TI(运算放大器设计指南-信号电源设计)

20 Analog Applications JournalAnalog and Mixed-Signal Products November 1999 Texas Instruments IncorporatedAmplifiers: Op Amps Single-supply op amp design Introduction Most portable systems have one battery, thus the popularity of portable equipment results in increased single-supply applications. Split- or dual-supply op amp circuit design is straightforward because op amp inputs and outputs are referenced to the normally grounded center tap of the supplies. In the majority of split-supply applications, sig- nal sources driving the op amp inputs are referenced to ground; thus with one input of the op amp referenced to ground, as shown in Figure 1, common-mode voltage and voltage bias problems are negligible. When signal sources are referenced to ground, single- supply op amp circuits exhibit a large input common-mode voltage (Figure 2). The input voltage is not referenced to the midpoint of the supplies like it would be in a split- supply application; rather, it is referenced to the lower- power supply rail. This circuit malfunctions when the input voltage is positive because the output voltage should go negative; this is hard to do with a positive supply. It operates marginally with small negative input voltages because most op amps cannot function when the inputs are connected to the supply rails. The constant requirement to account for input refer- ences makes it difficult to design single-supply op amp By Ron Mancini Senior Application Specialist, Operational Amplifiers PARAMETER NAME PARAMETER SYMBOL VALUE Input current IIN 0 Input offset-voltage VOS 0 Input impedance ZIN ¥ Output impedance ZOUT 0 Gain a ¥ Table 1. Ideal op amp parameter values + V - V RG RF VOUT + - VIN Figure 1. Split-supply op amp circuit + VRG RF VOUT VIN - + Figure 2. Single-supply op amp circuit Boundary conditions Use of a single-supply limits the output voltage range to the positive supply voltage. This limitation precludes negative output voltages when the circuit has a positive supply volt- age, but it does not preclude negative input voltages. As long as the voltage on the op amp input leads does not become negative, the circuit can handle negative input voltages. Beware of working with negative input voltages when the op amp is powered from a positive supply because op amp inputs are highly susceptible to reverse voltage break- down. Also, insure that all possible startup conditions do not reverse bias the op amp inputs when the input and supply voltage are of opposite polarity. Simultaneous equations Taking an orderly path to developing a circuit that works the first time means following these steps until the equa- tion of the op amp is determined. Use specifications and simultaneous equations to determine what form the op amp equation must have. Go to the section that illustrates that equation form (called a case), solve the equations to deter- mine the resistor values, and you have a working solution. A linear op amp transfer function is limited to the equa- tion of a straight line. (1) The equation of a straight line has four possible solu- tions depending upon the sign of m (the slope) and b (the intercept), thus simultaneous equations yield solu- tions in four forms. Four circuits are developed, one for each form of the equation of a straight line. The four equations, cases, or forms of a straight line are given in Equations 2 through 5 where electronic terminology has been substituted for math terminology. bmxy ––= circuits. This application note develops an orderly procedure for designing single-supply op amp circuits that leads to a working design every time. Application Note SLAA068, entitled, “Understanding Basic Analog—Ideal Op Amps,” develops the ideal op amp equations. The ideal op amp assumptions used to write ideal op amp equations are shown in Table 1 for your reference. www.ti.com/sc/docs/products/analog/tlc070.html Texas Instruments Incorporated Amplifiers: Op Amps 21 Analog Applications Journal November 1999 Analog and Mixed-Signal Products (2) (3) (4) (5) Given a set of two data points for VOUT and VIN, simulta- neous equations are solved to determine m and b for the equation that satisfies the given data. The sign of m and b determines the type of circuit required to implement the solution. The given data is derived from the specifications; i.e., a sensor output signal ranging from 0.1 volts to 0.2 volts must be interfaced into an analog-to-digital converter that has an input voltage range of 1 volt to 4 volts. These data points (VOUT = 1.0 V @ VIN = 0.1 V, VOUT = 4.0 V @ VIN = 0.2 V) are inserted into Equation 2, as shown in Equations 6 and 7, to obtain m and b for the specifications. (6) (7) Solving Equations 6 and 7 yields b = –2 and m = 30. Now m and b are substituted back into Equation 2, yield- ing Equation 8. (8) Notice that, although Equation 2 was the starting point, the form of Equation 8 is identical to Equation 3. The specifications or given data determine the sign of m and b, and starting with Equation 2, the final equation form is discovered after m and b are calculated. The next step is to develop a circuit that has an m = 30 and b = –2 to com- plete the problem solution. Circuits were developed for Equations 2 through 5, and they are given under the headings Case 1 through Case 4, respectively. Case 1 — VOUT = mVIN + b The circuit configuration that yields a solution for Case 1 is shown in Figure 3. 