《The Signal》 Bruce Trump_TI_slyt701

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TI 资深模拟电子工程师Bruce Trump的博文中与运算放大器相关知识的汇总,该文档来自于2018年1月初的TI官方邮件推送内容。文档代号slyt701。
Preface Learning analog seems like a daunting task. Analog engineers do not Each topic addressed in this book was originally published as a generally acquire their experience in a linear path from start to finish post on my blog The Signal, "which you can still visit on TI's E2E They zigzag a path through an obstacle course of hurdles. They Community. As such, you'l find that the lessons are short and to the acquire insights in small pieces -a bit here and a bite (not a byte) point; practical and intuitive; bite-sized and easy to digest. I needed there. Slowly, puzzle pieces fit into place, and hazy concepts come it to be that way because I'n a simple guy with little patience into focus Most of my blog posts sparked questions and other dialogue. I have We will never have the satisfaction of jumping a final hurdle or included links at the end of each topic to the original post tapping the final puzzle piece into place that just won 't happen comments were posted. I think you will find some valuable lessons Colleagues much smarter than I am cannot answer all of my taught there. Furthermore, this compendium does not include all of questions .. and I cannot answer all of yours my blogs. I've included links to other topics, at the end So this assemblage of little analog lessons is hopelessly incomplete. If you have any questions about the topics I cover here, or any other Still, I think you will find it helpful. It may fill some gaps in your precision-amplifier questions for that matter, I hope you will submit knowledge or stimulate your thinking them to the Precision Amplifiers forum on TIs E2E Community For reference, I created most of the images in this e-book using TI's TINA-TIM free software tool, downloadable from ti's website About the Author Bruce Trump As a boy experimenter and ham radio operator, Bruce Trump was drawn to electrical engineering. He never doubted that this was his career path s he worked on an early laser memory system and other analog system component k him to Ohio, where a After earning a bachelor's degree from lowa State University, his first industry job to His next stop was at Heath Co in Michigan designing Heathkits. He tackled a variety of projects there, including electronic clocks, megaphones, metal detectors, navigation calculators and high-power stereo amplifiers But hard-core analog was calling him. Burr-Brown, which was a leading provider of analog integrated circuits at that time, offered Bruce an opportunity in Tucson, Arizona, to hang out with and learn from real analog experts Texas Instruments acquired Burr-Brown in 2000, marking a new chapter in Bruce's analog career, which included roles in product development, product definition, applications engineering, technical literature, product promotion and business management When Bruce reflects on his career, he typically shares that his favorite activity was always dealing with customer application issues I always seemed to work this into whatever role I was currently playing. I particularly enjoyed developing customer seminars and datasheets. It was a challenge to clearly explain the inner workings and applications of precision analog components, "he said The signal Texas instruments Chapter I: Op Amp Voltage-Range Issues 1. Op amp voltage ranges: input and output, Figure 2 shows a so-called single-supply op amp. It has a C-M clearing some confusion range that extends to, and often slightly below, the negative rail That range allows its use in a wider range of circuits that operate System designers often have questions about the power supply close to ground. So an op amp that is not called"single supply input and output voltage-range capabilities of operational amplifiers is actually usable in some single-supply circuits, but a true single- op amps).It can be confusing, so here is my attempt to sort it out. supply type is more versatle First, common op amps do not have ground terminals. A standard op amp does not"know"where ground is, so it cannot know Positive rail (v+) whether it is operating from a dual supply (or a single power supply. As long as the power-supply input and output voltages are Single-supply op amp C-M range to or below 2.2V Single-supply op within their operating ranges, all is well egative rail. Examples amps often hay OPAZ34 LM324 Input common-mode Output a wider output Here are three critical voltage ranges to consider voltage range voltage voltage swing range