TEE 3111 LINEAR INTEGRATED CIRCUITS

INTRODUCTION

INTEGRATED CIRCUITS are tiny electronic circuits used to perform a specific electronic function such as amplification. It is usually combined with other components to form a more complex system.

Several integrated circuits (ICs) are made at a time on a thin wafer several centimeters wide and the wafer is subsequently sliced into individual ICs called chips.

In large scale integration (LSI) as many as 5000 circuit elements, such as resistors and transistors are combined in a square of silicon measuring about 1.3 cm on a side.

Larger-scale integration can produce a silicon chip with millions of circuit elements.

Chips are assembled into packages containing external leads to facilitate insertion into printed circuit boards for interconnection with other circuits or components.

In consumer electronics, ICs have made possible the development of many new products, including personal calculators and computers, digital watches and video games. They have also been used to improve or lower the cost of many existing products, such as appliances, television, and radios.

They are used extensively in industry, medicine, traffic control, environmental monitoring, and communications.

The ICs era begins in the early 1960s. Around 1965, the first integrated general-purpose operational amplifier was introduced. This low-cost, highly versatile device incorporated nine transistors and twelve resistors in a small package. It proved to have many advantages over comparable discrete component circuits in terms of reliability and performance.

Following this first IC, a huge variety was developed and broadly ICs today are classified into two groups: digital ICs and linear ICs.

This course was designed to familiarize the students with operational amplifiers and the main types of linear integrated circuits.

OPERATIONAL AMPLIFIERS

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Early operational amplifiers (op-amps) were used primarily to perform mathematical operations such as addition, subtraction, integration and differentiation, hence the term operational. Today’s devices are solid-state inexpensive devices with very high amplification capabilities.

A typical general purpose op amp is a direct coupled device able to accept either dc or ac input signals and process these voltages to produce a variety of both linear and nonlinear outputs.

SYMBOL AND TERMINALS

The standard symbol shown in Fig.1 (a) is a triangle that points in the direction of the signal flow. It has two input terminals, called the inverting input (-) and the noninverting input (+), and one output terminal. An op-amp operates with two dc supply voltages, one positive and the other negative, as shown in Fig.1 (b). For simplicity these terminals are left off the schematic symbol, but are understood to be there.

Each op amp has a part identification number (PIN), usually placed within the triangular symbol (Fig.1.1 (a)). The PIN refers to a particular op amp with specific characteristics.

Fig.1.1

THE IDEAL OP-AMP

In order to get an idea what an op-amp is an ideal model is considered. A practical op-amp falls short of the ideal characteristics, but it is much easier to understand and analyze the device from ideal point of view.

The characteristics of an ideal op-amp are:

1. Infinite input impedance (resistance).

The results of infinite resistance are:

- When an ideal op amp is powered in draws no supply current. The device wastes no power and therefore its temperature never increases.

- The op-amp can be connected directly to any circuit without fear of overloading.

2. Zero output impedance (resistance).

Therefore any load resistor could be connected.

3. Infinite voltage gain AOL.

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_

+

+V

-V

_

+

Invertinginput

Noninvertinginput

Output741 C

4. Infinite bandwidth.

All input voltage frequencies from 0 Hz to ∞ Hz are equally amplified by the device to produce an output voltage.

The equivalent circuit of an ideal op-amp shown in Fig.1.2 illustrates the above characteristics.

Fig.1.2

THE PRACTICAL OP-AMP

Internal Structure

The simplified internal structure of a practical op-amp is shown in Fig.1.3. It consists of three stages namely input, output and intermediate stage.

Dc power is obtained from a bipolar supply and applied to the external power supply terminals. Dc power is thus connected to each internal stage of the op-amp. Depending on the application input signals V(+) and V(-) can be applied to either input terminal or simultaneously to both inputs. The output voltage is monitored across the load resistor RL connected to the output terminal. It will depend on the op-amp’s input signals and characteristics of the op-amp.

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_

+

A.oVin

Fig.1.3

The input stage of the op-amp of Fig.1.3 is called differential amplifier. It has very high input resistance characteristics and a large voltage gain. When input signals V(+) and V(-) are applied, the difference voltage Ed is amplified by the gain of this stage and appears as the output voltage V1.

Level-Shifter Intermediate Stage performs two major functions. First, it shifts the dc voltage level of the voltage V1 to a value required to bias the output stage. Second, the level shifter allows the signal V1 to pass nearly unaltered and become input signal V2 for the output stage.

