module title : operational amplifiers topic title : op-amp ... · 2/03/2016 · • indentify the...

MODULE TITLE : OPERATIONAL AMPLIFIERS

TOPIC TITLE : OP-AMP PERFORMANCE

LESSON 5 : SELECTING AN OP-AMP

OA - 2 - 5

© Teesside University 2011

Published by Teesside University Open Learning (Engineering)

School of Science & Engineering

Teesside University

Tees Valley, UK

TS1 3BA

+44 (0)1642 342740

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________________________________________________________________________________________

INTRODUCTION________________________________________________________________________________________

This lesson discusses some of the criteria that might be relevant when

choosing an op-amp for a particular application. There are literally hundreds

of op-amps to choose from and we will need to be able to whittle the choice

down to find the optimum overall specification for our particular application.

The selection process is further complicated by manufacturers offering the

same or very similar op-amps but under their own manufacturer's codes.

An example is given of a web-based package that can help us in our selection.

The choice offered, though, is of course limited to that manufacturer’s range.

________________________________________________________________________________________

YOUR AIMS________________________________________________________________________________________

Upon completing this lesson you should be able to:

• indentify the need to match an op-amp to its source, load and

available power supply

• select an op-amp based upon parameter performance (bandwidth,

slew rate, quiescent current, noise, etc.)

• finalise a selection based on other features such as packaging, cost

and availability.

1

Teesside University Open Learning(Engineering)


________________________________________________________________________________________

THE IDEAL AND ACTUAL OP-AMP________________________________________________________________________________________

Before going on to look at how we might select a particular operational

amplifier from the thousands of available possibilities, it is worth our while

first pausing to refresh our memories as to how an ideal operational amplifier

should perform. TABLE 1 shows how the performance of an ideal op-amp

compares to an average practical op-amp. The ideal op-amp will, of course,

suit all occasions and applications.

TABLE 1 Ideal vs Practical Op-Amp

–

+

Ideal op-ampAVin

VoutVin

Rout = 0Rin = ∞

–

+

Practical op-amp

AVin

VoutVin

RoutRin

Open loop gain A

Bandwidth BW

Input impedance Zin

Output impedance Zout

Output voltage Vout

CMRR

Ideal

Infinite

Infinite

Infinite

0 Ω

Vout

= AD

(V+

– V–)

AD

= differentialmode gain

Infinite

Practical

105

10 - 100 Hz

>1 M Ω

10 - 100 Ω

Vout

= AD

(V+ – V

–) + A

C (V

+ + V

–)/2

AC

= common mode gain

10 - 100 dB

2



The extract given in TABLE 2, from the data sheet for the general purpose

'741' op-amp, gives some idea of the range of parameters that might be

relevant to a particular op-amp application. (The meaning of some of the

terminology is given in APPENDIX 1.)

In selecting an op-amp, the designer must sieve through the data sheets to find

parameters that will match the circuit’s specification. The task is of course eased

by experience and also by manufacturers’ software design tools that can list a

selection of their(!) devices that will be suitable for any given circuit. (See Self-

Assessment Question 5 as an example of the use of such a design aid.)

TABLE 2

3



________________________________________________________________________________________

WHERE TO BEGIN________________________________________________________________________________________

The starting point in selecting an op-amp is to examine the circuit that it is to

fit into. FIGURE 1 shows a simple op-amp circuit. There are three blocks that

are really external to the op-amp circuit itself and these have been highlighted

as:

• the source

• the load

• the power supply.

Usually these circuit blocks will have already been prescribed so that we have

to design around them. It is the demands set by the source, load and power

supply that will largely determine our choice of op-amp. Let us therefore

consider each of these blocks in turn.

FIG. 1

–

+

+VS

Powersupply

–VS

R3

R2

R1

VS

Signal source

RSRseries

C R

Load Outputsignal

4



________________________________________________________________________________________

THE SOURCE________________________________________________________________________________________

In this example the source consists of a Thévenin equivalent

(see opposite) of a voltage generator with its series resistance.

Typically the source will be a transducer (thermocouple,

microphone, strain gauge, photodiode, etc.; see APPENDIX 2

for further details) and it is the nature of the source voltage in

terms of its shape, frequency, bandwidth and amplitude that

will determine several of the most important parameters of the

op-amp.

Suggest some op-amp parameters the nature of the input voltage might define.

...................................................................................................................................................

...................................................................................................................................................