The total supply-voltage range. This is the total voltage 0.5V Voltage range of between the two supply terminals. For example, +15 V is a total his g= 1 circuit: Negative rail (V-) V-)+0.5Vto of 30 V The operating voltage range for an op amp rnight be 0.2V (often ground 6V to 36 V, for example. At the low-voltage extreme, this could be +3 V or +6V. At maximum it could be +18V or +36V or even V/30V. Yes, unbalanced supplies are OK, but only if Figure 2: Input and output voltage ranges of a typical single-supply op amp you heed the second and third bullet points below 2. The input common-mode voltage range(C-M range)is In a g= 1 buffer circuit, this op amp could produce an output swing generally specified relative to the positive and negative supply of 0.5 V from the V-rail (limited by the output capability) and 2.2V voltages, shown in Figure 1. In sorne equation-like form, the from the V+ rail (limited by the input c-m range C-M range of this hypothetical op amp would be described Figure 3 shows a" rail-to-rail"op amp. It can operate with an input as 2 V above the negative rail to 2. 5 V below the positive rail. voltage equal to or even slightly beyond both supply-voltage rails Something like this: (-)+2 V to(V+1-2.5 V. as shown in Figure 3. A rail-to-rail output means that the output 3. The output-voltage range(or output-swing capability) is, again voltage can swing very close to the rails, often within a 10-to commonly specitied relative to the rail voltages. In this case 100-mV range from the supply rails. Some op amps claim only )+1yto(+)-1.5V a rail-to-rail output, lacking the input characteristics shown in Figure 3. Rail-to-rail op amps are very commonly used on single Fiqures 1, 2 and 3 show aG= 1 buffer configuration. A key point 5-V supplies es and lower because they maximize the signal-voltage here: The output capability of the example in Figure 1 will be limited capability on their limited supply range to 2 V from the negative rail and 2.5 V from the positive rail, which is due to the limited-input C-M range. You would need to configure this op amp in a higher gain to deliver its full output-voltage range Positive rail (V+) 0.1V Positive rail (V+) Rail-to-rail op amps Rai-to-rail op amps have c-m range to or Output beyond both rails Total power supply 2.5V 1.5V Input common-mode voltage output voltage swing Examples: OPA320 voitage ra range operating range OPA365 Voltage range of from minimum Input common-mode Output 0.1V his g= 1 circuit to maximum voltage range voltage -+0.vtc range ∨o| tage range Negative rail (V-) (V+-0.1V of this g=1 02y ioften ground circuit: (V-)+2 V ta(+)2.5V Negative rail (V-) (could be ground Figure 3: input and output voltage ranges of a typical rail-to-rail op amp Figure 1: Input and output voltage ranges of a typical op amp used Rail-to-rail op amps are appealing because they ease signal-voltage on dual supplies(±) constraints but they are not always the best choice. Like other life choices, there are often trade-offs with other performance The example in Figure 1 is typical of an op amp generally used on attributes. But that is why you are an analog designer. Your life dual supplies. It would not be called a" single-supply type, but it is full of complex issues and trade-offs, and you love it! could operate as a single supply by staying within those ranges To see this original post with comments, click here The signal 4 Texas instruments 2. Rail-to-rail inputs: what you should know! Rail-to-rail operational amplifiers(op amps) are extremely popular The N-channel FETs operate with a common-mode voltage near and especially useful with low supply voltages. You should know how and slightly above the positive rail. Additional circuitry (not shown to accomplish rail-to-rail inputs and understand some trade-offs directs traffic, determining which input-stage signal the next stage w Figure 4 shows a typical dual-input, rail-to-rail stage comprising process. Most TI dual-input-stage op amps are designed so that the both N-channel and P-channel transistor pairs. P-channel field-effect approximately 1.3V from the positive rail. Above this transistors(FE [s) handle the signal through the lower portion of the voltage, there is insufficient gate voltage for the P-channel stage, so common-mode voltage range to slightly below the negative rail (or the signal path is redirected to the N-channel stage single-supply ground) V+ supply Offset Voltage vs Common-Mode voltage N-FETs active from 1v below -to above v+ E N-Chan Untrimmed Vin+ Rimmed P-FETs active fro To next sta below v- to near v+ (V+)2(+)-1V+ 01 Common-Mode