The Output Stage is a push-pull amplifier. The signal voltage V2 is coupled directly into the output stage. The most common output stage is a PNP/NPN push-pull transistor configuration. It allows the op amp to have a very low output resistance.

Packages

Three of the most common packages for op amps and integrate circuits are shown in Fig.1.4.

The metal can package (Fig.1.4(a)) is used where space is most important. The silicon chip is bonded to the bottom plane of the case and internally connected external leads are attached to it. As the name shows, this type of package is made out of metal and has high power dissipation abilities.

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Metal can packages are available with 3, 5, 8, 10 and 12 leads. A small metal tab is placed adjacent to one lead on the package as a lead designator. The leads are numbered anti-clockwise from the tab.

(a) (b) (c)

Fig.1.4

Figure 1.4(b) illustrates the dual-in-line (DIP) package. This case, made out of plastic or ceramics, is currently the most widely available and the most popular package configuration. The reason for that is ruggedness, ease of breadboarding, and adaptability to printed circuit board packaging.

The DIP package is available with 8 pins, called mini-DIP, as well as 14- and 16-pin dual-in-line packages.

As viewed from the top down, a small notch or dot is placed on one end of the DIP case to identify pin 1.As with the metal case, the pin designation is obtained by counting anti-clockwise from pin 1.

A surface-mounted technology-style package (SMT) is shown in Fig.1.4(c). These packages provide a higher circuit density.

Packing Code

Useful information can be obtained from the packing code stamped on the case by the manufacturer, Various manufacturers use different ways to code information on the package. Fig. 1.5 shows a typical packing code used by Motorola.

Fig.1.5

The top row of the code gives the following information:

- The letter code for the manufacturer, in this case MC indicates that Motorola. The letter code for Burr-Brown is BB, LM for National Semiconductors, etc.

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MC1741PC3296

- 1741 (or 741) is a PIN number identifying the device as a general purpose op-amp

- Letter P indicates a plastic 8-pin mini-DIP package- Letter ‘C’ stays for “Commercial Range”, indicating that the op-amp will operate

reliably in the temperature range 0 to 70oC. Letter ‘I’ instead of C indicates “Industrial Range” of -25 to + 80 oC, and letter ‘M’ indicating “Military Range” of -55 to +125 oC.

The second row gives the date of fabrication. The first two digits stay for the week, and the second two for the year of fabrication.

PIN Configuration

In order to properly connect an op-amp or a linear integrated circuit one should know the terminal configuration of a device. The name of each terminal (or pin) is given by the manufacturer. The pin-configuration for 741 general-purpose op-amp is shown in Fig.1.6.

Fig.1.6 Fig.1.7

Powering the Op Amp

Most op amps require both negative and positive power supply voltages for their operation. Such are obtained from a bipolar power supply. It is created by using two separate dc voltage sources, as illustrated in Fig.1.7. General purpose op-amps (like 741) are designed to operate with typical supply voltages of 15 V. Suitable voltages are 12 V or two 9-V batteries. The maximum specified power supply voltage for commercial-grade op amps is 18 V.

An op-amp is properly connected with pin 7 wired to VCC and pin 4 wired to VEE. With power applied in this way, the op-amp draws a supply current Isupply into pin 7, to establish bias currents for the internal transistors. For 741 general-purpose op amp the supply current is of 1.7 mA. If no other connections are made the device draws power, called power at idle:

Pidle = Vsupply x Isupply = 30 V x 1.7 mA = 51 mW

All this power is converted directly into heat. With typical power rating of 1W, the device is quite save under ‘idle’ conditions.

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1

2

3

4

8

7

6

5

Off-set Null

Inverting Input

Off-set Null

Non-inverting Input

VEE

VCC

NC

Output

VCC = +15 V

VEE = - 15 V

Output Terminal

A load on the op amp is modeled by RL (Fig.1.8(a)). One end of RL is connected to the output terminal 6of the op-amp. The other end is grounded. When Vo goes positive with respect to ground the output terminal sources current Io to the load. This load current is supplied by the positive supply VCC. Internally, the op- amp sources a current to the load when the NPN transistor of the push-pull output stage is conducting.

