OPERATIONAL AMPLIFIER

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O P E R A T I O N A L A M P L I F I E R

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On successful completion of this module, the student will be able to:

• Understand the internal block diagram of an Op-Amp.

• Describe the symbol and package types of an Op-Amp.

• Explain the function and locate the terminals of an Op-Amp.

• Know how to power up the Op-Amp chip.

• Recognize the connection of Op-Amp configurations.

• Compute the gain of different Op-Amp circuits.

• Explain briefly the operation of Op-Amp basic circuits.

Open-loop comparator circuit.

Closed-loop inverting amplifier circuit.

Closed-loop noninverting amplifier circuit.

• List the applications of different configuration circuits.

OBJECTIVES

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1. INTRODUCTION

The term Operational amplifier (Op-Amp) was originally used to describe a chain of high performance DC amplifiers that were used as a basis for the analogue type computers long ago. The very high gain Op-Amp of our days is a solid-state integrated circuit (IC) that is used in signal processing circuits, control circuits, and instrumentation. Of all analog (IC’s), the Op Amp is the most widely used in the widest variety of electronic circuits.

2. Operational Amplifier Block Diagram

The Op Amp consists of three stage amplifier circuits; all are interconnected and contained in a single IC. Referring to the block diagram in Figure 1, these three stages are:

a) First stage (Differential Amplifier): gives the Op-Amp its high input impedance. b) Second stage (Voltage Amplifier): gives the very high gain characteristics. c) Third stage (Emitter-Follower): gives the low output impedance characteristics.

3. Operational Amplifier Characteristics

Combined, these three stages circuits give the Op Amp its key characteristic: a) Very high input impedance.

b) Very low output impedance. c) Very high gain.

Therefore Op-Amp is a differential, voltage amplifier, high gain amplifier.

It is a differential amplifier: because it amplifies the difference between two voltages. It is a voltage amplifier: because the input and the output are voltages.

It is a high gain amplifier: because the gain is very high typically, over 100,000. Figure 1 Op Amp Block Diagram

Inverting Input (- VIN) +V - V Output Noninverting Input (+ VIN) Differential Amplifier Voltage Amplifier Output Amplifier

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4. Operational Amplifier Package

The entire op-amp circuit usually is placed within one of three basic packages, these are: • Dual In-Line through-hole Package (DIP) typically has 8 or 14 pins as in Figure 2a. • DIP Surface-Mount package (SMT) typically has 8 or 14 pins as in Figure 2b. • The TO-5 metal-can package is available with 8, 10, or 12 leads as in Figure 2c.

4.1 Op Amp Pins Identification

Like all ICs a notch or a dot is used to ease the pins-identification and placement of an Op-Amp. Figure 3(a) and (b) shows the most used markings system and are read as follows:

• The dot in one corner is located next to Pin-1.

• The notch at one end is located between Pins-1 and Pins-8 (on the left is Pin-1). • The rest of the pins are numbered proceeding anticlockwise from Pin 1.

Figure 3 Op Amp pins Identification

1 2 3 4 5 6 7 8 +VCC -VEE - + 741 b) Notched Package 1 2 3 4 5 6 7 8 +VCC -VEE - +

a) Dot marked Package

Figure 2 Op Amp packages

(b) OPA547FKTWT DIP SMT package (a) Op Amp 741

8-pins DIP package

(c) TO-5 741 8-Leads package

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4.2 Op Amp Pins Description

The pin connections of nearly all Op Amps are standard. Figure 4 shows the pin configuration of the type 741, the most common one being used worldwide.

• Pin 1 and Pin 5: Offset null input, are used to remove the Offset voltage. • Pin 2: Inverting input (-VIN), signals at this pin will be inverted at output Pin 6.

• Pin 3: Noninverting input (+VIN), signals at pin 3 will be processed without inversion. • Pin 4: Negative power supply terminal (-VEE).

• Pin 6: Output (VOUT) of the Op-Amp

• Pin 7: Positive power supply terminal (+VCC)

• Pin 8: No connection (N\C), it is just there to make it a standard 8-pin package

4.3 Op Amp Symbols

Figure 5(a) and (b) shows the triangle-shaped amplifier symbol used to represent the Op- Amp in an electronics schematic diagram. It comprises a total of 5 pins, as follows:

•

Two inputs (–VIN and +VIN) and one output (VOUT).

• Two power supply connections (+VCC ⇒ +VS) and (–VEE ⇒ –VS).

Figure 4 Op Amp pins Description 1 2 3 4 5 6 7 8 Offset Null -VEE N / C Output 741 +VCC Noninverting Input +VIN Inverting Input -VIN Offset Null

Figure 5 Op Amp Schematic Symbols -VIN + - +VIN VOU -VIN + - +VIN VOU +VS -VS

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5. Op Amp’s Power Supply Requirements

Most op-amp circuits require a dual power supply having two opposite polarity voltages (+VS & –VS) together with common ground as shown in Figure 6. Some circuits can be designed to work from a single supply as shown in Figure 7.

To power the chip from a dual power supply make the connections as follow: • The positive voltage of the supply (usually +5V to +15V) to pin-7 (+VS) of the chip. • The negative voltage of the supply (usually -5V to -15V) to pin-4 (-VS) of the chip.