voltage Figure 4: A typical dual-input rail-to-rail stage using both N-and P-channel transistor pairs The P and N input stages will have somewhat ditterent offset While unnoticed in most applications, this change in offset voltage voltages. If the common-mode voltage moves through this transition may be an issue if you require high accuracy. It can also cause (as it does with rail-to-rail G=1 operation), it creates a change in distortion in alternating current (AC) applications. But again, the offset. Some op amps are factory-trimmed by laser or electronic distortion will only occur if the common-mode input voltage crosses trimming, adjusted to reduce the offset of the input stages. This the transition between stages trimming reduces the change through the transition but stll leaves F igure 5 shows a second type of rail-to-rail input stage. An internal a residual bobble. Circuitry controlling the transition from the p to charge pump boosts the voltage powering a single P-channel input N input stage is referenced to the positive supply voltage, not to stage to approximately 2 V above the positive supply rail. This ground On a 3.3-V supply, the transition moves to an awkward point -midsupply voltage boost allows a single-input stage to perform seamlessly over the full rail-to-rail input-voltage range- below the bottom rail to above the top rail - with no transition glitch Offset Voltage vs Common-Mode voltage Internal Supply 2V above v+ Charge pump Untrimmed P-FET inputs Live rail-to-rail 0 Trimmed Vin (V+)-2(V+)-1V+ Common-Mode voltage Figure 5: A rail-to-rail input stage with an internal charge pump to boost the voltage, powering a single P-channel FET. The signal Texas instruments Charge pump". it sounds spooky to some designers. They are maximize output-voltage swing. TI's specifications for these devices noisy, right? But TI's most recent ones are remarkably quiet Charge generally look something like Table 1 pumps require very little current because they only power the input stage. There are no extra pins or capacitors-it is all internal. Charge- Parameter Conditions Min Typ Max Unit pump noise is below the broadband noise level; rarely can you see it Output in the time domain. Applications that analyze the spectral response Voltage output swing from RL= 10 kO 1525 below the broadband noise level, however, may see some artifacts both rails RL=2kQ 3550mV Not all applications need an op amp with rail-to-rail input Inverting op amp circuits or amplifiers in gain greater thar Table 1: Output specifications for rail-to-rail amplifiers. unity, for example, often do not require rail-to-rail input, yet still have rail-to-rail output. Do you really need a rail-to-rail input Table 1 makes it appear that the output wil never swing much closer than 15 mv from ground, and that last 15 mv can be critical amplifier? Many engineers prefer to use them so that they do not for accurate zero-based measurements. But wait: You really need to need to worry about exceeding the common-mode range. They carefully interpret all of the conditions of this specification, because use the same op amp in various points in their systems: some the assumption is that the load is connected halfway between the need rail-to-rail input; others not. Whatever your choice, with power-supply terminals knowledge of rail-to-rail types and trade-offs, you can select You will often find conditions cited at the top of the specifications more wisely. If in doubt, you are welcome to ask the engineers table, where you will see a statement like this: R connected to Vs/2 on the T E2E Community precision Amplifiers forum In this specified condition, the amplifier must sink current through Here are a few example op amps the load resistor as the output approaches ground. This reflects the OPA340 dual-input stage, trimmed offset, way the amplifier is tested, assuring that it can properly source and 5.5-MHz, rail-to-rail CMos sink current. It is a sensible and conservative way to test and specify the amplifier, but what if it is not the way your load is connected? OPA343 dual-input stage, untrimmed offset, Suppose your load is connected to ground as in Figure 6. The load 5.5-MHz, rail-to-rail CMOS resistor actually helps pull the output to ground, and the amplifier is OPA320 charge-pumped input stage, 20 MHZ not required to sink current rail-to-rail cMOs OPA322 charge-purnped input stage, untrimmed offset. 