Single Supply Op Amp Circuits Dr. Lynn Fuller Webpage:

Transcription

1 ROCHESTER INSTITUTE OF TECHNOLOGY MICROELECTRONIC ENGINEERING Single Supply Op Amp Circuits Dr. Lynn Fuller Webpage: 82 Lomb Memorial Drive Rochester, NY Tel (585) Department webpage: OpAmpSingleSupply.ppt Page 1

2 OUTLINE Introduction Basic Dual Supply Op Amp Circuits Power Supplies NJU 703X Op Amp LTC 6078 Op Amp Single Supply Op Amp Circuits Virtual Ground Inverting Amplifier Non Inverting Amplifier Comparator Multivibrator Current to Voltage Converter Differential Amplifier Page 2

3 INTRODUCTION This document discusses singlesupply, lowvoltage, railtorail, Operational Amplifier (Op Amp) circuits. Although all op amps can operate with single supply or dual supply, most engineers are familiar with dualsupply Op Amp circuits such as those shown on the following page. The dual supply allows the input and output to be easily referenced to zero volts. (analog ground = earth ground) Single supply Op Amps usually refers to low voltage Op Amps using voltages of 5, 3.3 or smaller and ground. Some types of Op Amps will not work at these voltages. (some Op Amps use BJT current source biasing that takes a couple of diode drops of voltage to work thus the output voltage of these Op Amps can only get within 1.4 volts of the supply rails. For example at 5 volts, output is limited between 1.4 volts and 3.6 volts and with 3.3 volts supply some Op Amps may not work at all. With single supply Op Amp circuits we also can not have negative output voltages. There are several techniques for working with these limitations. Page 3

4 VOLTAGE SUPPLIES Single Supply Dual DC Power Supply Multiple Output Supplies Page 4

5 CREATING A SPLIT SUPPLY FROM A SINGLE SUPPLY The simple voltage splitter draws a lot of power if R s are low. C s ensure AC short (for AC signals). Example: 20V single supply can be split giving / 10V. If R s are 10 ohms then I in each is 10/10=1 Amp and if the Op Amp draws only a few ma the voltages will be ~/ 10V. Resistor Power = IV=10 watts. (be sure to get resistors for 10 watts) If C=500uf the corner will be ~1/RC = 200r/s = 31.8hz good for AC signals above ~300hz Simple Voltage Splitter /2 V/2 R R C C Single Supply Common Ground Earth Ground Virtual Ground Signal Ground Analog Ground Chassis Ground Digital Ground Floating Ground Page 5

6 VIRTUAL GROUND / VOLTAGE SPLITTER 100K 100K Virtual Ground Using Op Amp Vout = V/2 Virtual ground is simply a voltage reference typically half of the supply voltage. This virtual ground can supply/sink only as much current as the maximum Op Amp output current. The output current can be increased as shown with the BJT s Page 6

7 TLE2426 RAIL SPLITTER (COMMERCIAL VIRTUAL GND) Page 7

8 OPERATIONAL AMPLIFERS The 741 Op Amp is a general purpose bipolar (BJT) integrated circuit that has input bias current of 80nA, and input voltage of / 15 supply maximum of / 18 volts. The output voltage can not go all the way to the and supply voltage. At a minimum supply of / 5 volts the output voltage can go ~6 volts pp. The newer Op Amps have railrail output swing and supply voltages as low as / 1.5 volts. The MOSFET input bias currents are ~ 1pA. The NJU7031 is an example of this type of Op Amp. Page 8

9 BASIC TWO STAGE CMOS OPERATIONAL AMPLIFIER 1. Low Voltage operation 2. Rail to Rail input and output voltages 3. Low Input bias ~ 1pA or smaller 4. Low Output Current (depends on M6 and M7) 5. Unity Gain Bandwidth depends on Cc Page 9

10 LOW VOLTAGE, RAILTORAIL OP AMP 1. 3 to 16 Volt operation 2. Rail to Rail input and output voltages 3. Low Input bias ~ 1pA 4. Output Current ~1mA 5. Unity Gain Bandwidth 1.5 MHz 6. Power Dissipation 1mA at 3 V = 3000uW Page 10