(a) (b)

Fig.1.8

When Vo goes negative as in Fig.1.8(b), the output terminal “sinks” a current Io from the load. This load current is supplied by the negative supply VEE. Internally, the op-amp sinks a current when the PNP transistor is conducting. The polarity of the voltage and the direction of the current of the output terminal are under control of the inputs, as will be discussed later.

The output voltage Vo can never go above the positive supply voltage VCC or below the negative supply voltage VEE. However the NPN and the PNP transistors need about 2V voltage drop to stay “On”. Therefore, the output voltage can never exceed (Vsupply – 2V). The minimum and maximum output voltages are called positive and negative saturation, +VSAT and VSAT. For a typical supply of 15 V, VSAT = 13V.

The output current is also limited. For example, manufacturers of general-purpose op amps, such as 741 guarantee output voltage swing of 12 V into loads of 10 k or greater. If RL is an open circuit and the load current as well as the output current will be of 0 A. With a load of 10 k, the output current will be 1.2 mA. This is the recommended value output current for best performance.When RL is reduced to 2 k, only 10 V swing is guaranteed, and the output current is 5 mA. This is the maximum current that the device should supply.

In practice a short-circuit connection at the output could occur. As this is equivalent to RL = 0 , the device could be destroyed because of the excessive current drawn. Op-amps like 741 are provided with short circuit protection to avoid destruction

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RL

+

_

Vo

6

NPN

PNP

On

Off

Io

(sourcing)

RL

_

+

Vo

6

NPN

PNP

Off

On

under such conditions. When the load resistance drops to a certain value, the output starts supplying a constant current between 15 and 30 mA. This current is called short circuit current , ISC. For 741 ISC = 25 mA for load resistances of 520 and less.

Under short-circuit conditions an op amp acts as a constant-current source, where the current supplied is independent of the load resistor value.

The power dissipated under short-circuit conditions is the power dissipated by the NPN or PNP transistor plus the power at idle. It is given as:

PD(SC) = VCE x ISC = 15V x 25 mA + 50 mW = 425 mW,

Providing the supply voltage is 15 V. Obviously, the device is protected from destruction.

Input Terminals

The parameters associated with the input terminals are:

Differential Input Resistance. This is the resistance (Ri) measured between the inverting and non-inverting terminal of an op amp, with one of them grounded. For 741 op amp RI = 2 M, but op amps whose input stage is built on FET transistors instead of BJTs, have input resistance of order of 1012 .

Input Bias Current. It is defined as the average value of currents flowing into both inverting IB(-) and noninverting IB(+) input terminals,

It ranges from 5 pA for FET input op amps to 80 nA for 741 op amp.

Generally, it can be assumed that the inputs of an op amp draw no current, or the bias current is of 0 A.

Input Offset Current Ideally, the two input bias currents (IB(+) and IB(-)) are equal, and thus their difference is zero. In a practical op-amp, however, the bias currents are not exactly equal. The input offset current is the difference of the two:

Its typical value is of about 20 nA.

Input Offset Voltage, Vos It results from the inability of ideally matched input transistors, therefore it exists in every practical op amp. It ranges from 2 to 6 mV for 741, but for some models it could be as small as 10 V. The effect of the input offset voltage is that the output can go to either +VSAT or - VSAT even when the input terminals are grounded.

For proper operation, op amps have a provision for eliminating the effect of the input offset voltage. It is done by connecting a 10 k variable resistance between the offset terminals at pins 1 and 5 and adjusting it until the output voltage is brought to 0 V (Fig.1.9)

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+

+VCC

7

4

- VEE

2

3

6

15

10 k

Fig.1.9

Differential Input Voltage, Ed. Op amps amplify the input voltage Ed, which is the difference voltage between the noninverting (+) and the inverting (-) inputs.

Ed = V(+) – V(-)

V(+) is the voltage measured at the noninverting input with respect to ground. V(-)

is the voltage measured at the inverting terminal with respect to ground.

Both input terminals can have voltages that are positive, negative, or at ground potential.

For proper operation the input voltages applied to the inverting and noninverting terminals should not exceed the saturation levels ( 13 V for 15 V supply).

Open-Loop Voltage Gain

The open-loop voltage gain AOL is the gain by which differential voltages are amplified by an op amp. It is specified by the manufacturer. For 741 AOL = 200 000. Thus the output voltage is given as:

Vo = AOL x Ed

However it must be kept in mind that Vo can never exceed VSAT and Io can never exceed ISC.