20 MHz. rai-to-rail CMOs Amplifier is always To see this original post with comments, click here sourcing current with 3. Swinging close to ground single-supply operation LOAD 3V resistive load, see text Rail-to-rail amplifiers can produce output voltages very close to ingle-supply ground-but how close? I am talking about complementary metal- operaton oxide semiconductor (CMOS)operational amplifiers (op amps that often are used in low-voltage designs when you are trying to Figure 6: Example of an amplifier load connected to ground The Signal 6 Texas instruments In this condition, most CMOS op amps can swing very close to an offset-voltage issue Of course, a small, negative input voltage ground-within a millivolt or two. The specifications may not highlight will bring the output very near o V, but your circuit may never have a this capability, but it is hinted at in Figure 7, showing output-voltag negative input voltage sWing as a function of output current. The graph could perhaps benefit from more resolution, but you can see that the output voltage exception. Load current and voltage are not in phase with a reactive converging on the specified voltage rails for this test is +2.75 V.For load, so the amplifier may have to sink current as the output voltage single-supply operation, the v- supply is equal too V approaches ground NoW I need to add a few provisions. Notice that in Figure 8, the feedback network is referenced to ground. You need to consider all (Referring to Cmos op amps, bipolar op amps cannot swing so close to ground. sources of load on the amplifier, not just RL. In this case, R1 + R2 are effectively additional ground-referenced loads in parallel to RL. Low-voltage battery-operated circuits are challenging, and it But if R1 is referenced to a positive voltage, the amplifier would have seems that we are always struggling to maximize voltage swings to sink current coming through the feedback network as the output With a good understanding of op amp capabilities, you may be neared O V. The output would not be able to swing quite so close able to squeak out additional output swing close to ground. If you to ground have questions about a specific amplifier or circuit configuration submit your question to the Precision Amplifiers forum on TIs In this same circuit, if the gain is high, the input offset voltage may E2E Communit affect your apparent output swing. For example, in G= 20, if the input offset voltage of the op amp is +1 mV, zero input will produce To see this original post with comments, click here a 20-mV output. That is not due to an output-swing limitation -it is OUTPUT VOLTAGE SWING VS OUTPUT CURRENT (Over Temperature) 2.75V +25°C 125°C Output swings to -40°c -1 supply voltages with zero load current 2.75V vs=±275V 20253035 Output Current(mA) Figure 7: Output-voltage swing shown as a function of output current. R110k Feedback network is additional load to ground Feedback is ground referenced. Reference to a positive voltage will require amp to sink V15 LOAD current Vin Figure 8: Single-supply op amp configuration with the feedback network referenced to ground The signal 7 Texas instruments Chapter I: Offset Voltage 4. Offset voltage and open-loop gain: Let us run some numbers. "If the dc open-loop gain is 100 dB, they are cousins that amounts to 1/10(100 dB/20)= 10 uV/. So for every volt of output swing from midsupply, the input voltage must change by 10 Everyone knows what offset voltage is, right? In the simplest G=1 HV. Think of it as an offset voltage that changes with the DC output circuit of Figure 9a, the output voltage is the offset voltage of the voltage With 9 V of output swing, that is a 90-WV change Maybe operational amplifier(op amp). The offset voltage is modeled as a that is insignificant in your circuits, maybe not direct current (DC) voltage in series with one input terminal. In unity gain, the offset is passed directly to the output with G= 1. In the The point is that thinking of finite open-loop gain as a changing offset high-gain circuit (Figure 9b), the output voltage is 1000 Vos. Right? voltage with a change in output voltage provides an intuitive way to size up the error. And the character of that error may matter, too. To test Well, nearly So, but not quite Understanding the"not quite"can offset voltage and open-loop gain, use a fancy two-amp loop circuit help you understand errors in your op amp circuits With it, you can control the output voltage and measure the offset In the first case, the output voltage was very near midsupply voltage. If you sweep the output voltage through its full output range (assuming dual supplies). This is the output voltage at which TI the change in offset voltage often looks something like Figure 10 defines and tests offset voltage. But in