11 NJU703X OP AMP DATA SHEET Page 11

12 LTC6078 OP AMP to 5.5 Volt operation 2. Rail to Rail input and output voltages 3. Low Input bias ~ 1pA 4. Output Current ~5mA 5. Unity Gain Bandwidth ~350Khz 6. Power dissipation 54 ua at 3 V = 162uW Page 12

13 SOME BASIC DUAL SUPPLY OP AMP CIRCUITS These dual supply circuits should be familiar: R2 R1 Vin V Inverting Amplifier Vo Vo= Vin R2/R1 R1 Vin V R2 Vo Vo= Vin (1 R2/R1) NonInverting Amplifier C Vin V Vo Vo= Vin Unity Gain Buffer Vin R Vo V Integrator Vo= 1/RC Vin dt Page 13

14 SOME BASIC ANALOG ELECTRONIC CIRCUITS These dual supply circuits should be familiar: V1 V2 R3 R1 R1 V Inverting Summer Vo= ( R3/R1) (V1 V2) Vo Rin V2 V1 Rin Rf Rf V Vo Vo= Rf/Rin (V1V2) Difference Amplifier Page 14

15 INVERTING AMPLIFIER EXAMPLE Inverting Amplifier R2 R1 Vo= Vin R2/R1 These Grounds are not the same Vin Vo 1. This is a DC and AC amplifier. 2. The input is referenced to the analog ground typically ½ of 3. The output voltage is referenced to the virtual ground or to earth ground. 4. If using a scope to measure Vo the scope ground is earth ground. If the Vin is ac you can AC couple the scope. 5. If the input Vin is DC you can measure the output relative to the analog ground using a multimeter (not the oscilloscope) Page 15

16 INVERTING AMPLIFIER EXAMPLES Single Supply DC Inverting Amplifier R1 R2 Vin Thermopile Gnd1 Gnd2 Vo Vo= Vin R2/R1 Gnd1 is analog ground ~1/2 of supply voltage. Vout can be take relative to Gnd1 or Gnd2 however there is a /2 DC added to Vo if relative to Gnd2. Page 16

17 INVERTING AMPLIFIER EXAMPLES offset The two 100K resistors create an analog ground ~1/2 V the gain = R2/R1, offset of V/2 or virtual ground Page 17

18 SINGLE RESISTOR SENSOR AMPLIFIER DESIGN R1 Sensor V R2 R4 V Vout NonInverting Amp Gain = 1 R3/R4 R3 Page 18

19 SINGLE SUPPLY NONINVERTING AMPLIFIER 3.3 Sensor =3.3 10K 10K To 10.07K 20K Vin 20K = K Vo 1. The two 20K resistors can be replaced by its Thevenin equivalent of V/2 and 10K 2. This sets up the analog ground at V/2 and the voltage gain to Vin is V/2 (or zero if referenced to analog ground) if the sensor is 10K 4. If the sensor is not exactly10k then Vo will have a value of 11 x (Vin ()/2) Page 19

20 RESISTOR R3 PARAMETER CHANGE (LIST) Resistor changes by / 0.1 ohm and supply voltage sweeps from 3.5V to 5V Page 20

21 NONINVERTING AMPLIFIER EXAMPLES Page 21

22 SINGLE SUPPLY COMPARATOR Vo Theoretical Vin Vref Vo 0 Vref Vin Page 22

23 SINGLE SUPPLYBISTABLE CIRCUIT WITH HYSTERESIS R R R Vo Theoretical V T Vo 0 V TL =1/3 V V TH = 2/3 V Vin Vin 1. The R s set up the threshold voltage at V/3 and 2V/3 2. Vout is either or Ground Page 23

24 SINGLE SUPPLY OSCILLATOR (MULTIVIBRATOR) R1 V T R3 R2 Vo V TH V TL Vo 0 t1 t C R Let R1 = 100K, R2=R3=100K and = 3.3 Then V TH = 2.2 when Vo = 3.3 V TL = 1.1 when Vo = 0 Page 24

