Texas instruments SLOA058 User Manual

Application Report

SLOA058– November 2000

A Single-Supply Op-Amp Circuit Collection

Bruce Carter Op-Amp Applications, High Performance Linear Products

One of the biggest problems for designers of op-amp circuitry arises when the circuit must be operated from a single supply, rather than ±15 V. This application note provides working circuit examples.

Contents

1 Introduction ................................................................................................................................... 3

1.1 Split Supply vs Single Supply.................................................................................................... 3

1.2 Virtual Ground........................................................................................................................... 4

1.3 AC-Coupling............................................................................................................................. 4

1.4 Combining Op-Amp Stages ...................................................................................................... 5

1.5 Selecting Resistor and Capacitor Values.................................................................................. 5

2 Basic Circuits ................................................................................................................................ 5

2.1 Gain .......................................................................................................................................... 5

2.2 Attenuation ............................................................................................................................... 6

2.3 Summing .................................................................................................................................. 9

2.4 Difference Amplifier .................................................................................................................. 9

2.5 Simulated Inductor.................................................................................................................... 9

2.6 Instrumentation Amplifiers ...................................................................................................... 10

3 Filter Circuits............................................................................................................................... 12

3.1 Single Pole Circuits................................................................................................................. 13

3.1.1 Low Pass Filter Circuits .............................................................................................................13

3.1.2 High Pass Filter Circuits.............................................................................................................13

3.1.3 All-Pass Filter............................................................................................................................14

3.2Double-Pole Circuits............................................................................................................... 15

3.2.1 Sallen-Key.................................................................................................................................15

3.2.2 Multiple Feedback (MFB)...........................................................................................................16

3.2.3 Twin T.......................................................................................................................................17

3.2.4 Fliege........................................................................................................................................20

3.2.5 Akerberg-Mossberg Filter .......................................................................................................... 22

3.2.6 BiQuad...................................................................................................................................... 24

3.2.7 State Variable............................................................................................................................25

4 References................................................................................................................................... 25

Appendix A – Standard Resistor and Capacitor Values ................................................................. 26

Figures

1 Split Supply (L) vs Single Supply (R) Circuits..................................................................................3

2 Single-Supply Operation at Vcc/2 ....................................................................................................4

3 AC-Coupled Gain Stages................................................................................................................6

4 Traditional Inverting Attenuation With an Op Amp...........................................................................6

5 Inverting Attenuation Circuit ............................................................................................................7

6 Noninverting Attenuation.................................................................................................................8

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7 Inverting Summing Circuit............................................................................................................... 9

8 Subtracting Circuit........................................................................................................................... 9

9 Simulated Inductor Circuit............................................................................................................. 10

10 Basic Instrumentation-Amplifier Circuit.......................................................................................... 11

11 Simulated Instrumentation-Amplifier Circuit................................................................................... 11

12 Instrumentation Circuit With Only Two Op Amps...........................................................................12

13 Low-Pass Filter Circuits.................................................................................................................13

14 High-Pass Filter Circuits................................................................................................................ 14

15 All-Pass Filter Circuit.....................................................................................................................14

16 Sallen-Key Low- and High-Pass Filter Topologies......................................................................... 16

17 Multiple-Feedback Topologies.......................................................................................................17

18 Single Op-Amp Twin-T Filter in Band-Pass Configuration............................................................. 18

19 Single Op-Amp Twin-T Filter in Notch Configuration..................................................................... 18

20 Dual-Op-Amp Twin-T Low-Pass Filter...........................................................................................19

21 Dual-Op-Amp Twin-T High-Pass Filter ..........................................................................................19

22 Dual-Op-Amp Twin-T Notch Filter .................................................................................................20

23 Low-Pass Fliege Filter ...................................................................................................................20

24 High-Pass Fliege Filter..................................................................................................................21

25 Band-Pass Fliege Filter.................................................................................................................21

26 Notch Fliege Filter.........................................................................................................................22

27 Akerberg-Mossberg Low-Pass Filter .............................................................................................22

