{"id":1168,"date":"2018-04-19T07:40:19","date_gmt":"2018-04-19T07:40:19","guid":{"rendered":"https:\/\/www.tina.com\/resources\/?page_id=1168"},"modified":"2018-07-06T22:51:54","modified_gmt":"2018-07-06T22:51:54","slug":"3-the-typical-op-amp","status":"publish","type":"page","link":"https:\/\/www.tina.com\/resources\/home\/practical-operational-amplifiers\/3-the-typical-op-amp\/","title":{"rendered":"3. The Typical Op-amp"},"content":{"rendered":"<div class=\"top-navigation\">\n<div class=\"current-page\"><img decoding=\"async\" src=\"https:\/\/www.tina.com\/images\/resource\/people-location-30.png\" \/>CURRENT &#8211; 3. The typical op-amp<\/div>\n<div class=\"previous-page\"><a href=\"https:\/\/www.tina.com\/resources\/home\/2-level-shifters\/\"><img decoding=\"async\" src=\"https:\/\/www.tina.com\/images\/resource\/leftarrow5.png\" \/>PREVIOUS- 2. Level shifters<\/a><\/div>\n<div class=\"next-page\"><a href=\"https:\/\/www.tina.com\/resources\/practical-operational-amplifiers\/4-manufacturers-specifications\/\">NEXT- 4. Manufacturers specifications<br \/>\n<\/a><\/div>\n<div class=\"clear\"><\/div>\n<\/div>\n<p><script>\nfunction setStyle(){ \nheight = document.getElementsByClassName('entry-header')[0].clientHeight;\ndocument.getElementsByClassName('entry-header')[0].style.marginTop = height +\"px\";\ndocument.getElementsByClassName('top-navigation')[0].style.top = Math.round(-(height-20)*2) +\"px\";\n}\nsetStyle();\nwindow.onresize = function(event) {\nsetStyle();\n};\n<\/script><\/p>\n<p align=\"justify\"><strong><em><span style=\"font-size: 22px;\">The Typical Op-amp<\/span><\/em><\/strong><\/p>\n<p>Most operational amplifiers are designed and constructed in accordance with the block diagram shown in Figure 8.<\/p>\n<div id=\"attachment_1169\" style=\"width: 487px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1169\" class=\"wp-image-1169 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure8.png\" alt=\"The Typical Op-amp\" width=\"477\" height=\"56\" srcset=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure8.png 477w, https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure8-300x35.png 300w\" sizes=\"auto, (max-width: 477px) 100vw, 477px\" \/><p id=\"caption-attachment-1169\" class=\"wp-caption-text\">Figure 8 \u2013 Typical configuration of an op-amp<\/p><\/div>\n<p>The differential amplifier and the voltage gain stage are the only stages which provide the voltage gain. The differential amplifier also provides the CMRR which is so important in the op-amp. The output of the differential amplifier is often connected to an emitter follower with a large emitter resistor so as to provide a high impedance load to the differential amplifier in order to obtain a high gain. Remember that a high-gain common-emitter amplifier suffers from much lower input impedance than a moderate gain CE amplifier. This then allows the use of a high gain CE amplifier to provide the additional gain. Linear op-amps are direct coupled to provide <em>ac<\/em> gain. This also eliminates the need for a coupling capacitor that is too large to be placed on an IC chip. Level shifters are required to insure that the output signal does not have any <em>dc<\/em> offset.\u00a0 Op-amps can be very accurately modeled by circuit simulation. We will demonstrate this using the TINACloud online circuit simulation.<\/p>\n<h6><em><span style=\"color: #000000; font-family: Bodoni MT,serif;\"><b><span lang=\"en-US\">3.1 Packaging<\/span><\/b><\/span><\/em><\/h6>\n<p>Op-amp circuits are packaged in standard IC packages, including cans, dual-in-line packages (DIP), and flat packs. Each of these packages has at least eight pins or connections. They are illustrated in Figures 9, 10, and 11.<\/p>\n<p>&nbsp;<\/p>\n<div id=\"attachment_1192\" style=\"width: 260px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1192\" class=\"wp-image-1192 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9M.png\" alt=\"The Typical Op-amp\" width=\"250\" height=\"182\" \/><p id=\"caption-attachment-1192\" class=\"wp-caption-text\">Figure 9 \u2013 Op-amp connection for can package (top view)<\/p><\/div>\n<div id=\"attachment_1193\" style=\"width: 261px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1193\" class=\"wp-image-1193 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-10M.png\" alt=\"The Typical Op-amp\" width=\"251\" height=\"183\" \/><p