...................................................................................................................................................

...................................................................................................................................................

________________________________________________________________________________________

Some possibilities include:

Gain, bandwidth, slew-rate, input offset voltage and currents.

As can be seen from TABLE 1, the op-amp has an inherently high input

resistance. Remember, though, that this is the input resistance to the op-amp,

not necessarily to the circuit of which it is part. In FIGURE 1 for example the

inverting input is at a virtual earth and so the input resistance as viewed from

the source (i.e. to the left of R1) will be R1. The value of R1 will have to be

chosen to be compatible with the source resistance RS.

5



FIGURES 2(a) and (b) show two contrasting examples. In (a) a thermocouple,

which is a voltage source, can be directly connected to the differential inputs of

an instrumentation amplifier. The high input resistance of the op-amp input

maintains the integrity of the input signal.

FIG. 2(a) Thermocouple Amplifier

FIG. 2(b) Photodiode Amplifier

–

+

–5V

+5V

VOUT

RF

ID

VREF

Photodiode

–

+Thermocouple

–5V

+5V

Thermocouplewires

VOUT

6



The example of (b) is of a photodiode, which is a current

source. The diode can be modelled by a Norton

equivalent generator as shown opposite. The diode is

reverse biased and its equivalent Norton resistance is of

the order of giga ohms. This resistance is certainly very

much greater than the input resistance to a general

purpose op-amp and connecting the diode directly to its differential input, as in

the case of the thermocouple, would tend to short-circuit the signal. The

circuit of FIGURE 2(b) is a much more satisfactory solution, where the op-

amp is used as a current-to-voltage converter, its output being VOUT = IDRF.

TYPES OF OP-AMP INPUT STAGES

Bipolar Input Stages

The original op-amp employed the basic bipolar junction transistor (BJT) input

stage of FIGURE 3(a). It consists of a 'long-tailed pair' of transistors that act

as a differential voltage amplifier. The amplifier is 'differential' because it

amplifies the difference between its two input voltages. A constant current

source in the 'tail' of the long-tailed pair means that the current through one

transistor can only increase at the expense of the current through the other.

Consider first of all that the voltages on the bases of both transistors are equal

(zero differential voltage). Assuming that the two transistors are identical then

the two collector currents will be equal. This means the output voltage will be

zero as the same voltage is dropped across each collector resistor. Now

consider if, say, the voltage on the base of the left hand transistor were to

increase. This will cause that transistor's collector current to increase and the

collector current in the right hand transistor will decrease by the same amount.

Thus the voltage across the left hand collector resistor will increase and the

voltage across the right hand collector resistor will fall by the same amount. If

the change in collector current is related to the change in base voltage by the

equation ΔIC = kΔVB then the output voltage will change by 2kΔVB.

7



Typically, an op-amp with a bipolar input stage will have input resistance of a

couple of mega ohms. More subtle designs can increase this to 20 MΩ or

more, but this will be at the expense of other parameters. In conclusion bipolar

inputs offer:

� Low offset voltage: 10 μV.

� Low offset voltage temperature drift: 0.1 μV/ºC.

� Well-Matched Bias Currents.

� Bias current decrease with temperature increase.

� High Bias Currents: 50nA – 10 μA.

FIG. 3(a)

–VCC

+VCC

Constant currentsource

VINDifferential

input

VOUT

8



FET Input Stages

FIGURE 3(b) shows field-effect transistors (FET) input stage. Its operation is

essentially similar to that of the BJT input. FETs have high input resistance

(hundreds or even thousands of MΩ). FETs also have much low bias currents

than BJTs. However the input offset voltage of a FET long-tailed pair is not as

good as that of a BJT.

FIG. 3(b)

There are two kinds of field-effect transistor: the MOSFET (Metal-Oxide-

Semiconductor FET) and the JFET (Junction FET). MOSFETS have the

superior input resistance.

The bias current of a FET op-amp is the leakage current of the gate, which is

of the order of a few tens of fA (10–15 A). However these leakage currents are

highly temperature sensitive and approximately double with every 10°C

increase in operating temperature. This means that the leakage current (say) of

50 fA at 20°C of a FET input will increase to about 50 pA at 125°C. Thus, in

high temperature applications bipolar op-amps might be the choice because

their input bias current decreases with increasing temperature.

Constant currentsource

VINDifferential

input

VOUT

9



In conclusion FET inputs offer:

� Very low bias current: 20 fA.