the second case, the output Note that the greatest change in offset voltage tends to occur at the may be several volts, assuming several millivolts of offset That output extremes, near the positive and negative rail. The op amp is requires a small additional dfferential voltage at the input ot the op "strain ing"to produce its maximurn output. The incremental open- amp to create the output swing (according to the open-loop gain of loop gain is higher in the middle and falls where the output nears that particular amplifier) the rails. As you plan your circuits, expect that this will be the case Offset voltage will increase more dramatically as you push the op amp to its swing limits 10Q 9.99kQ G=1 G=1000 >b(1000V (approximately Offset Voltage, v ( assumes± supplies) ( assumes± supplies Figure 9: Output offset voltage where G= 1 V/(a) andG= 1, 000 V/v(b). Ini speci Max spec OUT △V △V。s=90μV 50uV/div OUT △Vour=±4.5V=9V nvos Overdriven VouT=±48V 9V 100.000=100dB 90uV VouT (1V/div) Figure 10: Offset-voltage change shown as a function of output voltage The signal 8 Texas Instruments Not all op amp manufacturers specify Ao, the same way. TI tests Open-Loop its precision op amps for open-loop gain, which is averaged over Gain Conditions Min TypMaxUnit a generous output-swing range for good linear operation(the red 0peno)+05V<V<+)-05VR=10k00120 line in Figure 10). In the specifications table, it looks like Table 2 gmA()+b5V=≤0+5R=210、16 dB When the amplifier is overdriven(creating a larger offset voltage), the output will swing closer to the rails. Sometimes output swing differs Minimum Ao is assured with an Two load conditions shown from the conditions in Table 2. The output swing in Table 3, for output swing 0.5 V from rails Higher Ao, with 10 k load example, shows the output voltage with the input overdriven My op amp development group at Ti affectionately called this Table 2: Open-loop gain specifications shown with different loads and output voltage swings slam spec, " meaning that the input is overdriven and slammed as far as it can go to the rail Both types of specs are useful, depending on the requirements of Output Conditions Min Typ Max Unit your application. The key is to understand and carefully interpret Voltage output R=10k 0.2[015 V the specifications swing from rails R=2 k2 0.30.2 To see this original post with comments, click here Table 3: Example of an output voltage swing with the input overdriven. 5. SPICEing offset voltage: how to check the Before simulating, this is an opportunity to exercise best practices sensitivity of circuits to offset voltage with SPICE. What do you think the output current will be with 10 mV of input offset voltage It may not always be obvious how offset voltage will affect a circuit Direct current(DC) offsets are easy to simulate with a simulation Ofset voltage is modeled as a voltage source in series with one of program with integrated circuit emphasis (SPICE, but operational the input terminals. So in SPICE, you can merely insert a DC source amplifier (op amp macromodels only predict the effects of offset in series with one of the inputs to induce the effect of varying offset voltage of one unit. What about variation frorn device to device? voltage With V1 and v2 inputs connected to ground, ideally you would expect zero output current. But the offset voltage will supply The improved Howland current-source circuit(Figure 11)provides a small input: a DC simulation with Vx=0 and Vx=10 mV. Note the a good example. Its feedback to both input terminals may leave change in output current due to the change in Vx(Figure 12. There you wondering how the input offset voltage vos of the op amp may be other sources ot offset, so the delta in output current trom contributes to error. The oPA548 is a hefty power op amp with a these two Vx values reveals the contribution of offset voltage. Of 5-A maximum output and 60-v supply capability. It is frequently course, the offset could also be negative used in Howland circuits. But how will its 10-mV maximum offset voltage affect the output current of the circuit? The output offset with Vx=0 in the simulation comes from the offset voltage (2.56 mv included in the OPA548 macromodel-and would not be an additional contributor. most of tis macromodels have R110k an offset voltage approximately equal to the typical offset voltage value. In some circuits, other sources of output offsets could come 24V from input bias current and/or input offset current and would be additional contributors to total offset OPA548 What output offset current did you predict? The improved Howland