To power the chip from a single power supply make the connections as follow: • The positive voltage of the supply (+VS) to pin 7 and pin 4 grounded (Figure 7a); or • The negative voltage of the supply (-VS) to pin 4 and pin 7 grounded (Figure 7b).

What is the advantage of using dual power supply?

Using dual power supply will let the op amp to output true AC voltage. For instance having (+15V & –15V), will allow the output to swing between (+15V & –15V) as shown in Figure 8a; instead of +30V-to-0V as in the case of single power supply as shown in Figure 8b.

Figure 7 Single Supply Voltages connection -VIN + - +VIN VOUT -VS 7 4 -VIN + - +VIN VOUT +VS 7 4

(a) Single Positive Voltage (b) Single Negative Voltage

0V

+15V

-15V Output

Figure 8a Op Amp powered from Dual supply

+30V

0V Output

Figure 8b Op Amp powered from Single supply 30 V

30 V

Figure 6 Dual Supply Voltages connection -VIN + - +VIN VOUT +VS -VS 7 4 Common Ground

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6. OP-AMP CONFIGURATION

Feedback refers to connecting the output of the op-amp to its input, usually through resistors. According to the type of feedback employed, there are three basic circuit configurations shown in Figure 9:

a) Op Amp without Feedback (Open-loop comparator circuit). b) Op Amp with Negative Feedback.

c) Op Amp with Positive Feedback.

6.1 COMPARATOR

Figure 10a shows an op-amp as a comparator. The comparator is an op-amp configuration without any feedback. Its function is to compare two voltages and produce a signal that indicates which voltage is greater. Refer to Figure 10b; due to very large open-loop gain of an op-amp (AO), any difference (∆VIN) will always saturate the output (VO) at either of the power supply rails, (+VS) or (–VS), as follows:

•

When +VIN > –VIN ⇒ ∆VIN is positive ⇒ the output saturate at +VS ⇒ VO = +VS. • When +VIN < –VIN ⇒ ∆VIN is negative ⇒ the output saturate at –VS ⇒ VO = –VS. • VO will change its state (+VS –VS) when ∆VIN changes its sign (i.e. at ∆VIN = 0).

The VO will saturate at (+VS) or (–VS) If –VS ≤ VO ≥ +VS where VO = AO ∆VIN.

Figure 9 Types of Feedback

(a) Without Feedback (b) Negative Feedback (c) Positive Feedback

+VS VO +VIN = –VIN –VS +VIN > –VIN +VIN < –VIN 0 VO +VIN +VS –VS –VIN

(a) Comparator Circuit (b) Comparator Output Figure 10 Op-Amps as Comparator

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6.2 OP-AMP WITH NEGATIVE FEEDBACK

Negative feedback is used stabilize the gain and increase frequency response. The two basic amplifier circuits which utilize negative feedback are:

a) The non-inverting Amplifier. b) The inverting Amplifier.

6.2.1 NONINVERTING AMPLIFIER

Figure 11 shows how an op-amp can be configured as a noninverting amplifier. The input signal is applied to the noninverting input (+VIN). The output is fed back to the inverting input through the feedback circuit formed by RI and RF. The relations are expressed as follow:

6.2.2 INVERTING AMPLIFIER

Figure 12 shows how an op-amp can be configured as an inverting amplifier. The input signal is applied through a series input resistor RI to the inverting input. Also, the output is fed back through RF to the same input. The noninverting input is grounded. The relations are expressed as follow:

The negative sign indicate that the input is inverted at the output and hence the name inverting amplifier.

VO

VIN

RF

R1

Figure 11 Closed-Loop Noninverting Amplifier Circuit

Where; VO = Output voltage VF = Feedback voltage ANI = Noninverting Gain IN F F O NI F IN F IN O R R 1 V V A V R R R V + = =       + = R R -V V A V R R V IN F F O I IN IN F O       = =       − = Where; VO = Output voltage VIN = Input voltage AI = Inverting Gain VO VIN RF RIN

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VO

R2

R3

R1

C1

Figure 13 Astable Multivibrator

VIN

VO

Figure 17 Voltage Follower

6.3 OP-AMP WITH POSITIVE FEEDBACK

Positive feedback is generally associated with oscillation. An op amp can be configured to operate as an oscillator if suitable external components are connected and positive feedback used as shown in Figure 13.

7. OP-AMP APPLICATIONS

7.1 Positive Feedback Typical Application Relaxation oscillator (Astable multivibrator)

7.2 Negative Feedback Typical Application

7.2.1 Noninverting amplifier (Voltage Follower)

Figure 17 shows a noninverting amplifier with a unity gain (A =1) and it is called a VOLTAGE FOLLOWER. It has high input impedance and very low output impedance. It can be used for impedance matching.

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Figure 15 Inverting Op-Amp as Integrator R

C

VIN VO

Figure 16 Inverting Op-Amp as Differentiator C

R

VIN VO

Figure 14 Summing Amplifier

V1 VO V2 V3 0V R1 R2 R3 RF

7.2.2 Inverting amplifier Typical Applications

A) Summing amplifier (Adder).

