Menu
Home
People
Places
Arts
History
Plants & Animals
Science
Life & Culture
Technology
Reference.org
Operational transconductance amplifier
open-in-new
<h2 id="basic-operation">Basic operation</h2> <p>In the ideal OTA, the output current is a linear function of the differential input voltage, calculated as follows: </p> I o u t = ( V i n + − V i n − ) ⋅ g m {\displaystyle I_{\mathrm {out} }=(V_{\mathrm {in+} }-V_{\mathrm {in-} })\cdot g_{\mathrm {m} }} <p>where <i>V</i>in+ is the voltage at the non-inverting input, <i>V</i>in− is the voltage at the inverting input and gm is the <a href="/facts/Transconductance/lwh8krQa">transconductance</a> of the amplifier. </p><p>If the load is just a resistance of R load {\displaystyle R_{\text{load}}} to ground, the OTA's output voltage is the product of its output current and its load resistance: </p> V o u t = I o u t ⋅ R l o a d {\displaystyle V_{\mathrm {out} }=I_{\mathrm {out} }\cdot R_{\mathrm {load} }} <p>The voltage gain is then the output voltage divided by the differential input voltage: </p> G v o l t a g e = V o u t V i n + − V i n − = R l o a d ⋅ g m {\displaystyle G_{\mathrm {voltage} }={V_{\mathrm {out} } \over V_{\mathrm {in+} }-V_{\mathrm {in-} }}=R_{\mathrm {load} }\cdot g_{\mathrm {m} }} <p>The transconductance of the amplifier is usually controlled by an input current, denoted Iabc ("amplifier bias current"). The amplifier's transconductance is directly proportional to this current. This is the feature that makes it useful for electronic control of amplifier gain, etc. </p> <h2 id="non-ideal-characteristics">Non-ideal characteristics</h2> <p>As with the standard op-amp, practical OTA's have some non-ideal characteristics. These include: </p> <ul><li>Input stage <a href="/facts/Non-linearity/4aYItALC">non-linearity</a> at higher differential input voltages due to the characteristics of the input stage transistors. In the early devices, such as the CA3080, the input stage consisted of two bipolar transistors connected in the differential amplifier configuration. The transfer characteristics of this connection are approximately linear for differential input voltages of 20 mV or less.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> This is an important limitation when the OTA is being used open loop as there is no negative feedback to linearize the output. One scheme to improve this parameter is mentioned below.</li> <li>Temperature sensitivity of transconductance.</li> <li>Variation of input and output impedance, input bias current and input offset voltage with the transconductance control current Iabc.</li></ul> <h2 id="subsequent-improvements">Subsequent improvements</h2> <p>Earlier versions of the OTA had neither the Ibias terminal (shown in the diagram) nor the diodes (shown adjacent to it). They were all added in later versions. As depicted in the diagram, the anodes of the diodes are attached together and the cathode of one is attached to the non inverting input (Vin+) and the cathode of the other to the inverting input (Vin−). The diodes are biased at the anodes by a current (Ibias) that is injected into the Ibias terminal. These additions make two substantial improvements to the OTA. First, when used with input resistors, the diodes distort the differential input voltage to offset a significant amount of input stage non linearity at higher differential input voltages. According to National Semiconductor, the addition of these diodes increases the linearity of the input stage by a factor of 4. That is, using the diodes, the signal distortion level at 80 mV of differential input is the same as that of the simple differential amplifier at a differential input of 20 mV.<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> Second, the action of the biased diodes offsets much of the temperature sensitivity of the OTA's transconductance. </p><p>A second improvement is the integration of an optional-use output buffer amplifier to the chip on which the OTA resides. This is actually a convenience to a circuit designer rather than an improvement to the OTA itself; dispensing with the need to employ a separate buffer. It also allows the OTA to be used as a traditional op-amp, if desired, by converting its output current to a voltage. </p><p>An example of a chip combining both of these features is the National Semiconductor LM13600 and