OPERATIONAL AMPLIFIER

Transcription

1 MODULE3 OPERATIONAL AMPLIFIER Contents 1. INTRODUCTION Operational Amplifier Block Diagram Operational Amplifier Characteristics Operational Amplifier Package Op Amp Pins Identification Op Amp Pins Description Op Amp Symbols Op Amp s Power Supply Requirements OPAMP CONFIGURATION COMPARATOR OPAMP WITH NEGATIVE FEEDBACK NONINVERTING AMPLIFIER INVERTING AMPLIFIER OPAMP WITH POSITIVE FEEDBACK OPAMP APPLICATIONS Positive Feedback Typical Application Negative Feedback Typical Application Noninverting amplifier (Voltage Follower) Inverting amplifier Typical Applications Comparator Typical Applications are: PRACTICAL OPAMP DUAL POWER SUPPLIES Offset Null Adjustment of the µa Addition of Signals (Summing Amplifier) INSTITUTE OF APPLIED TECHNOLOGY 1

2 OBJECTIVES On successful completion of this module, the student will be able to: Understand the internal block diagram of an OpAmp. Describe the symbol and package types of an OpAmp. Explain the function and locate the terminals of an OpAmp. Know how to power up the OpAmp chip. Recognize the connection of OpAmp configurations. Compute the gain of different OpAmp circuits. Explain briefly the operation of OpAmp basic circuits. Openloop comparator circuit. Closedloop inverting amplifier circuit. Closedloop noninverting amplifier circuit. List the applications of different configuration circuits. INSTITUTE OF APPLIED TECHNOLOGY 2

3 1. INTRODUCTION The term Operational amplifier (OpAmp) 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 OpAmp of our days is a solidstate 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 OpAmp its high input impedance. b) Second stage (Voltage Amplifier): gives the very high gain characteristics. c) Third stage (EmitterFollower): gives the low output impedance characteristics. Inverting Input ( V IN ) V Differential Amplifier Voltage Amplifier Output Amplifier Output Noninverting Input ( V IN ) V Figure 1 Op Amp Block Diagram 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 OpAmp 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. INSTITUTE OF APPLIED TECHNOLOGY 3

4 4. Operational Amplifier Package The entire opamp circuit usually is placed within one of three basic packages, these are: Dual InLine throughhole Package (DIP) typically has 8 or 14 pins as in Figure 2a. DIP SurfaceMount package (SMT) typically has 8 or 14 pins as in Figure 2b. The TO5 metalcan package is available with 8, 10, or 12 leads as in Figure 2c. (a) Op Amp 741 8pins DIP package (b) OPA547FKTWT DIP SMT package Figure 2 Op Amp packages (c) TO Leads package 4.1 Op Amp Pins Identification Like all ICs a notch or a dot is used to ease the pinsidentification 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 Pin1. The notch at one end is located between Pins1 and Pins8 (on the left is Pin1). The rest of the pins are numbered proceeding anticlockwise from Pin 1. 1 V CC V CC V EE 4 5 V EE 4 5 a) Dot marked Package b) Notched Package Figure 3 Op Amp pins Identification INSTITUTE OF APPLIED TECHNOLOGY 4

5 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 (V IN ), signals at this pin will be inverted at output Pin 6. Pin 3: Noninverting input (V IN ), signals at pin 3 will be processed without inversion. Pin 4: Negative power supply terminal (V EE ). Pin 6: Output (UT ) of the OpAmp Pin 7: Positive power supply terminal (V CC ) Pin 8: No connection (N\C), it is just there to make it a standard 8pin package Offset Null Inverting Input VIN Noninverting Input VIN V EE N / C V CC Output Offset Null Figure 4 Op Amp pins Description 4.3 Op Amp Symbols Figure 5(a) and (b) shows the triangleshaped amplifier symbol used to represent the Op Amp in an electronics schematic diagram. It comprises a total of 5 pins, as follows: Two inputs ( V IN and V IN ) and one output (UT ). Two power supply connections (V CC V S ) and ( V EE V S ). V S V IN U V IN U V IN V IN (a) Without power connection V S (b) With power connection Figure 5 Op Amp Schematic Symbols INSTITUTE OF APPLIED TECHNOLOGY 5

6 5. Op Amp s Power Supply Requirements Most opamp circuits require a dual power supply having two opposite polarity voltages (V S & V S ) 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 pin7 (V S ) of the chip. The negative voltage of the supply (usually 5V to 15V) to pin4 (V S ) of the chip. V S V IN V IN 7 4 V S UT Common Ground Figure 6 Dual Supply Voltages connection To power the chip from a single power supply make the connections as follow: The positive voltage of the supply (V S ) to pin 7 and pin 4 grounded (Figure 7a); or The negative voltage of the supply (V S ) to pin 4 and pin 7 grounded (Figure 7b). V S V IN 7 UT V IN 7 UT V IN 4 V IN 4 V S (a) Single Positive Voltage (b) Single Negative Voltage Figure 7 Single Supply Voltages connection 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 30Vto0V as in the case of single power supply as shown in Figure 8b. 15V 30V Output 0V 30 V Output 30 V 15V Figure 8a Op Amp powered from Dual supply 0V Figure 8b Op Amp powered from Single supply INSTITUTE OF APPLIED TECHNOLOGY 6