� Offset voltage down to 50 μV (not as good as a BJT).

� Offset voltage temperature drift of ~5 μV/°C (not as good as a BJT).

� bias current doubles every 10°C.

10



________________________________________________________________________________________

THE LOAD________________________________________________________________________________________

The demands of the load will determine

the required output properties of the op-

amp. The load can be generally

represented by a RC parallel

combination. The capacitance can be

due to the nature of the load itself and/or

be caused by the track or cable

connecting the op-amp to the load.

At low frequencies the load will be essentially resistive (FIGURE 4(a))

whereas at high frequencies the capacitance will predominate (FIGURE 4(b)).

FIG. 4 Effect of Output Impedance

Ro

Resistive load

(a)

Vo

RLAVD VIN

Ro

Capacitive load

(b)

Vo

CLAVD VIN

Rseries

C R

LoadOutputsignal

11



The load acts, along with the output resistance of the op-amp, Ro, as a potential

divider so that the output voltage is given by:

where

Thus

where AVD(LF) is the low frequency gain (f << fc) and is given by

and the corner frequency

At high frequencies (f >> fc),

V A Vf

fo VD LF inc≈ ×( )

fC

R R

R R

c

LL o

L o

=

+

1

2π

A A VR

R RVD LF VD inL

L o( ) ≈ ×

+

V A VZ

Z R

ZR

j R C

VA

j

o VD inL

L o

LL

L L

oVD LF

1 +

1 +

= ××

=

= ( )

ω

ff

f

V

c

in×

12



TYPES OF OP-AMP OUTPUT STAGES

Bipolar Output Stages

FIGURE 5 shows the essential features

of a bipolar op-amp's output stage. It

uses two complementary emitter

followers in a 'push-pull' arrangement.

The circuit action is very simple. On

positive excursions of the input signal

(from a previous stage in the op-amp),

the upper, npn, transistor will operate

to produce a replica of the positive half

cycle. On negative excursions of the

input signal, the lower, pnp, transistor

will operate to produce a replica of the

negative half cycle. The purpose of the

two diodes is to reduce 'cross-over distortion' that would be otherwise caused

by the input signal having to exceed the VBE of the transistor before it begins to

operate.

Note that the output is taken from between the transistors' emitters; this means

that the transistors are working in emitter-follower mode. This confers the op-

amp with the required low output resistance. However the configuration

allows the maximum output voltage to swing only to within VBE

(approximately 0.7 volts) of the supply voltage. This might not be an issue for

large values of supply voltage but does represent a large overhead when

operating off, say, a 5 V supply.

Note that only one output transistor is conducting at a time. This means that in

the absence of an input signal neither transistor is conducting, so there is no

output power dissipation in the idle condition. This is an important

consideration in integrated circuits as it reduces heating in the chip.

+VCC

npn

VIN

Bipolar op-amp output stage

VOUT

FIG. 5

–VCC

pnp

13



FET Output Stages

FIGURE 6(a) shows a CMOS output stage. The output is taken from the

drains of the two CMOS transistors that form an inverting amplifier. When the

input signal (from a previous stage of the op-amp) is positive the lower

transistor is on and the output voltage is pulled low. When the input signal is

negative the upper transistor is on and the output voltage is pulled high.

In theory the arrangement can give rail-to-rail output voltages but this will

only occur on no load. In practice when operating to a very high resistance

load, the output can reach within tens of millivolts of either supply rail. The

excursion of output voltage is limited by the channel 'on' resistance of the

transistor. This resistance is of the order of 10 to 100 Ω. The channel

resistance causes a straight forward IR volt-drop, so the greater the load current

the lower the output voltage (and consequently the overall gain).

FIGURE 6(b) gives the equivalent output circuit using a complementary pair

of JFETS. This arrangement can also approach rail-to-rail output voltages

when operating with small load currents.

FIG. 6(a) FIG. 6(b)

+VDD

VIN

CMOS op-amp output stage

VOUT

–VDD

+VDD

VIN

JFET op-amp output stage

VOUT

–VDD

D

D

14



'RAIL-TO-RAIL' OUTPUT OPERATION

In many applications, especially when operating on low supply voltages, the

output is required to swing right up to the rail voltages. As just mentioned,

above, both the CMOS and JFET output configurations can approach rail-to-

rail operation on very high resistance loads.

