displaystyle A_{\text{i}}={\frac {i_{\text{out}}}{i_{\text{in}}}}} </td><td> ∞ {\displaystyle \infty } </td><td> ∞ {\displaystyle \infty } </td><td></td></tr><tr><th><a href="/facts/Gain_(electronics)/CWbehg27">Voltage gain</a></th><td> A v = v out v in {\displaystyle A_{\text{v}}={\frac {v_{\text{out}}}{v_{\text{in}}}}} </td><td> g m R S g m R S + 1 {\displaystyle {\frac {g_{m}R_{\text{S}}}{g_{m}R_{\text{S}}+1}}} </td><td> ≈ 1 {\displaystyle \approx 1} </td><td> g m R S ≫ 1 {\displaystyle g_{m}R_{\text{S}}\gg 1} </td></tr><tr><th><a href="/facts/Input_impedance/AQCLaTj9">Input impedance</a></th><td> r in = v in i in {\displaystyle r_{\text{in}}={\frac {v_{\text{in}}}{i_{\text{in}}}}} </td><td> ∞ {\displaystyle \infty } </td><td> ∞ {\displaystyle \infty } </td><td></td></tr><tr><th><a href="/facts/Output_impedance/80brK3Cv">Output impedance</a></th><td> r out = v out i out {\displaystyle r_{\text{out}}={\frac {v_{\text{out}}}{i_{\text{out}}}}} </td><td> R S ∥ 1 g m = R S g m R S + 1 {\displaystyle R_{\text{S}}\parallel {\frac {1}{g_{m}}}={\frac {R_{\text{S}}}{g_{m}R_{\text{S}}+1}}} </td><td> ≈ 1 g m {\displaystyle \approx {\frac {1}{g_{m}}}} </td><td> g m R S ≫ 1 {\displaystyle g_{m}R_{S}\gg 1} </td></tr></tbody></table> <p>The variable <i>gm</i> that is not listed in the schematic is the <a href="/facts/Transconductance/lwh8krQa">transconductance</a> of the device (usually given in units of <a href="/facts/Siemens_(unit)/kLUHhcCj">siemens</a>). </p> <h2 id="see-also">See also</h2> <ul> <li>Electronics portal</li></ul> <ul><li><a href="/facts/Negative_feedback_amplifier/zQSRBReJ">Negative feedback amplifier</a></li> <li><a href="/facts/Buffer_amplifier/KLM5GzpF">Buffer amplifier</a></li> <li><a href="/facts/Two-port_network/cqUrBTYq">Two-port networks</a></li> <li><a href="/facts/Hybrid-pi_model/VwzM3JAk">Hybrid-pi model</a></li> <li><a href="/facts/Common_collector/kL82QNn2">Common collector</a></li> <li><a href="/facts/Common_emitter/GBVl6DPa">Common emitter</a></li> <li><a href="/facts/Common_base/KK7aHT5e">Common base</a></li> <li><a href="/facts/Common_source/8UDNcIpv">Common source</a></li> <li><a href="/facts/Common_gate/I6fF7PaU">Common gate</a></li> <li><a href="/facts/Common_emitter/GBVl6DPa">Emitter degeneration</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Common Drain Amplifier or Source Follower Archived 2013-12-28 at the Wayback Machine—Circuit analysis, low frequency, high frequency, and impedance calculations. <a href="http://webpages.eng.wayne.edu/cadence/ECE7570/doc/cdrain3.pdf" target="_blank">http://webpages.eng.wayne.edu/cadence/ECE7570/doc/cdrain3.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> </ol>