its successor, the <a href="/facts/LM13700/e34HC8K2">LM13700</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Current_differencing_transconductance_amplifier/4ubxNDHb">Current differencing transconductance amplifier</a></li> <li><a href="/facts/Transimpedance_amplifier/Y5JKdvE3">Transimpedance amplifier</a> – converts current to voltage</li></ul> <h2 id="notes">Notes</h2> <h2 id="external-links">External links</h2> <ul><li><a href="https://web.archive.org/web/20141010084814/http://www.schmitzbits.de/ota3080.html">A Short Discussion of the Operational Transconductance Amplifier (OTA)</a></li> <li><a href="https://web.archive.org/web/20110927084725/http://users.ece.gatech.edu/~lanterma/studentprojects/TeamOTAComparison.pdf">Comparison of Operational Transconductance Amplifiers (archive)</a></li> <li>Examples: <a href="http://www.intersil.com/content/dam/Intersil/documents/ca30/ca3080-a.pdf">CA3080 (obsolete product)</a>, <a href="http://pdfserv.maxim-ic.com/en/ds/MAX435-MAX436.pdf">MAX 435 (obsolete product)</a> <a href="https://web.archive.org/web/20120717014904/http://pdfserv.maxim-ic.com/en/ds/MAX435-MAX436.pdf">Archived</a> 2012-07-17 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>, <a href="http://pdfserv.maxim-ic.com/en/ds/MAX435-MAX436.pdf">MAX 436 (obsolete product)</a> <a href="https://web.archive.org/web/20120717014904/http://pdfserv.maxim-ic.com/en/ds/MAX435-MAX436.pdf">Archived</a> 2012-07-17 at the <a href="/facts/Wayback_Machine/nmQ3a6JC">Wayback Machine</a>, <a href="http://www.ti.com/lit/ds/symlink/lm13700.pdf">LM13700</a>, <a href="http://focus.ti.com/lit/ds/symlink/opa860.pdf">OPA860</a>, <a href="http://focus.ti.com/lit/ds/symlink/opa861.pdf">OPA861</a></li> <li><a href="https://web.archive.org/web/20160116211924/http://www.elektor-labs.com/project/discrete-otas-for-synth-diy-elektor-formant-upgrades.14314.html">Discrete OTAs for Synth-DIY & Elektor-Formant-Upgrades (archive)</a></li> <li><a href="https://www.researchgate.net/publication/373719052_Voltage_Offset_Compensation_in_CMOS_OTAs_Novel_Approaches_for_Temperature_Drift_Improvement">Reducing voltage offset of the integrated CMOS OTA (archive)</a></li> <li><a href="https://www.researchgate.net/publication/373718601_OTA_Slew-Rate_and_Bandwidth_Enhancement_Based_on_Dynamic_Input-overdriven_Current_Mirror">slew-rate and bandwidth improvement of the CMOS OTA based on the frequency overdrived current mirror (archive)</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>"Performance Evaluation of Different Types of CMOS Operational Transconductance Amplifier" (PDF). Archived from the original (PDF) on 2013-03-23. <a href="https://web.archive.org/web/20130323074116/http://ijsr.net:80/archive/v2i3/IJSRON2013566.pdf" target="_blank">https://web.archive.org/web/20130323074116/http://ijsr.net:80/archive/v2i3/IJSRON2013566.pdf</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>CA3080 <a href="https://web.archive.org/web/20150130132158/http://www.intersil.com/en/products/amplifiers-and-buffers/all-amplifiers/amplifiers/CA3080.html" target="_blank">https://web.archive.org/web/20150130132158/http://www.intersil.com/en/products/amplifiers-and-buffers/all-amplifiers/amplifiers/CA3080.html</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Jung, W.G., IC Op-Amp Cookbook (Howard W. Sams -Bobbs-Merrill First Ed. 1974) p. 440 et seq. <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>"LM13700 Dual Operational Transconductance Amplifiers With Linearizing Diodes and Buffers" (PDF). Texas Instruments. 15 December 2015. Retrieved 26 January 2016. <a href="http://www.ti.com/lit/ds/symlink/lm13700.pdf" target="_blank">http://www.ti.com/lit/ds/symlink/lm13700.pdf</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Jung, W.G., IC Array Cookbook(Hayden, 1980) p. 40-41. <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Data Sheet for LM 13700 – Graph of Distortion v. Differential Input Voltage (National Semiconductor, June 2004) p. 6. <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>"LM13700 Dual Operational Transconductance Amplifiers With Linearizing Diodes and Buffers" (PDF). Texas Instruments. 15 December 2015. Retrieved 26 January 2016. <a href="http://www.ti.com/lit/ds/symlink/lm13700.pdf" target="_blank">http://www.ti.com/lit/ds/symlink/lm13700.pdf</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> </ol>