The bipolar output of FIGURE 5, however can

never approach closer than VBE of the rail

voltage. FIGURE 7 shows an alternative bipolar

arrangement that (almost) gives rail-to-rail

output. This configuration uses the transistors in

common-emitter (CE) mode rather than as the

emitter-followers of FIGURE 5. (FIGURE 7 is

CE mode because the input is applied between

the base and the emitter and the output is taken

from between the collector and the emitter. Note

that transistors’ collectors are commoned at the

output.) The output voltage is now limited to

just be the saturation voltage, VCE(SAT), of the output transistor. For low load

currents (less than 100 μA), VCE(SAT) is of the order of 10 mV but at higher

load currents VCE(SAT) falls dramatically, e.g. to 500 μV at a load current of

50 mA.

The CE mode of operation also has the benefit of having a higher gain than the

emitter follower mode, which has unity gain. Thus an op-amp with a CE

output will have a higher open-loop gain than the corresponding op-amp with

an emitter-follower output. There is, however, a major disadvantage of

operating the output transistors in CE mode. The output stage has a much

higher output resistance than the emitter-follower configuration. This

handicap is, though, in practice countered by its higher open-loop gain as the

op-amp is invariable used with applied negative feedback, which has the effect

pnp

+VCC

npn

VINVOUT

FIG. 7

–VCC

15



of lowering the output resistance. Thus for the same closed-loop the CE

output op-amp can still produce an output resistance comparable to its emitter-

follower cousin.

________________________________________________________________________________________

THE SUPPLY________________________________________________________________________________________

If the avalible power supply is already specified, then it can be a principal

determinate in the choice of op-amp. For example it could be a dual supply of

a given voltage (e.g. ±24, ±15 or ±9 volts). A high voltage would

immediately rule out many types of op-amp. Conversely, op-amps with a high

supply voltage rating may fail to perform at lower voltages (say ±5 V).

The supply voltage will so define the available output voltage swing. If an

output span of ±9 V is required then clearly the supply voltage should be well

in excess of this (say ±12 V).

Matters are further complicated by the wide use of portable, battery-operated

equipment. This has lead to the development of single-supply op-amps

(FIGURE 8) that generally are designed to operate at lower voltages, typically

5 V and below.

FIG. 8

–

+

+VS

Dual power supply

–VS

–

+

+VS

Single power supply

VREF

16



In general, we require the output voltage swing of an op-amp to swing

symmetrically about the mid-point of the supply voltage. For a dual-supply

this will normally be about ground (0 volts) as [VS + (–VS)]/2 = 0. The

problem with a single-supply is that the ground potential will be one of the

supplies, so some form of biasing must be used to centre the op-amp's output

voltage at the mid-point of the supply. In FIGURE 8 this is achieved by

feeding a reference voltage, VREF = VS/2, into the non-inverting input of the

op-amp. (See Self-Assessment Question 1.)

The design of single-supply op-amp circuits is more complicated than that for

dual-supply. For more details refer to the literature, e.g. Texas Instruments'

Application Note SLOA030: "Single-Supply Op-amp Design Techniques."

(http://focus.ti.com/lit/an/sloa030a/sloa030a.pdf)

________________________________________________________________________________________

OTHER FACTORS AFFECTING THE CHOICE OF OP-AMP________________________________________________________________________________________

POWER CONSUMPTION

The demands of battery operated equipment have lead to the development of

'micropower' op-amps that consume just a few hundred nA in the quiescent

state. To achieve such low current values, however, high-valued resistors must

be used and this has an adverse affect upon the op-amp's performance in terms

of speed and noise.

Some 'micropower' op-amps are designed to conserve battery life by being able

to shut down when not in use. This is achieved by incorporating a shutdown

terminal that, when pulled to ground, will disabled the amplifier. The output

is placed in a high impedance mode and the supply current reduced to just a

few nA.

17



BANDWIDTH / PULSE RESPONSE

We deal with bandwidth and pulse response under the same heading as they

are closely related.

Five parameters that describe the frequency behaviour of an op-amp are:

• unity-gain bandwidth

• gain bandwidth product (GBW)

• gain margin

• phase margin

• full power bandwidth (FPBW).

Three parameters relating to pulse response are:

• slew rate

• rise time

• overshoot.

