2 op-amp concepts

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2 op-amp concepts

  1. 1. Operational Amplifiers: Basic Concepts Prof. Greg Kovacs Department of Electrical Engineering Stanford University
  2. 2. Design Note: The Design Process• Definition of function - what you want.• Block diagram - translate into circuit functions.• First Design Review.• Circuit design - the details of how functions are accomplished. – Component selection – Schematic – Simulation – Prototyping of critical sections• Second Design Review.• Fabrication and Testing.EE122, Stanford University, Prof. Greg Kovacs 2
  3. 3. EE122 Parts KitEE122, Stanford University, Prof. Greg Kovacs 3
  4. 4. Parts Kit Guide LT1175 LT1011 Adjustable Comparator Negative Voltage AD654 V-F Regulator LT1167 Converter Instrum. Sockets Amp 7805 +5V RegulatorYellow LEDs MJE3055 NPN Power TransistorBright Red LEDs 555 Timer IRLZ34 Logic-Level PowerLM334 Temp. Sensor MOSFET (N-Chan) CdS Cell LT1036 Voltage Regulator +12/+5 SFH300-3B LT1033 Voltage Phototransistor Regulator 3A Negative, LT1056 Op- LTC1064-2 Adjustable Amps Switched Capacitor Filter EE122, Stanford University, Prof. Greg Kovacs 4
  5. 5. Each set of five pins are shorted together internally so you can make multiple connections to one i.c. pin or component lead... Each of the long rows of pins is shorted together so you can use them as power supply and ground lines... Protoboards...EE122, Stanford University, Prof. Greg Kovacs 5
  6. 6. USE decoupling capacitors (typically0.1 µF) from each power supply rail toground. This is essential to preventunwanted oscillations.The capacitors locally source and sinkcurrents from the supply rails of thechips, preventing them from “talking”to each other and their own inputs! EE122, Stanford University, Prof. Greg Kovacs 6
  7. 7. Point-To-Point Soldering On A Ground-Planed BoardEE122, Stanford University, Prof. Greg Kovacs 7
  8. 8. What Is an Op-Amp Anyway? OBJECTIVES (Why am I sitting in this classroom?) • To obtain a practical understanding of what operational amplifiers (“op-amps”) are and some applications they can be used for. • To understand the basic op-amp circuit configurations. • To understand the basic characteristics (good and bad) of op-amps before measuring some of them in the lab. • To keep your parents happy! "We dont need need no education.... We dont no education...."EE122, Stanford University, Prof. Greg Kovacs 8
  9. 9. The Ideal Op-Amp 1) The input impedance is infinite - i.e. no current ever V1 flows into either input of the op-amp. A(V2-V1) + 2) The output impedance is V2 - zero - i.e. the op-amp can drive any load impedance to any voltage. 3) The open-loop gain (A) is infinte.The Op-Amp produces an outputvoltage that is the difference between 4) The bandwidth is infinite.the two input terminals, multiplied bythe gain A... 5) The output voltage is zero when the input voltage difference is zero. EE122, Stanford University, Prof. Greg Kovacs 9
  10. 10. Types of Op-Amps • Single • Low power • Dual • Low noise • Quad • Low offset • High power • High voltage NICE • High speed HAT!Traditional costumes ofanalog circuit designers. EE122, Stanford University, Prof. Greg Kovacs 10