id=\"caption-attachment-1193\" class=\"wp-caption-text\">Figure 10 &#8211; Op-amp connection 14-pin DIP (Top View)<\/p><\/div>\n<div id=\"attachment_1196\" style=\"width: 260px\" class=\"wp-caption alignleft\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1196\" class=\"wp-image-1196 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9-11MM.png\" alt=\"The Typical Op-amp\" width=\"250\" height=\"182\" \/><p id=\"caption-attachment-1196\" class=\"wp-caption-text\">Figure 11 \u2013 Op-amp connection for 10-pin flat pack (Top view)<\/p><\/div>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>&nbsp;<\/p>\n<p>When constructing a circuit, it is important to identify the various leads correctly (they are usually not numbered). The figures illustrate the location of pin 1. In the <em>can package<\/em> of Figure 9, pin 1 is identified as the first pin to the left of the tab, and the pins are numbered consecutively counterclockwise looking from the top. In the <em>dual-in-line package<\/em> of Figure 10, the top of the package has an indentation to locate pin 1, and the pins are numbered down on the left and up on the right. Note that more than one op-amp (typically 2 or 4) is packaged in one DIP.<\/p>\n<p>In the <em>flat pack<\/em> of Figure 11, pin 1 is identified by a dot and the pins are numbered as in the DIP.<\/p>\n<h6><em><span style=\"color: #000000; font-family: Bodoni MT,serif;\"><b><span lang=\"en-US\">3.2 Power Requirements<\/span><\/b><\/span><\/em><\/h6>\n<p>Many op-amps require both a negative and a positive voltage source. Typical voltage sources range from \u00b15 V to \u00b125 V. Figure 12 shows typical power supply connections to the op-amp.<\/p>\n<p>The maximum output voltage swing is limited by the <em>dc<\/em> voltage supplied to the op-amp. Some operational amplifiers can be operated from a single voltage source. The manufacturer&#8217;s specifications define the limits of operation in those cases where the op-amp uses only one power supply.<\/p>\n<div id=\"attachment_1205\" style=\"width: 260px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1205\" class=\"wp-image-1205 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9-12.png\" alt=\"Op-amps, typical op-amps\" width=\"250\" height=\"182\" \/><p id=\"caption-attachment-1205\" class=\"wp-caption-text\">Figure 12 \u2013 Power supply connections<\/p><\/div>\n<p>The maximum output voltage swing is limited by the <em>dc<\/em> voltage supplied to the op-amp. Some operational amplifiers can be operated from a single voltage source. The manufacturer&#8217;s specifications define the limits of operation in those cases where the op-amp uses only one power supply.<\/p>\n<h6><em><span style=\"color: #000000; font-family: Bodoni MT,serif;\"><b><span lang=\"en-US\">3.3 The 741 Op-amp<\/span><\/b><\/span><\/em><\/h6>\n<p>The \u03bcA741 op-amp is illustrated in the equivalent circuit of Figure 13. It has been produced since 1966 by most IC manufacturers, and although there have been many advances since its introduction, the 741 is still widely used.<\/p>\n<div id=\"attachment_1209\" style=\"width: 544px\" class=\"wp-caption alignnone\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-1209\" class=\"wp-image-1209 size-full\" src=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9-13.png\" alt=\"operational amplifiers, typical op-amps\" width=\"534\" height=\"282\" srcset=\"https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9-13.png 534w, https:\/\/www.tina.com\/resources\/wp-content\/uploads\/2018\/04\/Practical-op-ampFigure9-9-13-300x158.png 300w\" sizes=\"auto, (max-width: 534px) 100vw, 534px\" \/><p id=\"caption-attachment-1209\" class=\"wp-caption-text\">Figure 13 \u2013 The 741 op-amp<\/p><\/div>\n<p>The 741 op-amp has <em>internal compensation <\/em>which refers to the RC network which causes the high-frequency amplitude response to fall off. Because the amplifier has high gain (on the order of 10<sup>4<\/sup> to 10<sup>5<\/sup> at low frequencies) and because parasitic capacitances in the transistors allow <em>parasitic feedback<\/em>, the op-amp would become unstable and oscillate if it were not for the internal compensation. Two cascaded difference amplifiers drive a complementary symmetry power amplifier through another voltage amplifier.