The slew rate of an op-amp is the maximum rate at which its output voltage

can change without distortion. Slew rate is linked to the FPBW of the op-amp

as the latter is the maximum frequency at which slewing does not occur at the

rated voltage output, Vp. The two parameters are related by the equation:

There is price to pay for an op-amp's high speed (or wide bandwidth) in terms

of the trade-off between speed and power consumption. As a general rule the

higher the speed the greater the power consumption. Conversely, low-power

devices will tend to lack speed.

FPBW =slew rate

p2πV

18



Most op-amps are internally compensated to make them unconditionally

stable. This means that their open-loop gain reaches unity before the phase

shift is 180°. The use of uncompensated op-amps, though, can give a flatter

amplitude response and wider bandwidth. Uncompensated op-amps still

require external compensation to give stability but the compensation can be

adapted to meet the demands of the specific circuit application to give

maximum flatness of response, the highest possible bandwidth or the best

achievable pulse response.

PRECISION

A device is precise if it gives the same output at all times for the same input

conditions. Precision is often a required criterion in a design. Environmental

factors such as temperature can affect precision by altering offset voltages and

currents. As mentioned earlier, bipolar input op-amps give superior

performance in terms of offset voltage than FET inputs. However FET inputs

have very much reduced input bias currents. Input bias current flow through

the source resistance and the feedback network to create an offset voltage and

drift. But the input bias current of FETs doubles every 10°C increase in

temperature which might rule them out in high temperature applications.

LOW NOISE

Noise is broadly defined as any unwanted signal. There are two sources of

noise: external and internal, FIGURE 9. External noise can arise from nearby

power sources, pick-up from leads, radio frequency interference, from the

power supply, etc. Internal noise is generated within the device itself and is

present irrespective of the presence of an input signal. All these sources

combine to give a noisy output. Here, however, we shall just consider the

internal noise self-generated within the op-amp circuit.

19



FIG. 9

INTERNAL NOISE

The two main sources of internal noise in an op-amp are shot noise and

thermal (or Johnson) noise.

Shot noise exists because electric current consists of the motion of umpteen

charge carriers (electrons in a wire, electrons and holes in a semiconductor).

When viewed on mass, the cumulative motion of these charge carriers gives

the flow of current that of a uniform fluid, but when viewed at the atomic level

the individual motion of the charge carriers is found to randomly fluctuate.

The motion of current in a conductor could be likened to the flow of water in a

stream with a stony bottom. When the water is deep the flow seems uniform

despite the disorder at stream’s bottom. If the stream is shallow, however, the

effect of its uneven bottom becomes apparent as the flow ripples over the

stones. A similar effect occurs as electric current flows through any medium

(be it a metallic conductor, semiconductor, pn junction or whatever) because of

the medium’s unavoidable imperfections. At very low current levels the flow

of current is in fits and starts, as illustrated in FIGURE 10. Shot noise has

been likened to the noise of lead shot being poured onto a metal sheet.

Although the effect of shot noise only becomes significant at very low levels

of current, such low levels can be found in an op-amp.

Pick-up on input leads

Radio frequencyinterference

Internal noise

Power supply noise

Noisy output

20



(a) Large current flow (b) Low current flow

FIG. 10

Thermal noise is caused by the thermal agitation of the charge carriers at any

temperature above absolute zero. Thermal energy is imparted to the carriers

that give them a random kinetic energy, making them dart about in a

completely haphazard fashion. FIGURE 11 illustrates the random motion of a

conducting electron within the atomic lattice of a conductor. Although trapped

in the lattice structure, the atoms of the conductor are also in a state of thermal

agitation which causes them to vibrate about a mean position. As the atoms of

the lattice vibrate they will jostle the electrons.

When a voltage is applied across a conductor the conducting electrons will

move in the direction of the resultant electric field. This motion is however

impressed upon the thermal motion. The electrons do not move in a straight

line but tend to drift in the general direction of the applied field.

21



FIG. 11

Cacluation of Noise Currents and Voltages

Shot noise is a source of current noise as it represents random variations in the

average value of a current. The rms1 current value of shot noise is given by the

equation:

Thermal noise is a source of voltage noise as it represents random variations

in the average value of a voltage. The rms value of thermal noise is given by

the equation:

________________________________________________________________________________________

1The average value of a shot noise current or a thermal noise voltage is zero. This is because the origins of

such noise is due to completely random events and the currents or voltages produced are as likely to be

positive in amplitude as they are to be negative.