  11. 11. A Bit of History...• The first Op-Amps were invented during the time of the Second World War...• Dr. C. A. Lovell of Bell Telephone Laboratories introduced the Op-Amp...• George A. Philbrick independently introduced a single vacuum tube operational amplifier in 1948.• SO, Op-Amps are NOT new!• The ever-popular 741 monolithic Op-Amp was designed by Dave Fullagar in 1967....EE122, Stanford University, Prof. Greg Kovacs 11
  12. 12. The First "Real" OpAmp -> The K2-W +300 VDC 510K NE-68 220K 680K 1M 7.5pF 12AX7- Vin 12AX7 NE-68 Vout+ Vin 221K 9.1K 220K 2.2M 120K 4.7M 500pF -300 VDC EE122, Stanford University, Prof. Greg Kovacs 12
  13. 13. K2-XAEE122, Stanford University, Prof. Greg Kovacs 13
  14. 14. The K2-W Tube OpAmp• Invented by Julie Loebe and George Philbrick (early 1950s)• The first "mass production" OpAmp...• Cost (in 1950s) approximately $22.00...• Basic specifications comparison to 741 and LT1037... Parameters K2-W OpAmp 741 OpAmp LT1037 OpAmp Power Supplies +/- 300 VDC, +/- 15V +/- 15V 6.3 VAC (filaments) Open-Loop 1.5X104 5X10 4 30X106 Gain Vout Swing +/- 50V +/- 12V +/- 13.5 V Iout +/- 1 mA 25 mA 25 mA Idrain 5 mA (no load) 1.7 mA 2.7 mA RL(min) 50 KΩ none (SC none (SC protect) protect) Slew Rate +/- 12 V/µSec +/- 0.5 V/µS 15 V/µS EE122, Stanford University, Prof. Greg Kovacs 14
  15. 15. Good Op-Amp Web Sites • www.linear-tech.com • www.national.com • www.burr-brown.com • www.maxim-ic.com • www.intersil.comEE122, Stanford University, Prof. Greg Kovacs 15
  16. 16. Cool New Project/Design Website: www.designnotes.com Submit your favorite circuit or program. Every month the best design entry (judged by your peers), wins $100 in cash! Monthly winners are eligible for the $1200 Grand Prize!EE122, Stanford University, Prof. Greg Kovacs 16
  17. 17. Courtesy Mr. Charlie Kiers.EE122, Stanford University, Prof. Greg Kovacs 17
  18. 18. Op-Amp Datasheets for EE122: Some Example Devices• LM741 (basic)• LT1056 (JFET input)• LMC660 (CMOS - low power)• LT1220/1221 (fast)• LM675 (medium power)• LM12 (high power)EE122, Stanford University, Prof. Greg Kovacs 18
  19. 19. The Ideal Op-Amp In SPICE Use a voltage-controlled voltage source: EXXXXXXX N+ N- NC+ NC- GAIN Eout 3 0 1 2 100K 2 V1 A(V2-V1) 3 + 1 V2 - 0EE122, Stanford University, Prof. Greg Kovacs 19
  20. 20. Feedback: What is it and Where Can I Get Some? NEAT THINGS YOU CAN DO TO AN AMPLIFIER BY USING FEEDBACK (OF THE NEGATIVE KIND...) 2 • The gain of the circuit is made less sensitive to the values of individual components. • Nonlinear distortion can be reduced. • The effects of noise can be reduced. • The input and output impedances of the amplifier can be modified. • The bandwidth of the amplifier can be extended.EE122, Stanford University, Prof. Greg Kovacs 20
  21. 21. WHAT CAN YOU DO WITH OP-AMPS? • Feed the hungry. • Save the dolphins. • Amplify signals. • Differentiate signals. • Heal the sick. • Pay off the deficit. • Buffer signals. • Sum multiple signals. • End global warming. • Make music very loud! • Integrate signals. EE122, Stanford University, Prof. Greg Kovacs 21
  22. 22. Jim WilliamsEE122, Stanford University, Prof. Greg Kovacs 22