7 6. OPAMP CONFIGURATION Feedback refers to connecting the output of the opamp 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 (Openloop comparator circuit). b) Op Amp with Negative Feedback. c) Op Amp with Positive Feedback. (a) Without Feedback (b) Negative Feedback (c) Positive Feedback 6.1 COMPARATOR Figure 9 Types of Feedback Figure 10a shows an opamp as a comparator. The comparator is an opamp 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 openloop gain of an opamp (A O ), any difference ( V IN ) will always saturate the output ( ) at either of the power supply rails, (V S ) or ( V S ), as follows: When V IN > V IN V IN is positive the output saturate at V S = V S. When V IN < V IN V IN is negative the output saturate at V S = V S. will change its state (V S V S ) when V IN changes its sign (i.e. at V IN = 0). V S V S V IN V IN 0 V IN > V IN V IN = V IN V IN < V IN V S V S (a) Comparator Circuit (b) Comparator Output Figure 10 OpAmps as Comparator The will saturate at (V S ) or ( V S ) If V S V S where = A O V IN. INSTITUTE OF APPLIED TECHNOLOGY 7

8 6.2 OPAMP 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 noninverting Amplifier. b) The inverting Amplifier NONINVERTING AMPLIFIER Figure 11 shows how an opamp can be configured as a noninverting amplifier. The input signal is applied to the noninverting input (V IN ). The output is fed back to the inverting input through the feedback circuit formed by R I and R F. The relations are expressed as follow: V A O NI R IN R F = V R IN VO R F = = 1 V R IN Where; = Output voltage F V F = Feedback voltage A NI = Noninverting Gain F R 1 R F V IN Figure 11 ClosedLoop Noninverting Amplifier Circuit INVERTING AMPLIFIER Figure 12 shows how an opamp can be configured as an inverting amplifier. The input signal is applied through a series input resistor R I to the inverting input. Also, the output is fed back through R F to the same input. The noninverting input is grounded. The relations are expressed as follow: V A O I R = R V = V O F F IN V R = R IN F IN Where; = Output voltage V IN = Input voltage A I = Inverting Gain R IN R F V IN Figure 12 ClosedLoop Inverting Amplifier Circuit The negative sign indicate that the input is inverted at the output and hence the name inverting amplifier. INSTITUTE OF APPLIED TECHNOLOGY 8

9 6.3 OPAMP 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. C 1 R 1 R 2 R 3 Figure 13 Astable Multivibrator 7. OPAMP APPLICATIONS 7.1 Positive Feedback Typical Application Relaxation oscillator (Astable multivibrator) 7.2 Negative Feedback Typical Application 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. It is capable of driving several loads. V IN Figure 17 Voltage Follower INSTITUTE OF APPLIED TECHNOLOGY 9

10 7.2.2 Inverting amplifier Typical Applications A) Summing amplifier (Adder). Figure 14 shows how an opamp can be connected as an Adder. R 1 R F V 1 V 2 V 3 R 2 R 3 0V Figure 14 Summing Amplifier B) Integrator Figure 15 shows how an opamp can be connected as an Integrator. C R V IN Figure 15 Inverting OpAmp as Integrator C) Differentiator Figure 16 shows how an opamp can be connected as a Differentiator. R C V IN Figure 16 Inverting OpAmp as Differentiator 7.3 Comparator Typical Applications are: Crossover detectors Analog to digital converters (ADC) Counting applications (e.g. count pulses that exceed a certain voltage level). INSTITUTE OF APPLIED TECHNOLOGY 10

11 8. PRACTICAL 8.1 OPAMP DUAL POWER SUPPLIES In general opamps are designed to be powered from a dual voltage supply. The dual power supply is a DC source with twopolarity 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 (V S and V S ) of the power supplies. Single Power Supply Single Power Supply 15V Common 15V Figure 18 Dual Power Supply Power Supply Ranges Positive Supply Rail (V S ): 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. Figure 19 Task: 1) Connect twosingle power supply (of equal output) in series as shown in Figure 19. 2) Take the link between the V S & the V S terminals as common (Ground = G). 3) Measure the voltages between (V S & G), ( V S & G), and (V S & V S ). 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. INSTITUTE OF APPLIED TECHNOLOGY 11

12 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 (V IN and V IN ). Referring to Figure 20, the output is given by: = A O V IN ; Where A O is the gain without feedback and called the openloop voltage gain. And V IN = (V IN ) ( V IN ); consequently there are three cases: Casea: V IN > V IN is positive. Caseb: V IN < V IN is negative. Casec: V IN = V IN is zero. Practically Casec is not true, because when V IN = V IN = 0, there is a slight V IN V IN V S V S Common Ground Figure 20 Differential Amplifier amount of voltage at the output is known as offset voltage as shown in Figure 21. V IN = 0 For (V IN = V IN = 0); 0 Figure 21 Offset Voltage Then offset voltage can be defined as the slight amount of voltage that appears at the output when the voltage differential ( V IN ) between the input pins is 0 V. Offset null adjustments differ with the application (i.e. Inverting or NonInverting Amplifier). Offsetnull 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 opamp. INSTITUTE OF APPLIED TECHNOLOGY 12

13 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 pin6 and ground with a dc voltmeter. 9) Adjust the potentiometer until the output voltage read 0. This is the zero null state. V S V N/C µa V S V IN = 0 Figure 22 Offset Null adjustment Note: This is a recommended null procedure for the ua741 type opamp. Always look for, and follow the particular procedure as specified by that chip manufacturer. Procedures may become obsolete or updated and changed when improved opamp versions come on the market. INSTITUTE OF APPLIED TECHNOLOGY 13

14 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 = (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 V 1 and V 2 derived from potentiometers on the OAT343 deck and V 3 from an external signal generator. 3) Use the table shown in Figure 24 to note down your observations. Figure 23 Summing Amplifier 4) Set V 3 to zero and adjust V 1, V 2 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 V 1 and V 2 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 V 1 V 2 V 1 V 2 Output ( ) 10V 8V 2V 10V 2V 8V 2V 8V 10V 8V 8V 0 Figure 24 Result Table INSTITUTE OF APPLIED TECHNOLOGY 14