V kTBRthermal = 4

I eI Bshot DC= 2

Vibrating lattice

Random motion ofconducting electron

22



where:

• T is the absolute temperature (K)

• k is Boltzmann’s constant (1.38 × 10–23 J K–1)

• e is the electronic charge (1.6 × 10–19 C)

• B is the system bandwidth (Hz)

• IDC is the average direct current (A)

• R is the resistance of the conductor (Ω).

Consideration of these equations shows that:

• Noise is limited by limiting the bandwidth. Systems having wide

bandwidths let through more noise than those having a narrow

bandwidth.

• Shot noise current is independent of temperature.

• Thermal noise voltage increases with increasing temperature.

• Thermal noise voltage increases with increasing conductor

resistance.

The above observations suggest that to minimise noise we should:

• restrict the bandwidth

• keep temperatures low

• keep resistances low.

FIGURE 12 shows the noise model for an op-amp. Although the noise is

internally generated, it is convenient to represent the op-amp as being noiseless

with its internally generated sources of noise presented at its inputs. The

sources of voltage noise are shown lumped together as single source on the

non-inverting input. The current noise sources are shown on both inputs as

their most significant causes are from the input bias currents.

23



FIG. 12

UNITS OF MEASUREMENT OF NOISE

Manufacturers' data sheets often only quote the op-amp’s noise voltage. The

equation has units of volts but the value is dependent

upon the bandwith. As the operating frequency range depends on the

application, the data sheet quotes the noise voltage density in terms of volts

per root hertz (V/√Hz or VHz–1/2). Typical values of noise voltage for general

purpose op-amps are in the range of 10 to 20 nV/√Hz whereas low-noise

devices can have values of less than 2 nV√Hz. Current noise can vary from

1pA/√Hz for a bipolar input to less than 1 fA/√Hz for a FET device.

Bipolar input op-amps can generate significant voltage and current noise. The

source of their voltage noise is the thermal noise of the input transistors’ base

resistance. The current noise is generated by shot noise from the base-emitter

junction. The current noise only becomes manifest when it generates a voltage

by flowing through a resistance and can be minimised by keeping the input

impedance low. FET op-amps have significant voltage noise but their current

noise is usually insignificant as their bias currents are so low. At present the

best noise performance is achieved with bipolar-input op-amps rather than with

FET inputs. To take advantage of the bipolar’s low noise voltage, however, the

V kTBRthermal = 4

Current noise

–

+

Voltage noise

24



associated circuit's resistances must be kept low. This will create increased

supply current and power disipation. There is nearly always a trade off

between parameters!

Finally, FIGURE 13 shows how the resistors in a simple op-amp circuit add to

the op-amp’s noise. In calculating the total noise performance of the circuit,

the contribution of the thermal noise (VTH1, VTH2, VTH3) from these resistors

must be taken into account along with the op-amps voltage noise VN (current

noise has be ignored).

Self-Assessment Question 2 gives an example of how noise can affect

performance.

FIG. 13

–

+

R3VTHR3

R1VTHR1

R2VTHR2 VN

25



OPERATING TEMPERATURE

The effect of temperature on offset voltages and currents was mentioned in the

discussion of input stages. Some applications will require operation in an

extended temperature environment; say –40°C to +125 °C, as might possibly

be demanded by the needs of the automotive or military industries. If a

temperature stipulation is made then this can considerably narrow the range of

suitable op-amps.

PACKAGING

Op-amps come in all types of packaging, FIGURE 14 shows some

possibilities. The package chosen will depend upon the number of op-amps

required per package (single, dual and quad), the size of the package and the

method of manufacture and soldering.

Some high impedance FET op-amps are packaged in glass-sealed hermetic

packages and can have special problems in terms of track layout and

connection if the device’s properties are not to be impaired.

If a high output power is required then a metal can package might be required

perhaps with an associated heat sink.

FIG. 14

26



COST AND AVAILABILITY

Whilst not an engineering issue, cost is certainly a most practical one,

particularly in volume production. Availability might be limited to existing

stock or to that held by a preferred supplier.

27



________________________________________________________________________________________

SELF-ASSESSMENT QUESTIONS________________________________________________________________________________________

1. Determine the quiescent output of the single-supply op-amp of

FIGURE 15.

FIG. 15

2. An op-amp circuit has a voltage gain of 40 dB, an output voltage swing of

±2.5 V and an equivalent noise voltage input of 1.5 μV. Show if its noise

performance is adequate if its output is presented to a 16-bit ADC.