  23. 23. Analog Hacker’s BibleEE122, Stanford University, Prof. Greg Kovacs 23
  24. 24. THE VOLTAGE FOLLOWER V- = VOUT V- VOUT VIN V+ VOUT = AV+ - V-What is it good for? A V+ VOUT = 1+ABuffering a high-impedance signal to "beat"Heisenberg... You dont load it down when you 1measure it...It has the best bandwidth of any op-amp circuit.Some op-amps need to be COMPENSATED forstable unity-gain operation (more later....).EE122, Stanford University, Prof. Greg Kovacs 24
  25. 25. THE INVERTING AMPLIFIER R2 i fb R1 R2 AV = −VIN V- VOUT i in V+ R1The V- terminal is referred to as a "virtual ground"... Why is that? VOUT = A( 0 − V− ) thus, Due to NEGATIVE FEEDBACK!!!! VOUT VOUT V− = ≈ =0 A ∞ THIS IS A KEY POINT!!!EE122, Stanford University, Prof. Greg Kovacs 25
  26. 26. Op-Amp Application: CLIPPER (or "Fuzz Box")Mellow Tunes Heavy Metal IN R2 OUT R1 Input signal must be large enough to turn on diodes... EE122, Stanford University, Prof. Greg Kovacs 26
  27. 27. THE NON-INVERTING AMPLIFIER R2 i2 R1 V- V+ VOUT i1 VIN A key point to note here is that the V- node is not a virtual ground in this configuration! The important thing to consider is that the voltage difference between V+ and V- is kept near zero. In other words, V- VIN.  R2  AV = 1+  R1 EE122, Stanford University, Prof. Greg Kovacs 27
  28. 28. THE SUMMING AMPLIFIER R1V1 Rf i 1 R2V2 if R3 V-V3 R4 V+ VOUTV4 Rf Rf Rf VOUT = −V1 − V2 − L − VNVn Rn R1 R2 RN or,  Vi  VOUT = −Rf ∑ i  Ri What is it good for?Summing multiple input signals in anyproportion desired (determined by the relativevalues of the input resistors.Averaging signals (do you know how?). EE122, Stanford University, Prof. Greg Kovacs 28
  29. 29. Noise Canceling CircuitEE122, Stanford University, Prof. Greg Kovacs 29
  30. 30. Instrumentation Amplifier • Very high input impedance. • Gain can be set with only one resistor. • Can optimize CMRR. R4  2R2  A V = − 1+  R3  R1 Source: Sedra, A. S., andSmith, K. C., “MicroelectronicCircuits,” Oxford, 1998. For one-resistor gain adjust, set R4 = R3 and fix R2. EE122, Stanford University, Prof. Greg Kovacs 30
  31. 31. Op-Amp Application: EKG Filter (0.04 - 150 Hz) Instrumentation AmplifierSource: Webster, J. G.,“Medical Instrumentation:Application and Design,”Houghton Mifflin, 1978. EE122, Stanford University, Prof. Greg Kovacs 31
  32. 32. A Safe Heart Signal InterfacePolar™ heart-rate transmitter - provides magnetically coupled bursts of 5 kHz energy thatmark the start of each heartbeat (i.e., you don’t get the actual waveform).www.polarusa.comEE122, Stanford University, Prof. Greg Kovacs 32
  33. 33. Polar™ OEM ReceiverEE122, Stanford University, Prof. Greg Kovacs 33
  34. 34. THE INTEGRATORNeed R2 to make the integrator "leaky"... R2Otherwise small DC offsets wouldcharge it up (and up, and up!!!!). C1 For DC inputs: R1 VIN V- VOUT VOUT V+ = - R2 i in VIN R1 For AC inputs:What is it good for? iin = ifbTriangle wave generation.Ramp generation (scopes!). vin = - C1 v out dMath (yuk) as it used to be done! R1 dt What kind of filter is this? vout = - 1 vindt R1 C1 EE122, Stanford University, Prof. Greg Kovacs 34