<\/p>\n<p>The 741 op-amp consists of three stages: an input differential amplifier, an intermediate single-ended high gain amplifier, and an output buffering amplifier. Other circuitry important to its operation is a level shifter to shift the <em>dc<\/em> level of the signal so that the output can swing both positive and negative, bias circuits to provide reference currents to the various amplifiers, and circuits that protect the op-amp from short circuits at the output. The 741 is internally compensated by means of an on-chip capacitor-resistor network.<\/p>\n<p>The op-amp is further improved by adding more stages of amplification, isolating the input circuits, and adding more emitter followers at the output to decrease the output impedance. Other improvements result in increased CMRR, higher input impedance, wider frequency response, decreased output impedance and increased power.<\/p>\n<h6><strong><em>Bias Circuits<\/em><\/strong><\/h6>\n<p>Several constant sources can be seen in the 741 op-amp of Figure 13. Transistors <em>Q<sub>8<\/sub><\/em> and <em>Q<sub>9<\/sub><\/em> are the current source for <em>I<sub>EE<\/sub><\/em> of the differential amplifier formed by <em>Q<sub>1<\/sub><\/em>, <em>Q<sub>2<\/sub><\/em>, <em>Q<sub>3<\/sub><\/em>, and <em>Q<sub>4<\/sub><\/em>. Transistors <em>Q<sub>5<\/sub><\/em>, <em>Q<sub>6<\/sub><\/em>, and <em>Q<sub>7<\/sub><\/em>, are the active loads substituting for the <em>R<sub>C<\/sub><\/em> resistors of the differential amplifier. Transistors <em>Q<sub>10<\/sub><\/em>, <em>Q<sub>11<\/sub><\/em>, and <em>Q<sub>12<\/sub><\/em> form the bias network for the differential amplifier current sources.\u00a0 Transistors <em>Q<sub>10<\/sub><\/em> and <em>Q<sub>11<\/sub><\/em> form a Widlar current source for this bias network with the other transistors acting as a current mirror.<\/p>\n<h6><strong><em>Short Circuit Protection<\/em><\/strong><\/h6>\n<p>The 741 circuit includes a number of transistors that are normally cut off and conduct only in the event that a large current exists at the output. The bias on the output transistors is then changed to reduce this current to an acceptable level. In the circuit of Figure 13, this short-circuit protection network consists of transistors Q<sub>15<\/sub> and <em>Q<sub>22<\/sub><\/em> and resistor <em>R<sub>11.<\/sub><\/em><\/p>\n<h6><strong><em>Input Stage<\/em><\/strong><\/h6>\n<p>The input stage of the 741 op-amp is required to provide voltage gain, level shifting, and a single-ended differential amplifier output. The complexity of the circuitry causes a large offset voltage error. In contrast to this, the standard resistor-loaded differential amplifier causes less offset voltage error. However, the standard amplifier has limited gain which means that more stages would be required to achieve the desired amplification. The resistor-loaded differential amplifiers are used in op-amps which have less voltage drift than the 741.<\/p>\n<p>BJTs used in the input stage require large bias currents, introducing offset current problems. To reduce the offset current error, other op-amp types use MOSFETs in the input stage.<\/p>\n<p>The input stage of the 741 is a differential amplifier with an active load formed by transistors <em>Q<sub>5<\/sub><\/em>, <em>Q<sub>6<\/sub><\/em>, and <em>Q<sub>7<\/sub><\/em> and resistors <em>R<sub>1<\/sub><\/em>, <em>R<sub>2<\/sub><\/em>, and <em>R<sub>3<\/sub><\/em>. This circuit provides a high resistance load and converts the signal from differential to single-ended with no degradation of gain or common-mode rejection ratio. The single-ended output is taken from the collector of <em>Q<sub>6<\/sub><\/em>. The input stage level shifter consists of lateral <em>pnp<\/em> transistors, <em>Q<sub>3<\/sub><\/em> and <em>Q<sub>4<\/sub><\/em>, which are connected in a common-base configuration.<\/p>\n<p>Use of the lateral transistors, <em>Q<sub>3<\/sub><\/em> and <em>Q<sub>4<\/sub><\/em>, results in an added advantage. They help protect the input transistors, <em>Q<sub>1<\/sub><\/em> and <em>Q<sub>2<\/sub><\/em>, against emitter-base junction breakdown. The emitter-base junction of an <em>npn<\/em> transistor will break down when the reverse bias exceeds about 7 V. Lateral transistor breakdown does not occur until the reverse bias exceeds about 50 V. Since the transistors are in series with <em>Q<sub>1<\/sub><\/em> and <em>Q<sub>2<\/sub><\/em>, the breakdown voltage of the input circuit is increased.