28 Akerberg-Mossberg High-Pass Filter.............................................................................................23

29 Akerberg-Mossberg Band-Pass Filter............................................................................................23

30 Akerberg-Mossberg Notch Filter....................................................................................................24

31 Biquad Low-Pass and Band-Pass Filter ........................................................................................24

32 State-Variable Four-Op-Amp Topology .........................................................................................25

Tables

1 Normalization Factors .....................................................................................................................8

2 A Single-Supply Op-Amp Circuit Collection

1 Introduction

There have been many excellent collections of op-amp circuits in the past, but all of them focus exclusively on split-supply circuits. Many times, the designer who has to operate a circuit from a single supply does not know how to do the conversion.

Single-supply operation requires a little more care than split-supply circuits. The designer should read and understand this introductory material.

1.1 Split Supply vs Single Supply

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All op amps have two power pins. In most cases, they are labeled V they are labeled V

and GND. This is an attempt on the part of the data sheet author to

CC+

and V

, but sometimes

CC-

categorize the part as a split-supply or single-supply part. However, it does not mean that the op amp has to be operated that way—it may or may not be able to operate from different voltage rails. Consult the data sheet for the op amp, especially the absolute maximum ratings and voltage-swing specifications, before operating at anything other than the recommended power-supply voltage(s).

Most analog designers know how to use op amps with a split power supply. As shown in the left half of Figure 1, a split power supply consists of a positive supply and an equal and opposite

negative supply. The most common values are ±15 V, but ±12 V and ±5 V are also used. The input and output voltages are referenced to ground, and swing both positive and negative to a

limit of V

, the maximum peak-output voltage swing.

OM±

A single-supply circuit (right side of Figure 1) connects the op-amp power pins to a positive voltage and ground. The positive voltage is connected to V

, and ground is connected to V

CC+

CC-

or GND. A virtual ground, halfway between the positive supply voltage and ground, is the reference for the input and output voltages. The voltage swings above and below this virtual ground to the limit of V which are specified in data sheets as V

. Some newer op amps have different high- and low-voltage rails,

OM±

and V

respectively. It is important to note that there

OL,

are very few cases when the designer has the liberty to reference the input and output to the virtual ground. In most cases, the input and output will be referenced to system ground, and the designer must use decoupling capacitors to isolate the dc potential of the virtual ground from the input and output (see section 1.3).

+SUPPLY +SUPPLY

-SUPPLY

HALF_SUPPLY

Figure 1. Split Supply (L) vs Single Supply (R) Circuits

A common value for single supplies is 5 V, but voltage rails are getting lower, with 3 V and even lower voltages becoming common. Because of this, single-supply op amps are often rail-to-rail devices, which avoids losing dynamic range. Rail-to-rail may or may not apply to both the input and output stages. Be aware that even though a device might be specified as rail-to-rail, some

A Single-Supply Op-Amp Circuit Collection 3

SLOA058

Ω

specifications can degrade close to the rails. Be sure to consult the data sheet for complete specifications on both the inputs and outputs. It is the designer’s obligation to ensure that the voltage rails of the op amp do not degrade the system specifications.

1.2 Virtual Ground

Single-supply operation requires the generation of a virtual ground, usually at a voltage equal to Vcc/2. The circuit in Figure 2 can be used to generate Vcc/2, but its performance deteriorates at low frequencies.

R1 and R2 are equal values, selected with power consumption vs allowable noise in mind. Capacitor C1 forms a low-pass filter to eliminate conducted noise on the voltage rail. Some applications can omit the buffer op amp.

In what follows, there are a few circuits in which a virtual ground has to be introduced with two resistors within the circuit because one virtual ground is not suitable. In these instances, the

resistors should be 100 kW or greater; when such a case arises, values are indicated on the schematic.

1.3 AC-Coupling

A virtual ground is at a dc level above system ground; in effect, a small, local-ground system has been created within the op-amp stage. However, there is a potential problem: the input source and output load are probably referenced to system ground, and if the op-amp stage is connected to a source that is referenced to ground instead of virtual ground, there will be an unacceptable dc offset. If this happens, the op amp becomes unable to operate on the input signal, because it must then process signals at and below its input and output rails.