  35. 35. OP-AMP INTEGRATOR SIMULATION R2Op-Amp Integrator Simulation*YOU fill in the component values! C1R1 1 2 ?CI2 2 3 ? 1 R1 2R2 2 3 ? VIN V-E1 3 0 0 2 100K 3 VOUT i in V+Vin 1 0 pulse(-1 1 0 5nS 5nS 500uS 1mS).TRAN 100uS 10mS 0.probe.end V(3) V(1) 10V V 0.0V -10V 0.0S 2mS 4mS 6mS 8mS 10mS tran3 TIME mS EE122, Stanford University, Prof. Greg Kovacs 35
  36. 36. Insect Larva Containing CandyEE122, Stanford University, Prof. Greg Kovacs 36
  37. 37. THE DIFFERENTIATOR R2R1 is needed to limit the high-frequency C1 ifgain (noise may be small, but it can have R1 V- VINa very large derivative!). V+ VOUT i in vout = - R2 C1 dvin Design the circuit to be used dt below this frequency fmax = 1 2π R1 C1 What kind of filter is this? EE122, Stanford University, Prof. Greg Kovacs 37
  38. 38. INTEGRATOR/DIFFERENTIATOR SIMULATION V(3) V(1) V(6) 2V V 0.0V -2V 10mS 11mS 12mS tran2 TIME mSWhy dont you get out what you put in?EE122, Stanford University, Prof. Greg Kovacs 38
  39. 39. "REAL" OP-AMPS DO EAT QUICHE What You WANT What You GET1) The input impedance is infinite 1) The input impedance is infinite NO, but it is often GIGA NO, but it is often GIGA --i.e. no current ever flows into i.e. no current ever flows into or TERA !! or TERA either input of the op-amp. either input of the op-amp.2) The output impedance is zero -- 2) The output impedance is zero NO, but is can be a few NO, but is can be a few i.e. the op-amp can drive any i.e. the op-amp can drive any ohms in many cases! ohms in many cases! load impedance to any voltage. load impedance to any voltage.3) The open-loop gain (A) is 3) The open-loop gain (A) is NO, but it is usually NO, but it is usually infinte. infinte. several million! several million!4) The bandwidth is infinite. 4) The bandwidth is infinite. NO, usually several MHz. NO, usually several MHz.5) The output voltage is zero 5) The output voltage is zero NO, offset voltages exist, NO, offset voltages exist, when the input voltage when the input voltage but can be trimmed. but can be trimmed. difference is zero. difference is zero. EE122, Stanford University, Prof. Greg Kovacs 39
  40. 40. SLEW RATEOp-Amps can only swing their outputs Simulated slew-rate-limited so fast... unity-gain amplifier with aIf you try an make them go faster, they UA741 op-amp try, but you get a limiting rate of change, the SLEW RATE! 3.9 µS to swing 2 V 2.0V gives a slew rate of 0.5 V/µS 0.0V -2.0V 0us 5us 10us 15us 20usMeasuring the V(1) V(2)SLEW RATE of Timea lobster using Exit Add_trace Remove_trace X_axis Y_axis Plot_controla piece of Display_control Macros Hard_copy Cursor Zoom LabelBungie-cord... EE122, Stanford University, Prof. Greg Kovacs 40
  41. 41. Slew Rate Example - Rising LM741 (slow) and LT1056, ±15V Supplies, 2k Load, 1 VPP Squarewave Input (locally terminated into 50 ).EE122, Stanford University, Prof. Greg Kovacs 41
  42. 42. Slew Rate Example - Falling LM741 (slow) and LT1056, ±15V Supplies, 2k Load, 1 VPP Squarewave Input (locally terminated into 50 ).EE122, Stanford University, Prof. Greg Kovacs 42
  43. 43. “Ultra Stealth Airplane”EE122, Stanford University, Prof. Greg Kovacs 43
  44. 44. Gain-Bandwidth Product is Constant This animation shows the how the bandwidth of an op-amp in the inverting configuration increases as the gain is decreased.EE122, Stanford University, Prof. Greg Kovacs 44