3. State why a video amplifier might use an uncompensated op-amp.

–

+

C1

R2

10 kΩ

V2

VCC

VCC

10 kΩ

V1

R1

C2

28



4. FIGURE 16 shows TLV2771 op-amp used to interface a piezoelectric

sensor to an ADC in a measurement application. The equivalent circuit of

the sensor is a Norton model where CP = 10 nF and RP = 20 MΩ. The

circuit is to operate off two AA batteries (nominally a 3 V supply).

Also given below are some of the principle features of the op-amp.

State reasons why this op-amp is suited for this application.

FIG. 16

–

+

Cf

VCC = 3V

R2

High slew rate 10.5 V/msWide bandwidth 5.1 MHzHigh output drive1 mA of supply current per channelExcellent dc precisionRail-to-rail output swing360 mV input offset voltage17nV/√Hz input noise voltage0.6 fA/√Hz input noise current2pA input bias current1012 Ω differential input resistanceOperates from a 2.5-V to 5.5-V single-supply voltageMicro power shutdown mode (I

DD<1 mA)

TLV2770 Operational amplifier with shutdown

TLV2771Cpqp Rp

Piezoelectric sensor

10 MΩ

R1

VCC2

Vo = ×VCC

2qp

(Cp + Cc) [ ]R1 + R

2

R1+

29



5. The web site address below gives access to an example of an op-amp

design aid by Analogue Devices (there are several such web examples

from various manufacturers).

http://designtools.analog.com/dtAPETWeb/dtAPETWizard.aspx

Access the site and use its ‘Parametric Search’ feature to find an op-amp

(by Analogue Devices of course!) that most closely matches the criteria

listed below.

Supplyvoltage

Vos

Ib

Smallsignal

bandwidth

Slewrate

Quiescentcurrent

perpackage

Amplifiersper

package

V Noisedensity

Unitpric(US)

= 5 V 10 mVor

better

100 nAor

better

1 MHzat least

10 V/μsor

better

2 mAor better

2 10nV/√Hzor better

$2 USor less

for1000

30



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NOTES________________________________________________________________________________________

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31



________________________________________________________________________________________

ANSWERS TO SELF-ASSESSMENT QUESTIONS________________________________________________________________________________________

1. Starting from first principles:

• V2 = A(V+ – V–) where A is the open loop gain of the op-amp.

• As A is very large, V+ ≈ V–.

• But V+ = VCC/2, so V– = VCC/2.

• Also for dc values, V– = V2.

• Hence V2 = VCC/2.

The quiescent output has been fixed at half the supply voltage as required

by the simple expedient of using a potential divider to half VCC.

2. 40 dB corresponds to a gain of 100.

Thus the output noise is 100 × 1.5 μV = 150 μV.

The op-amp has an output span of 5 V.

The ADC can resolve down to 5/216 = 76 μV.

Thus the least two significant bits of the ADC's output would be swamped

by noise.

The noise performance of the op-amp is inadequate for this application.

N.B. This simple circuit does though have serious drawbacks. The most

obvious one is that of power supply rejection. Any variation in VCC is

directly reflected in the bias applied to the non-inverting input. This will

then appear as a change in the output voltage. Less obviously, the circuit

can become unstable if driving a low output resistance. The associated

large output current can cause fluctuations in VCC which then is fed back to

the input via the potential divider. (In audio amplifiers such oscillations are

referred to as 'motor-boating' because of the resulting audible 'put-put'

output noise.)

32



3. A video amplifier requires a wide bandwidth and also a flat amplitude

response. The response of an uncompensated op-amp can be better

optimised to meet these criteria than a compensated op-amp.

4. The most salient point about the signal source is that it is of a very high

impedance. The table below shows the op-amp's properties and

comments made, where possible, as to its suitability.

5. The spreadsheet below has been generated by the selection software. Not

all the selections meet the criteria exactly, these have been highlighted. If

these op-amps are ruled out then there are just three left in the running:

AD8646, AD8616 and AD8647. The designer would now have to make a

decision based upon general performance, cost, packaging options,

availability and perhaps personal preference based upon experience.

PropertyHigh output drive1 mA of supply current per channelExcellent dc precisionRail-to-rail output swing2 pA input bias current360 mV input offset voltage17nV/√Hz input noise voltage0.6 fA/√Hz input noise current1012 Ω differentialinput resistanceOperates from a 2.5V to 5.5Vsingle-supply voltageMicro power shutdown mode (I

DD<1 mA)

CommentPossible advantage if driving an ADCLow power consumptionOf value in this measurement applicationImportant in the low-voltage applicationIndicates a high input impedanceWould need more informationWould need more informationWould need more informationImportant for a voltage amplifierconnected to a high impedance sourceImportant for battery operation

Important for battery operation

???