2V30V INOUT -= bm += )2.0(4 bm += )1.0(1 bm --= INOUT VV bm +-= INOUT VV bm -= INOUT VV bm += INOUT VV The circuit equation is written using the voltage divider rule and superposition. (9) (10) (11) Case 2 — VOUT = mVIN – b The circuit shown in Figure 4 yields a solution for Case 2. The circuit equation is obtained by taking the Thevenin equivalent circuit looking into the junction of R1 and R2. After the R1, R2 circuit is replaced with the Thevenin equivalent circuit, the gain is calculated with the ideal gain equation. (12) (13) (14) ÷ ł ö ç Ł æ ++ ÷ ł ö ç Ł æ + = 21G F 21 2 REF RRR R RR R Vb 21G 21GF RRR RRRR + ++ =m ÷ ÷ ł ö ç ç Ł æ + ÷ ł ö ç Ł æ + - ÷ ÷ ł ö ç ç Ł æ + ++ = 21G F 21 2 REF 21G 21GF INOUT RRR R RR R V RRR RRRR VV ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + = G GF 21 1 REF R RR RR R Vb ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + = G GF 21 2 R RR RR R m ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + + ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + = G GF 21 1 REF G GF 21 2 INOUT R RR RR R V R RR RR R VV + V RG RF VOUT VIN R1 R2VREF TLV2471 + - Figure 3. Schematic for Case 1 — VOUT = mVIN + b + V RG RF VOUT VINVREF R2 R1 TLV2471 + - Figure 4. Schematic for Case 2 — VOUT = mVIN – b Continued on next page www.ti.com/sc/docs/products/analog/tlc080.html www.ti.com/sc/docs/products/analog/tlv2470.html Texas Instruments IncorporatedAmplifiers: Op Amps 22 Analog Applications JournalAnalog and Mixed-Signal Products November 1999 Case 3 — VOUT = –mVIN + b The circuit shown in Figure 5 yields the transfer function desired for Case 3. The circuit equation is obtained with superposition. (15) (16) (17) Case 4 — VOUT = –mVIN – b The circuit shown in Figure 6 yields a solution for Case 4. The circuit equation is obtained by using superposition to calculate the response to each input. The individual responses to VIN and VREF are added to obtain Equation 18. (18) (19) (20) Conclusion Single-supply op amp design is more complicated than split-supply op amp design, but with a logical design approach excellent results are achieved. Single-supply design was considered technically limiting because the older op amps had limited capability. Op amps such as the TI TLV247x, TLC07x, and TLC08x have excellent ÷ ł ö ç Ł æ = G2 F REF R R Vb G1 F R R =m ÷ ł ö ç Ł æ - ÷ ł ö ç Ł æ -= G2 F REF G1 F INOUT R R V R R VV ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + = G GF 1 1 REF R RR RR R Vb G F R R =m ÷ ł ö ç Ł æ + ÷ ł ö ç Ł æ + + ÷ ł ö ç Ł æ -= G GF 21 1 REF G F INOUT R RR RR R V R R VV VREF + V RG RF VOUT VIN R1 R2 TLV2471 + - Figure 5. Schematic for Case 3 — VOUT = –mVIN + b + V RG1 RF VOUT VIN VREF RG2 TLV2471 + - Figure 6. Schematic for Case 4 — VOUT = –mVIN – b Continued from previous page single-supply parameters; thus, when used in the correct applications, these op amps yield rail-to-rail performance far surpassing their split-supply counterparts. More in- depth information concerning single-supply op amp design can be found in Texas Instruments Application Note SLOA030, entitled, “Single-Supply Op Amp Design Techniques.” www.ti.com/sc/docs/products/analog/tlc070.html www.ti.com/sc/docs/products/analog/tlc080.html www.ti.com/sc/docs/products/analog/tlv2470.html IMPORTANT NOTICE Texas Instruments Incorporated and its subsidiaries (TI) reserve the right to make corrections, modifications, enhancements, improvements, and other changes to its products and services at any time and to discontinue any product or service without notice. Customers should obtain the latest relevant information before placing orders and should verify that such information is current and complete. All products are sold subject to TI's terms and conditions of sale supplied at the time of order acknowledgment. TI warrants performance of its hardware products to the specifications applicable at the time of sale in accordance with TI's standard warranty. Testing and other quality control techniques are used to the extent TI deems necessary to support this warranty. Except where mandated by government requirements, testing of all parameters of each product is not necessarily performed. TI assumes no liability for applications assistance or customer product design. Customers are responsible for their products and applications using TI components. To