Common Mode Rejection

It is the ability of an op amp to reject unwanted (noise) signals. Such signals are of the same magnitude and polarity and appear on the both inputs of the device. Common mode rejection means that such signals will not appear on the output to distort the desired signal. Thus the output due to common mode input is ideally of 0 V.The measure of an op amp’s ability to reject common mode signals is a parameter called common-mode rejection ratio (CMRR):

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or in dB,

where Acm is the common mode gain of an op amp, which is very small (much less than one) for practical devices. Ideally it is zero.Thus, the higher CMRR is, the better.

Frequency Response Characteristics

A practical op amp does not amplify all frequencies by the same voltage gain. Fig.1.10 shows the different amplification of 741 for different frequencies.

Fig.1.10

At very low frequencies, the open-loop gain is constant at 200 000. As the input signal frequencies increase above 5 Hz, AOL decreases at a constant rate until the gain is 1 or 0 dB at some particular frequency fT, called unity-gain frequency.

The frequency fT defines the unity-gain bandwidth of the device. For 741 this value is 1 MHz.

Above 5 Hz, the product of AOL and the frequency is constant and equal to the unity-gain frequency:

fT = AOL x f,where f is the input signal frequency.

Slew Rate

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Fig.1.10

Slew rate, SR, is the speed at which the output voltage of an op amp can change in response to an abrupt change of the input voltage:

.

The maximum slew rate of 741 is 0.5 V/s, which classifies it as a relatively slow device. Distortion will result if the output is to change faster than the maximum slew rate.

Op amps with slew rate of 10 V/s, 60 V/s and as high as 375 V/s are available.

Equivalent Circuit

The equivalent circuit of a practical op amp is shown in Fig.1.11. It illustrates the very high but finite input resistance and the small, but different of 0 output resistance of the voltage source.

Dual and Quad Operational Amplifiers

The variety of op-amp manufactured nowadays is enormous. The demands for better performance and reduced sizes of electronic system led to larger scale of integration and improved parameters of ICs.

Very often more than one op-amp would be housed in a same package. Usually two or for op-amps of the same type would be packed in the same case. In the case of two, the IC is called dual, and in the case of four, the IC is called quad.

The two or the four op-amps can be wired and used independently from each other.

Examples for dual and quad op-amps are 727 and 747.

FET –Input Operational Amplifiers

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Zin

Zout

_

+

VinVout

AOL Ed

Fig.1.11

In same cases, as stated before, the input stage of an op-amp is implemented on Field-Effect Transistors instead of Bipolar-Junction Transistors. It leads to improved parameters, such as increased input resistance and reduced offset current Example for a FET-input op-amp is LF 356.

Data Sheet

In order to use an IC properly, its pin configuration, parameters and their ranges should be known. Such information is obtained from the manufacturer’s Data Sheet.

A typical data sheet includes:

- Short list of main features;- Pin configuration (also called connection diagram);- Internal structure;- Specification tables for the various modifications;- Typical performance curves;- Mechanical drawings of the package (packages);- Typical applications.

There could be variations in presentation of the information from one device from another or from one manufacturer to another, but the format is more or less the same.

At the end of this chapter shortened versions of data sheets of some common, general-purpose op amps are given. Those should be carefully studied and compared.

At should be noted that the text refers to MC1741 and the data sheet included is AD 741. It is not a problem, for one is manufactured from Motorola and the other from Analog Devices, but the parameters are identical.

TUTORIAL SHEET 1

A. FILL-IN BLANKS

1. The op-amp’s functional block diagram shows three stages: _____________, _______________, and _______________.

2. The op-amp symbol is a _______________ that points in the direction of signal flow.

3. A genera-purpose op-amp such as the _____________ has a minimum of ___________ terminals.

4. An ideal op-amp has _______________ input resistance and _________ output resistance.

5. General-purpose op-amps use _________________ power supplies whose voltage is measured with respect to ground (power supply common).

6. Differential input voltage Ed is the difference between ________ and _________. Both are measured with respect to _____________.

7. Idle power drawn by a 741 has a typical value of ___________.8. The op-amp limits output current to a maximum of _____________.9. The maximum values for output voltage are limited to values called

___________ and ____________.

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B. TRUE/FALSE

1. _________ When RL = 200 , IL = Vo/ RL.2. _________ When RL = 200 , Vo = ISC x RL.3. _________ When Ed = 0.1 V and V = 15 V, output Vo goes to 20 000 V.4. _________ When Ed = - 0.1 V and V = 15 V, Vo = - 13 V.5. _________ When the output of an op –amp is short circuited it is heated by an

idle power of 50 mW plus a load currennt power of ISC x VCC.