0.1 µF

+Vcc

Vcc/2

R1 100 k

R2 100 k

+Vcc

Ω

Figure 2. Single-Supply Operation at VCC/2

The solution is to ac-couple the signals to and from the op-amp stage. In this way, the input and output devices can be referenced to ground, and the op-amp circuitry can be referenced to a virtual ground.

When more than one op-amp stage is used, interstage decoupling capacitors might become unnecessary if all of the following conditions are met:

• The first stage is referenced to virtual ground.

• The second stage is referenced to virtual ground.

4 A Single-Supply Op-Amp Circuit Collection

• There is no gain in either stage. Any dc offset in either stage is multiplied by the gain in both, and probably takes the circuit out of its normal operating range.

If there is any doubt, assemble a prototype including ac-coupling capacitors, then remove them one at a time. Unless the input or output are referenced to virtual ground, there must be an input-decoupling capacitor to decouple the source and an output-decoupling capacitor to decouple the load. A good troubleshooting technique for ac circuits is to terminate the input and output, then check the dc voltage at all op-amp inverting and noninverting inputs and at the op-amp outputs. All dc voltages should be very close to the virtual-ground value. If they are not, decoupling capacitors are mandatory in the previous stage (or something is wrong with the circuit).

1.4 Combining Op-Amp Stages

Combining op-amp stages to save money and board space is possible in some cases, but it often leads to unavoidable interactions between filter response characteristics, offset voltages, noise, and other circuit characteristics. The designer should always begin by prototyping separate gain, offset, and filter stages, then combine them if possible after each individual circuit function has been verified. Unless otherwise specified, filter circuits included in this document are unity gain.

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1.5 Selecting Resistor and Capacitor Values

The designer who is new to analog design often wonders how to select component values. Should resistors be in the 1-Ω decade or the 1-MΩ decade? Resistor values in the 1-kΩ to 100-kΩ range are good for general-purpose applications. High-speed applications usually use resistors in the 100-Ω to 1-kΩ decade, and they consume more power. Portable applications usually use resistors in the 1-MΩ or even 10-MΩ decade, and they are more prone to noise. Basic formulas for selecting resistor and capacitor values for tuned circuits are given in the various figures. For filter applications, resistors should be chosen from 1% E-96 values (see Appendix A). Once the resistor decade range has been selected, choose standard E-12 value capacitors. Some tuned circuits may require E-24 values, but they should be avoided where possible. Capacitors with only 5% tolerance should be avoided in critical tuned circuits—use 1% instead.

2 Basic Circuits

2.1 Gain

Gain stages come in two basic varieties: inverting and noninverting. The ac-coupled version is shown in Figure 3. For ac circuits, inversion means an ac-phase shift of 180°. These circuits work by taking advantage of the coupling capacitor, CIN, to prevent the circuit from having dc gain. They have ac gain only. If CIN is omitted in a dc system, dc gain must be taken into account.

It is very important not to violate the bandwidth limit of the op amp at the highest frequency seen by the circuit. Practical circuits can include gains of 100 (40 dB), but higher gains could cause the circuit to oscillate unless special care is taken during PC board layout. It is better to cascade two or more equal-gain stages than to attempt high gain in a single stage.

A Single-Supply Op-Amp Circuit Collection 5

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+Vcc

Vout

INVERTING Gain = – R2/R1

R3 = R1||R2 for minimum error due to input bias current

NONINVERTING

Gain = 1 + R2/R1 Input Impedance = R1||R2

for minimum error due to input bias current

Vin

Cin

Vcc/2

Cin

+Vcc

Vout

2.2 Attenuation

The traditional way of doing inverting attenuation with an op-amp circuit is shown in Figure 4, in

INVERTING Gain = – R2/R1

R3 = R1||R2 for minimum error due to input bias current

which R2 < R1. This method is not recommended, because many op amps are unstable at gains of less than unity. The correct way to construct an attenuation circuit1 is shown in Figure 5.