Figure 14 shows how an op-amp can be connected as an Adder.

B) Integrator

Figure 15 shows how an op-amp can be connected as an Integrator.

C) Differentiator

Figure 16 shows how an op-amp can be connected as a Differentiator.

7.3 Comparator Typical Applications are: • Crossover detectors

• Analog to digital converters (ADC)

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Figure 19

8. PRACTICAL

8.1 OPAMP DUAL POWER SUPPLIES

In general op-amps are designed to be powered from a dual voltage supply.

The dual power supply is a DC source with two-polarity voltages (+V & -V) called the supply rails and one common ground. Sometimes the dual power supply configuration is referred to as a split power supply. Figure 18 shows how the two single power supplies are connected together to form a dual power supply. Figure 19 shows how the common lead is connected to the (+VS and –VS) of the power supplies.

Power Supply Ranges

• Positive Supply Rail (+VS): Typically it range from +5V to +15V dc with respect to ground.

• Negative Supply Rail (–VS): is typically in the range of –5V to –15Vdc with respect to ground.

Task:

1) Connect two-single power supply (of equal output) in series as shown in Figure 19. 2) Take the link between the +VS & the –VS terminals as common (Ground = G). 3) Measure the voltages between (+VS & G), (–VS & G), and (+VS & –VS).

4) Compare these reading to the output of a single power supply. 5) Try to build a dual power supply using an even number of dry cells.

Single Power Supply

Figure 18 Dual Power Supply

Common +15V

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VO -VIN +VIN +VS -VS - +

Figure 20 Differential Amplifier

Common Ground

8.2 Offset Null Adjustment of the µA741

Differential amplifier Theory

Basically an op amp is a differential voltage amplifier; it amplifies the difference between the two input voltages (+VIN and -VIN). Referring to Figure 20, the output VO is given by: VO = AO ∆VIN;

Where AO is the gain without feedback and called the open-loop voltage gain. And ∆VIN = (+VIN) – (–VIN); consequently there are three cases: Case-a: +VIN > -VIN ⇒ VO is positive. Case-b: +VIN < -VIN ⇒ VO is negative. Case-c: +VIN = -VIN ⇒ VO is zero.

Practically “Case-c” is not true, because when +VIN = -VIN = 0, there is a slight

amount of voltage at the output is known as offset voltage as shown in Figure 21.

Then offset voltage can be defined as the slight amount of voltage that appears at the output when the voltage differential (∆VIN) between the input pins is 0 V.

Offset null adjustments differ with the application (i.e. Inverting or Non-Inverting Amplifier). Offset-null potentiometers are not placed on design schematics as they would detract from a design.

Procedure:

1) Figure 22 shows how µA741 is connected for Offset Null Adjustment. 2) Connect the supply voltage (+12V & –12V) to pins 7 & 4 respectively. 3) Make sure that the power is same as the design application.

4) Adjust the 10K potentiometer to its center position.

5) Connect the potentiometer outside leads between pins 1 and 5 of the op-amp. -

+

For (+VIN = -VIN = 0);

VO ≠ 0

VIN = 0

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6) Connect the wiper of the potentiometer to the negative supply voltage. 7) Ensure that input signals are zero by connecting pins 2 and 3 to the ground. 8) Measure the output between pin-6 and ground with a dc voltmeter.

9) Adjust the potentiometer until the output voltage read 0. This is the zero null state.

Note:

This is a recommended null procedure for the uA741 type op-amp. Always look for, and follow the particular procedure as specified by that chip manufacturer. Procedures may become obsolete or updated and changed when improved op-amp versions come on the market.

Figure 22 Offset Null adjustment -VS N/C VO +VS VIN = 0 µ µµ µA741 1 2 3 4 5 6 7 8 V

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8.3

Addition of Signals (Summing Amplifier)

Theory

Suppose that two inputs v1 and v2 are applied to the circuit of Figure 23, and if all resistors have the same resistance value then

VO = - (v1 + v2),

This gives the (negative) sum of the inputs. Additional inputs can be applied and the same rule applies.

Procedure

1) Arrange the circuit as shown in Figure 23 and in the Patching Diagram.

2) With the DC inputs V1 and V2 derived from potentiometers on the OAT343 deck and V3 from an external signal generator.

3) Use the table shown in Figure 24 to note down your observations.

4) Set V3 to zero and adjust V1, V2 to the pairs of values set out in the table of Figure 24 and take measurements to complete the table.

5) Apply +8 V to V1 and V2 and measure the output voltage.

It is possible to avoid the amplifier limiting addition result for any combination of values V1, V2 within the range ±10V by choosing either:

R1 = R2 = 200 kΩ, R0 = 100 kΩ R1 = R2 = 100 kΩ, R0 = 50 kΩ

In both cases the gain for signals V1 and V2 will be reduced to – (1/2)

Results Table V1 V2 V1 + V2 Output (VO) +10V -8V +2V +10V -2V +8V -2V -8V -10V +8V -8V 0

Figure 23 Summing Amplifier

OPERATIONAL AMPLIFIER

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