<\/p>\n<h6><em><strong>Intermediate Stage<\/strong><\/em><\/h6>\n<p>The intermediate stages in most op-amps provide high gain through several amplifiers. In the 741, the single-ended output of the first stage is connected to the base of <em>Q<sub>16<\/sub><\/em> which is in an emitter follower configuration. This provides a high input impedance to the input stage which minimizes loading. The intermediate stage also consists of transistors <em>Q<sub>16<\/sub><\/em> and <em>Q<sub>17<\/sub><\/em>, and resistors <em>R<sub>8<\/sub><\/em> and <em>R<sub>9<\/sub><\/em>. The output of the intermediate stage is taken from the collector of <em>Q<sub>17<\/sub><\/em>, and provided to <em>Q<sub>14<\/sub><\/em> through a phase splitter. The capacitor in the 741 is used for frequency compensation which is discussed in subsequent chapters of this text.<\/p>\n<h6><strong><em>Output Stage<\/em><\/strong><\/h6>\n<p>The output stage of an op-amp is required to provide high current gain to a low-output impedance. Most op-amps use a complementary symmetry output stage to increase efficiency without sacrificing current gain. The maximum achievable efficiency for the complementary symmetry, class B amplifier is 78% . The single-ended output amplifier has a\u00a0maximum efficiency of only 25%. Some op-amps use Darlington pair complementary symmetry to increase their output capability. The complementary symmetry output stage in the 741 consists of <em>Q<sub>14<\/sub><\/em> and <em>Q<sub>20<\/sub><\/em>.<\/p>\n<p>The small resistors, <em>R<sub>6<\/sub><\/em> and <em>R<sub>7<\/sub><\/em>, provide current limiting at the output. The Darlington pair, <em>Q<sub>18<\/sub><\/em> and <em>Q<sub>19<\/sub><\/em>, is used in place of the diode in the diode-compensated complementary symmetry output stage as described in Chapter 8. The Darlington pair arrangement is favored over the two transistors connected as a diode since it can be fabricated in a smaller area. The current source substituting for the bias resistor in the complementary symmetry circuit is realized by one part of transistor <em>Q<sub>13<\/sub><\/em>. Transistors <em>Q<sub>22<\/sub><\/em>, <em>Q<sub>23<\/sub><\/em>, and <em>Q<sub>24<\/sub><\/em> are part of a level shifter arrangement which insures that the output voltage is centered around the zero axis.<\/p>\n<div class=\"current-page\"><img decoding=\"async\" src=\"https:\/\/www.tina.com\/images\/resource\/people-location-30.png\" \/>CURRENT &#8211; 3. The typical op amp<\/div>\n<div class=\"previous-page\"><a href=\"https:\/\/www.tina.com\/resources\/home\/2-level-shifters\/\"><img decoding=\"async\" src=\"https:\/\/www.tina.com\/images\/resource\/leftarrow5.png\" \/>PREVIOUS- 2. Level shifters<\/a><\/div>\n<div class=\"next-page\"><a href=\"https:\/\/www.tina.com\/resources\/practical-operational-amplifiers\/4-manufacturers-specifications\/\">NEXT- 4. Manufacturers specifications<br \/>\n<\/a><\/div>\n<div class=\"clear\"><\/div>\n","protected":false},"excerpt":{"rendered":"<p>CURRENT &#8211; 3. The typical op-amp PREVIOUS- 2. Level shifters NEXT- 4. Manufacturers specifications The Typical Op-amp Most operational amplifiers are designed and constructed in accordance with the block diagram shown in Figure 8. The differential amplifier and the voltage gain stage are the only stages which provide the voltage gain. The differential amplifier also [&hellip;]<\/p>\n","protected":false},"author":1,"featured_media":0,"parent":917,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"yst_prominent_words":[421,418,424,428,414,335,423,417,425,270,244,420,422,412,427,416,413,419,426,415],"class_list":["post-1168","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/pages\/1168","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/comments?post=1168"}],"version-history":[{"count":56,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/pages\/1168\/revisions"}],"predecessor-version":[{"id":3612,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/pages\/1168\/revisions\/3612"}],"up":[{"embeddable":true,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/pages\/917"}],"wp:attachment":[{"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/media?parent=1168"}],"wp:term":[{"taxonomy":"yst_prominent_words","embeddable":true,"href":"https:\/\/www.tina.com\/resources\/wp-json\/wp\/v2\/yst_prominent_words?post=1168"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}