25 OP AMP COMPARATOR WITH HYSTERESIS RC OSCILLATOR C2 Represents Scope Probe Page 25

26 CAPACITANCE CHANGE TO VOLTAGE i R V i C Co = Average value of C Cm = amplitude of C change C = Co Cm sin (2pft) V is constant across C 9V TL081 9 V Vo Vo = i R i = d (CV)/dt i = V Cm 2 p f cos (2pft) Vo = 2pf V R Cm cos (2pft) amplitude of Vo Page 26

27 DUAL SUPPLY OP AMP DC TO VOLTAGE Page 27

28 SINGLE SUPPLY VERSION OF SIGNAL PROCESSING i 5.6 MEG Vdc=9V i 100K C 100K 9V TL081 Vo earth ground Page 28

29 SINGLE SUPPLY MICROPHONE DC TO VOLTAGE Page 29

30 SINGLE SUPPLY PHOTO DETECTOR I TO V AMP 470K Vishay BPW46 Digikey No ND Light I 5 Vout NJU7024 The voltage across the diode is zero volts in the dark and the current is zero In the light I is 5uA (in direction shown, i.e. out of pside) What is Vout? Page 30

31 SIGNAL CONDITIONING FOR TEMPERATURE SENSOR Vout ~ 2mV/ C 3.3V 20K R1 p I n 0.2 < Vout < 0.7V Page 31

32 RESISTIVE PRESSURE SENSOR 5 Volts R1 R3 R2 R4 Vo2 5 Volts R1=427 R3=427 Vo1=2.5v Vo2=2.5v Vo1 Gnd R2=427 R4=427 Resistors on a Diaphragm Gnd No Pressure Vo2Vo1 = 0 Page 32

33 WHEATSTONE BRIDGE AND DIFFERENTIAL AMP Vo R1 R2 Vs Rst R3 R4 Rsb Vo Rin Rin Rf Rf Vo1 1. The R1=R2=R3=R4 make a Wheatstone bridge and are sensor resistors that will change in response to pressure. 2. Vo and Vo should be equal to each other and ~Vs/2 with no pressure. Page 33

34 SENSOR AND INTERNAL ELECTRONICS Page 34

35 SUMMARY Low voltage Op Amps are often used with a single supply. Some circuits work just fine with single supply such as the comparator. Other circuits use a virtual ground typically ½ of the supply voltage. Since signal generators and oscilloscopes are referenced to earth ground. Op Amp circuits need to consider this if powered by a single supply referenced to earth ground. In that case earth ground and virtual ground are at different voltages. Page 35

36 REFERENCES 1. MOSFET Modeling with SPICE, Daniel Foty, 1997, Prentice Hall, ISBN Operation and Modeling of the MOS Transistor, 2nd Edition, Yannis Tsividis, 1999, McGrawHill, ISBN UTMOST III Modeling ManualVol.1. Ch. 5. From Silvaco International. 4. ATHENA USERS Manual, From Silvaco International. 5. ATLAS USERS Manual, From Silvaco International. 6. Device Electronics for Integrated Circuits, Richard Muller and Theodore Kamins, with Mansun Chan, 3 rd Edition, John Wiley, 2003, ISBN ICCAP Manual, Hewlet Packard 8. PSpice Users Guide. 9. Using Single Supply Operational Amplifiers from Microchip 10. Designing Single Supply, LowPower Systems from Analog Devices 11. Designing Circuits for Single Supply Operation from Linear Technology 12. Single Supply Design from TI 13. Design TradeOffs for SingleSupply Op Amps from Maxium Page 36

37 HOMEWORK SINGLE SUPPLY OP AMP CKTS 1. Do SPICE analysis for a single op amp with dual supply to amplify the output for a single resistor sensor similar to that shown on page 18 (but with only one op amp). 2. If you want to measure a small value (0.1pF) slowly changing capacitance, what circuit could be used? Show it works using SPICE. Page 37

38 WHEATSTONE BRIDGE AND DIFFERENTIAL AMP Vo R1 R2 Vs Rst R3 R4 Rsb Vo Rin Rin Rf Rf Vo1 1. The R1=R2=R3=R4 make a Wheatstone bridge and are sensor resistors that will change in response to pressure. 2. Vo and Vo should be equal to each other and ~Vs/2 with no pressure. Page 38