C. CIRCLE THE CORRECT ANSWER

1. Typical bias currents for the 741 are (50 nA, 5nA).2. Op-amps with FET inputs have (higher, lower) input resistance than BJT

inputs.3. The op-amp’s output is a (single-ended, differential) voltage.4. Input voltage at the non-inverting input should never go (above VCC, above

VEE).5. Open-loop gain of an op-amp (decreases, increases) by 20 dB as frequency

is decreased by a decade.D. MATCHING EXERCISE

1. ________ 13 V a. Open – loop gain2. ________ 50 mW b. ISC

3. ________ 50 nA c. + VSAT

4. ________ 25 mA d. Ibias

5. ________ AOL e. Idle power

E. PROBLEMS

1. (a) Name the three basic stages internal to a general-purpose op amp.(b) Describe briefly the function of each stage.

2. Draw the symbol of an op-amp and label each terminal.3. Name the three package styles used for housing ap-amps.4. If the package code on a mini-DIP package is MC 1301 PM/1297, list the

meanings of the code.5. The following questions apply to an ideal op amp:6.

(a) What is the ‘input resistance’?(b) How much current do the input terminals draw?(c) What is the ‘output resistance’?(d) What is the voltage gain AOL?(e) What is the bandwidth?

7. Determine the power dissipated by a 741C op amp at idle if the bipolar supply used is:(a) 9 V; (b) 12 V; 18 V. In each case the supply current remains the same.

8. The output voltage is a negative value:(a) Is the op amp sinking or sourcing current?(b) Which push-pull transistor in the output stage of the op amp is not

conducting?9. Repeat Problem 8. for a positive value of the output voltage.10. Estimate the value of both + VSAT and – VSAT if the power supply for a 741C

op-amp is:(a) 9 V; (b) 12 V; 18 V.

11. (a) What is the short-circuit current for a 741 op amp?

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(b) Will ISC flow if RL = 300 ?(c) What will Vo equal if RL = 100 ?

12. What is the value of RL that will just initiate short-circuit protection for the 741 if the dc supply is 15 V?

13. Determine the total power dissipated by a 741 op amp in short-circuit protection when RL is:(a) 500 ; (b) 300 ; (c) 100 .(d) Does the temperature of the op amp increase or decrease as RL is lowered?

14. What is the differential input resistance Ri of a 741op amp?15. With zero volts on both inputs, an op-amp idealy should have an output:

(a) equal to the positive supply voltage; (b) equal to the negative supply voltage;(c) equal to 0;(d) equal to Ed x AOL.

16. Assume that the power to a 741 op-amp is VCC = +12 V and VEE = - 12 V. What is the maximum value of input signal that can be applied to either input terminal?

17. Determine both the magnitude and polarity of Ed if V(+) = + 1 V and V(-) is:(a) 0 V; (b) + 13 V; (c) – 0.6 V.

18. Repeat Problem 17 if V(+) is changed to – 0.3 V.19. Solve for the output voltage Vo for each of the values of Ed in Problems 17

and 18. Assume that the 741 op-amp is powered by a 18-V supply.20. Solve for the output voltage Vo for each of the values of Ed in Problems 17

and 18. Assume that a 741-op amp is used with a load of 150 .21. Of the values listed, the most realistic value for AOL is:

(a) 1; (b) 3 000; (c) 90 dB; (d) 100 000.22. Use Fig.1.9 to determine AOL if the input signal frequency is:

(a) 100 Hz; (b) 10 Hz.23. A certain op-amp has bias currents of 50 A and 49.3 A. The input offset

current is:(a) 700 nA; (b) 700 A; (c) 99.3 A; (d) 49.65 A; (e) none of these.

24. Find the time for Vo to change from + 6 V to –6 V if the slew rate of the op-amp is:(a) 10 V/; (b) 60 V/; (c) 375 V/.

25. The output of a particular op-amp increases 8 V in 12 s. The slew rate is:(a) 96 V/s; (b) 0.67 V/s; (c) 1.5 V/s; (d) none of these.

26. The purpose of offset nulling is to:(a) reduce the gain;(b) equalize the input signals(c) zero the output error voltage(d) b and c

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