minimize the risks associated with customer products and applications, customers should provide adequate design and operating safeguards. TI does not warrant or represent that any license, either express or implied, is granted under any TI patent right, copyright, mask work right, or other TI intellectual property right relating to any combination, machine, or process in which TI products or services are used. Information published by TI regarding third-party products or services does not constitute a license from TI to use such products or services or a warranty or endorsement thereof. Use of such information may require a license from a third party under the patents or other intellectual property of the third party, or a license from TI under the patents or other intellectual property of TI. Reproduction of information in TI data books or data sheets is permissible only if reproduction is without alteration and is accompanied by all associated warranties, conditions, limitations, and notices. Reproduction of this information with alteration is an unfair and deceptive business practice. TI is not responsible or liable for such altered documentation. Resale of TI products or services with statements different from or beyond the parameters stated by TI for that product or service voids all express and any implied warranties for the associated TI product or service and is an unfair and deceptive business practice. TI is not responsible or liable for any such statements. Following are URLs where you can obtain information on other Texas Instruments products and application solutions: TI Worldwide Technical Support Internet TI Semiconductor Product Information Center Home Page support.ti.com TI Semiconductor KnowledgeBase Home Page support.ti.com/sc/knowledgebase Product Information Centers Americas Phone +1(972) 644-5580 Fax +1(972) 927-6377 Internet/Email support.ti.com/sc/pic/americas.htm Europe, Middle East, and Africa Phone Belgium (English) +32 (0) 27 45 54 32 Netherlands (English) +31 (0) 546 87 95 45 Finland (English) +358 (0) 9 25173948 Russia +7 (0) 95 7850415 France +33 (0) 1 30 70 11 64 Spain +34 902 35 40 28 Germany +49 (0) 8161 80 33 11 Sweden (English) +46 (0) 8587 555 22 Israel (English) 1800 949 0107 United Kingdom +44 (0) 1604 66 33 99 Italy 800 79 11 37 Fax +(49) (0) 8161 80 2045 Internet support.ti.com/sc/pic/euro.htm Japan Fax International +81-3-3344-5317 Domestic 0120-81-0036 Internet/Email International support.ti.com/sc/pic/japan.htm Domestic www.tij.co.jp/pic Asia Phone International +886-2-23786800 Domestic Toll-Free Number Toll-Free Number Australia 1-800-999-084 New Zealand 0800-446-934 China 800-820-8682 Philippines 1-800-765-7404 Hong Kong 800-96-5941 Singapore 800-886-1028 Indonesia 001-803-8861-1006 Taiwan 0800-006800 Korea 080-551-2804 Thailand 001-800-886-0010 Malaysia 1-800-80-3973 Fax 886-2-2378-6808 Email tiasia@ti.com Internet support.ti.com/sc/pic/asia.htm ti-china@ti.com C011905 Safe Harbor Statement: This publication may contain forward- looking statements that involve a number of risks and uncertainties. These “forward-looking statements” are intended to qualify for the safe harbor from liability established by the Private Securities Litigation Reform Act of 1995. These forward- looking statements generally can be identified by phrases such as TI or its management “believes,” “expects,” “anticipates,” “foresees,” “forecasts,” “estimates” or other words or phrases of similar import. Similarly, such statements herein that describe the company's products, business strategy, outlook, objectives, plans, intentions or goals also are forward-looking statements. All such forward-looking statements are subject to certain risks and uncertainties that could cause actual results to differ materially from those in forward-looking statements. Please refer to TI's most recent Form 10-K for more information on the risks and uncertainties that could materially affect future results of operations. We disclaim any intention or obligation to update any forward-looking statements as a result of developments occurring after the date of this publication. Trademarks: All trademarks are the property of their respective owners. Mailing Address: Texas Instruments Post Office Box 655303 Dallas, Texas 75265 © 2005 Texas Instruments Incorporated Products Amplifiers amplifier.ti.com Data Converters dataconverter.ti.com DSP dsp.ti.com Interface interface.ti.com Logic logic.ti.com Power Mgmt power.ti.com Microcontrollers microcontroller.ti.com Applications Audio www.ti.com/audio Automotive www.ti.com/automotive Broadband www.ti.com/broadband Digital control www.ti.com/digitalcontrol Military www.ti.com/military Optical Networking www.ti.com/opticalnetwork Security www.ti.com/security Telephony www.ti.com/telephony Video & Imaging www.ti.com/video Wireless www.ti.com/wireless SLYT189