Vcc/2

Figure 3. AC-Coupled Gain Stages

+Vcc

Cin

Vin

Vcc/2

Figure 4. Traditional Inverting Attenuation With an Op Amp

Vout

This circuit is taken from the design notes of William Ezell

6 A Single-Supply Op-Amp Circuit Collection

Rf 2

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INVERTING Component values

normalized to unity

Vin

Cin

Vcc/2

RinA 1

RinB 1

+Vcc

Vout

Figure 5. Inverting Attenuation Circuit

A set of normalized values of the resistor R3 for various levels of attenuation is shown in Table 1. For nontablated attenuation values, the resistance is:

INO

( )

223−

INO

To work with normalized values, do the following:

• Select a base-value of resistance, usually between 1 kW and 100 kW for Rf and Rin.

• Divide Rin in two for RinA and RinB.

• Multiply the base value for Rf and Rin by 1 or 2, as shown in Figure 5.

• Look up the normalization factor for R3 in the table below, and multiply it by the base-value

of resistance.

For example, if Rf is 20 kΩ, RinA and RinB are each 10 kΩ, and a 3-dB attenuator would use a

12.1-kΩ resistor.

A Single-Supply Op-Amp Circuit Collection 7

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1.0000

0.5

0.9441

8.4383

0.8913

4.0977

0.7943

0.9311

0.7079

1.2120

3.01

0.7071

1.2071

3.52

0.6667

1.000

0.6310

0.8549

0.5623

0.6424

0.5012

0.5024

6.02

0.5000

0.4467

0.4036

0.3981

0.3307

0.3548

0.2750

9.54

0.3333

0.2500

0.3162

0.2312

0.2512

0.1677

12.04

0.2500

0.1667

13.98

0.2000

0.1250

0.1778

0.1081

15.56

0.1667

0.1000

16.90

0.1429

0.08333

0.1259

0.07201

18.06

0.1250

0.07143

19.08

0.1111

0.06250

0.1000

0.05556

0.0562

0.02979

0.0316

0.01633

0.0100

0.005051

0.0032

0.001586

0.0010

0.0005005

Vout

Table 1. Normalization Factors

DB Pad Vout/Vin R3

Noninverting attenuation can be performed with a voltage divider and a noninverting buffer as shown in Figure 6.

NONINVERTING Component values

normalized to unity

Vin

Figure 6. Noninverting Attenuation

8 A Single-Supply Op-Amp Circuit Collection

Cin

+Vcc

Vcc/2

2.3 Summing

Vout = – R2(Vin1/R1 + Vin2/R2 +

An inverting summing circuit (Figure 7) is the basis of an audio mixer. A single-supply voltage is seldom used for real audio mixers. Designers will often push an op amp up to, and sometimes beyond, its recommended voltage rails to increase dynamic range.

Noninverting summing circuits are possible, but not recommended. The source impedance becomes part of the gain calculation.

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INVERTING

Vin3/R3)

= R1A||R1B||R1C||R2

for minimum error due to input bias current

2.4 Difference Amplifier

Just as there are summing circuits, there are also subtracting circuits (Figure 8). A common application is to eliminate the vocal track (recorded at equal levels in both channels) from stereo recordings.

For R1 = R3 and R2 = R4: Vout = (R2/R1)(Vin2 – Vin1) R1||R2 = R3||R4 for minimum error due to input bias current

Vin1

Cin1

Cin2

Vin2

CIin3

Vin3

R1A

R1B

R1C

Vcc/2

Figure 7. Inverting Summing Circuit

Cin1

Vin1

Vin2

Cin2

+Vcc

Vout

2.5 Simulated Inductor

The circuit in Figure 9 reverses the operation of a capacitor, thus making a simulated inductor. An inductor resists any change in its current, so when a dc voltage is applied to an inductance, the current rises slowly, and the voltage falls as the external resistance becomes more significant.

Vcc/2

Figure 8. Subtracting Circuit

A Single-Supply Op-Amp Circuit Collection 9

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