R5 o=(V2-V1)/R5 +24V is essentially a difference amplifier (four resistors around an op amp) with an added resistor, R5. This unity-gain difference amplifier V2HA Load (equal resistors) causes the input diference voltage (V2-V1)to be R310k R499990 impressed on R5; the resuting current flows to the load. The offset voltage, however, is applied oirectly to the noninverting input and is Figure 11: An example circuit-an improved Howland current source amplified by +2-like a noninverting amplifier (G= 1 +2/R1). Thus, 10-mV offset voltage creates 20 mV across R5, producing a 20- mA output current offset. A-10-mV offset would create a-20-mA R110k R210k output current (current sinking from the load -24V Maybe you see it intuitively, maybe you don't. Either way, SPICE can 5.1mA Simulated offset 10mV251mA provide confirmation voltage Change of 20mA To see this original post with comments, click here oV and 10mV Vx 24V Offset due to offset in macro model 310kR49999Q Figure 12: Output offset current due to op amp ofifset-voltage in an improved Howland current source The signal Texas instruments 6. Where are the trim pins Some background Notice that the trim circuitry in this example is referenced to the on offset-voltage trim pins V+ supply. Some op amps have trim circuitry referenced to the V supply terminal Connecting the wiper of the potential to the wrong In 2012, my colleague Soufiane Bendaoud published an article rail or to ground on a dual supply will surely cause problems. Some Pushing the Precision Envelope. " In it, he discussed various designers attempt tricky active circuitry to drive these pins. While technologies that TI uses to"trim or adjust the offset voltage of its this is possible, ground-referenced circuitry connected to the trin amplifiers to very low values. It got me thinking about offset voltage pins can create power-supply rejection problems trim pins. Where do they go? It is best to use the trim pins only to null the offset of the first Newer operational amplifiers (op amps lack the offset voltage trim pins once found on virtually all op amps. There are many factors at and its offset dominates that of the comolete signal chain. If used work in this change. Better, lower-offset amplifiers, autocalibrated to correct other large sources of offset in the chain, you could system designs, pressure to reduce assembly and adjustment introduce an unwanted temperature drift costs, tiny surface-mount packages -all combine to reduce the use of offset trim pins. Still, many of our best-selling op amps have trim Lacking trim pins, there are other ways to trim offsets in your pins, and knowledge and best practices of how to use(or not use /stem. You could inject or su ariable voltages from a them are fading potentiometer or other control signal into various points in your signal chain Figure 14 shows examples. The trimming voltages This much is easy: if you do not use the trim pins, leave them open shown here should be derived from the power supplies. Regulated circuit, with no connection. Do not connect them to ground supplies are probably sufficient Unregulated supplies, such as Figure 13 shows a common type of internal trim circuitry. Trim batteries, may not be sufficiently constant or stable pins connect to a tapped portion of the input-stage load circuiti The improved offset voltage of modern amplifiers often climinates Adjusting the potentiometer skews the balance of the load plus or the need for trimming. Still, there are times when some type of minus a few millivolts of input offset voltage Datasheets generally offset adjustment is required. You can be ready with techniques recommend a value for the potential, but it is not critical. A muc whether with trim pins or add-on circuitry higher resistance potentiometer will cause the change in offset voltage to occur toward the extremes of rotation. Too low a value To see this original post with comments, click here will reduce the adjustment range. Potentials in the range of +100 percent to 50 percent of the recommended value will likely function satisfactoril Trim d potentiometer V+ 0.1F OPA277 20k9 Input stage load circuitry Offset trim 8 Offset Trim 7 8 Output OPA277 6 NC(1 Inputs 0.1uF 8-Pin DiP So-8 Figure 13: Typical internal circuitry where trim pins connect to the input-stage load circuitry Trimming without Trim Pins R1 1k R210k Vin h R31 R2 9k Ro 100K Vout R1 1k V- R3 1M Vout Rp 100k es V+ R41 In Figure 14: Examples of offset-correction voltages injected into various points of the signal chain The signal Texas instruments

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