33



Part#

QueryParameter

AD8646AD8616

AD8647OP262

AD8648AD8615

AD8656

AD8652AD8692AD8606AD8666

Smallsignalbandwidth

1 MHz

24 MHz24 MHz

24 MHz15 MHz

22 MHz23 MHz

28 MHz

50 MHz10 MHz10 MHz4 MHz

Vos

=>10 mV/μs

600 μV23 μV

600 μV25 μV

700 μV80 μV

50 μV

100 μV400 μV20 μV600 μV

Ib

=<100 nA

300 fA200 fA

300 fA260 nA

200 fA200 fA

10 pA

1 pA200 fA200 fA200 fA

V noiseDensity

=<10nV/√Hz

6nV/√Hz6nV/√Hz

6nV/√Hz9.5nV/√Hz

6nV/√Hz7nV/√Hz

4nV/√Hz

5nV/√Hz8nV/√Hz6.5nV/√Hz10nV/√Hz

Vcc

- Vee

=5 V2.7 V-5.5 V2.7 V - 6 V2.7 V5.5 V2.7 V- 12 V2.7 V5.5 V2.7 V - 6 V2.7 V5.5 V2.7 V5.5 V2.7 V - 6V2.7 V - 6V5 V - 16 V

Iq

peramplifier

=<2 mA

2 mA2 mA

2 mA775 mA

2 mA1.7 mA

4.5 mA

9 mA1.05 mA1.2 mA1.55 mA

Amplifiersperpackage

2

22

22

41

2222

US price1000-4999

=<$2

$0.61$1.29

$0.71$1.47

$0.88$0.76

$1.11

$1.99$0.64$1.19$0.93

Slew rate

=>10 V/μs

11 V/μs12 V/μs

11 V/μs13 V/μs

12 V/μs12 V/μs

11 V/μs

41 V/μs5 V/μs5 V/μs3.5 V/μs

34



________________________________________________________________________________________

SUMMARY________________________________________________________________________________________

In the selection of an op-amp for a given application there will generally be

certain constraints that will narrow the selection process. For example, if the

design demands a 2.5 V single-supply then this will dramatically limit the

available choice. Having found all the devices that meet this criterion, the task

now is to make a decision based upon the other design criteria (slew rate, noise

performance, etc.) and upon cost and any packaging requirements.

The table below attempts to summarise some of the factors that can be of

influence in the choice of an op-amp. There is no particular factor that is

dominant – this will depend upon the application and the associated system

specification.

SupplySingle-supplyDual supplyBattery/mains

Signal sourceImpedanceCurrent/voltageSignal magnitude

LoadImpedanceRail-to-railVoltage swing

ParametersBandwidthNoiseSlew rateQuiescent currentGain

OtherCostpackageAvailabilityTemperature

35



________________________________________________________________________________________

APPENDIX 1: SOME OP-AMPS TERMINOLOGY________________________________________________________________________________________

36



________________________________________________________________________________________

APPENDIX 2: SOME TRANSDUCER CHARACTERISTICS________________________________________________________________________________________

Transducer

Thermistor

Thermocouple

Resistance temperaturedetector (RTD)(In bridge cicuit)

Float level sensor

Load cell(Strain gauge)

Photodiode

Transducer

ΔR: Resistance changes with temperatureR=k exp(–ve temp coefficient)Highly non-linearΔV: Thermoelectric voltageV

T = kT

10 μV/°C to 100 μV/°CLinearΔR: Resistance changes with temperatureR = R

0(+ temp coefficient)LinearΔR: Resistance changes with levelOutput of mV to several voltsLinearΔR: Resistance changes with applied strainΔR = eGR0

LinearΔI: Current changes with light intensity1 pA to 1 μA

BT( )

( )1 + αT + βT 2)

Typical Outputresistance50 Ω to 1 M Ω

20 Ω to 20 k Ω

100 Ω to 1000 Ω

100 Ω to 2 k Ω

120 Ω to 1 k Ω

109 Ω

37



module title : operational amplifiers topic title : op-amp ... · 2/03/2016 · • indentify the...

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