  45. 45. OPEN-LOOP CHARACTERISTICS OF "REAL" OP-AMPS 3 dB frequency fu is VERY LOW!1.0M This frequency is determined by the "Dominant Pole" of the100K op-amp. If negative feedback is applied, fu may be shifted to much 10K 1.0h 3.0h 10h 30h 100h higher frequencies V(5)/ V(1) Frequency fu Unity-gain frequency fT can be100 VERY HIGH (many MHz)! For unity-gain connected op-1.0 amps, fu is the same as fT. For any other gain, fT can be determined by using the GAIN-10m BANDWIDTH PRODUCT 100Kh 300Kh 1.0Mh 3.0Mh 10Mh V(5)/ V(1) Frequency fT fT fU = Closed − Loop _Gain EE122, Stanford University, Prof. Greg Kovacs 45
  46. 46. STABILITY AND COMPENSATIONWith negative feedback, if the input of the amplifier receives a -180° out-of- phase replica of the output signal (via the feedback circuit) you end up with OSCILLATIONS!!!!All op-amps have a high-frequency roll-off determined by several poles. This means that eventually, you will hit -180° phase! The key to STABILITY is to ensure that this This effect is happens when the gain has fallen off worse at lower to less than 1! gains because MORE SIGNAL ISThis can be accomplished by FED BACK! DELIBERATELY rolling off the amplifier using a COMPENSATION CAPACITOR! EE122, Stanford University, Prof. Greg Kovacs 46
  47. 47. MODELING "REAL" OP-AMPS The WORSE the op-amp, the more work it takes to model it! IDEAL OP-AMP: Eout 3 0 1 2 100K CRAPPY, OBSOLETE OP-AMP:* UA741 operational amplifier "macromodel" subcircuit gcm 0 6 10 99 2.574E-9* connections: non-inverting input iee 10 4 dc 10.16E-6* | inverting input hlim 90 0 vlim 1K* | | positive power supply q1 11 2 13 qx* | | | negative power supply q2 12 1 14 qx* | | | | output r2 6 9 100.0E3* | | | | | rc1 3 11 7.957E3.subckt UA741 1 2 3 4 5 rc2 3 12 7.957E3* re1 13 10 2.740E3c1 11 12 4.664E-12 re2 14 10 2.740E3c2 6 7 20.00E-12 ree 10 99 19.69E6dc 5 53 dx ro1 8 5 150de 54 5 dx ro2 7 99 150dlp 90 91 dx rp 3 4 18.11E3dln 92 90 dx vb 9 0 dc 0dp 4 3 dx vc 3 53 dc 2.600egnd1 98 0 3 0 0.500000 ve 54 4 dc 2.600egnd2 99 98 4 0 0.500000 vlim 7 8 dc 0fb1 7 99 vb 10610000.000000 vlp 91 0 dc 25fb2 7 99 vc -10000000.000000 vln 0 92 dc 25fb3 7 99 ve 10000000.000000 .model dx D(Is=800.0E-18)fb4 7 99 vlp 10000000.000000 .model qx NPN(Is=800.0E-18 Bf=62.50)fb5 7 99 vln -10000000.000000 .endsga 6 0 11 12 137.7E-6EE122, Stanford University, Prof. Greg Kovacs 47
  48. 48. EXAMPLE OF USING A "REAL" OP-AMP MACROMODELX1 1 2 3 4 2 UA741 NOTE:Vplus 3 0 15V 1) You declare an "instance"Vminus 0 4 15V of your macromodel with aVin 1 0 AC 1 0 name that begins with "X".AC DEC 100 1hz 10MEG.probe 2) You have to explicitly.end define the power supplies. 1.0M 100 10m 1.0h 100h 10Kh 1.0Mh 10Mh V(5)/V(1) FrequencyEE122, Stanford University, Prof. Greg Kovacs 48
  49. 49. SPAM ZAPPED WITH PHOTONS!EE122, Stanford University, Prof. Greg Kovacs 49
  50. 50. CONCLUSION • Op-Amps are useful for lots of things. • Op-Amps deliver a lot of performance for peanuts! • Op-Amp circuits are generally fairly intuitive if you remember the basic "rules" of op-amp operation!EE122, Stanford University, Prof. Greg Kovacs 50

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