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  • AB45 Operational Amplifier

    (Schmitt Trigger & Comparators)

    Operating Manual Ver.1.1

    An ISO 9001 : 2000 company

    94-101, Electronic Complex Pardesipura, Indore- 452010, India Tel : 91-731- 2570301/02, 4211100 Fax: 91- 731- 2555643 e mail : [email protected] Website : www.scientech.bz Toll free : 1800-103-5050

    mailto:[email protected]://www.scientech.bz

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    RoHS Compliance

    Scientech Products are RoHS Complied. RoHS Directive concerns with the restrictive use of Hazardous substances (Pb, Cd, Cr, Hg, Br compounds) in electric and electronic equipments. Scientech products are Lead Free and Environment Friendly. It is mandatory that service engineers use lead free solder wire and use the soldering irons upto (25 W) that reach a temperature of 450C at the tip as the melting temperature of the unleaded solder is higher than the leaded solder.

    AB45 Operational Amplifier

    (Schmitt Trigger & Comparators)

    Table of Contents

    1. Introduction 4 2. Theory 6 3. Experiments

    Experiment 1 10 Study of Operational Amplifier as Comparator and Zero Crossing Detector

    Experiment 2 12 Study of Operational Amplifier as Schmitt Trigger

    4. Data Sheet 14 5. Warranty 15 6. List of Accessories 15

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    Introduction AB45 is a compact, ready to use Operational Amplifier experimental Board. This is useful for students to study Op-amp as Comparator, Zero crossing Detector and Schmitt trigger. It can be used as stand alone unit with external DC power supply or can be used with Scientech Analog Lab ST2612 which has built in DC power supply, AC power supply, function generator, modulation generator, continuity tester, toggle switches, and potentiometer.

    List of Boards : Model Name AB01 Diode characteristics (Si, Zener, LED) AB02 Transistor characteristics (CB NPN) AB03 Transistor characteristics (CB PNP) AB04 Transistor characteristics (CE NPN) AB05 Transistor characteristics (CE PNP) AB06 Transistor characteristics (CC NPN) AB07 Transistor characteristics (CC PNP) AB08 FET characteristics AB09 Rectifier Circuits AB10 Wheatstone bridge AB11 Maxwells Bridge AB12 De Sautys Bridge AB13 Schering Bridge AB14 Darlington Pair AB15 Common Emitter Amplifier AB16 Common Collector Amplifier AB17 Common Base Amplifier AB18 RC-Coupled Amplifier AB19 Cascode Amplifier AB20 Direct Coupled Amplifier AB21 Class A Amplifier AB22 Class B Amplifier (push pull emitter follower) AB23 Class C Tuned Amplifier AB24 Transformer Coupled Amplifier AB25 Phase Locked Loop (FM Demodulator & Frequency Divider / Multiplier) AB26 FET Amplifier AB27 Voltage Controlled Oscillator AB28 Multivibrator (Mono stable/Astable) AB29 F-V and V-F Converter AB30 V-I and I-V Converter AB31 Zener Voltage Regulator AB32 Transistor Series Voltage Regulator AB33 Transistor Shunt Voltage Regulator

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    AB35 DC Ammeter AB37 DC Ammeter (0-2mA) AB39 Instrumentation Amplifier AB41 Differential Amplifier (Transistorized) AB42 Operational Amplifier (Inverting / Non-inverting / Differentiator) AB43 Operational Amplifier (Adder/Scalar) AB44 Operational Amplifier (Integrator/ Differentiator) AB49 K Derived Filter AB51 Active filters (Low Pass and High Pass) AB52 Active Band Pass Filter AB54 Tschebyscheff Filter AB56 Fiber Optic Analog Link AB57 Owens Bridge AB58 Andersons Bridge AB59 Maxwells Inductance Bridge AB64 RC Coupled Amplifier with Feedback AB66 Wien Bridge Oscillators AB67 Colpitt Oscillator AB68 Hartley Oscillator AB80 RLC Series and RLC Parallel Resonance AB82 Thevenins and Maximum Power Transfer Theorem AB83 Reciprocity and Superposition Theorem AB84 Tellegens Theorem AB85 Nortons theorem AB88 Diode Clipper AB89 Diode Clampers AB90 Two port network parameter AB91 Optical Transducer (Photovoltaic cell) AB92 Optical Transducer (Photoconductive cell/LDR) AB93 Optical Transducer (Phototransistor) AB96 Temperature Transducer (RTD & IC335) AB97 Temperature Transducer (Thermocouple) AB101 DSB Modulator and Demodulator AB102 SSB Modulator and Demodulator AB106 FM Modulator and Demodulator

    and many more

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    Theory Operational amplifier is a direct-coupled high-gain amplifier usually consisting of one or more differential amplifiers and usually followed by a level translator and an output stage. The output stage is generally a push-pull or push-pull complementary-symmetry pair. An operational amplifier is available as a single integrated circuit package.

    The operational amplifier is a versatile device that can be used to amplify DC as well as AC input signals and was originally designed for performing mathematical operations such as addition, subtraction, multiplication, and integration. Thus the name operational amplifier seems from its original use for these mathematical operations and is abbreviated to op-amp. With the addition of suitable external feedback components, the modern day op-amp can be used for a variety of applications, such as AC and DC signal amplification, active filters, oscillators, comparators, Schmitt trigger, regulator, integrator, differentiator.

    Comparator (Voltage Level Detector) : An op-amp comparator is a circuit which compares an arbitrary input signal against a fixed reference voltage. The output of the comparator circuit switches between the two saturation voltages depending on the value of arbitrary input signal w.r.t the reference voltage (if the input amplitude is less than the reference voltage, output is at one saturation level and vice-versa).

    As shown in figure 1 a non-inverting comparator circuit. A fixed reference voltage Vref (say 1V or 2V) is applied to the () input (shown in figure 2a for Vref = 1V and figure 2b Vref = 1V), and the other time varying signal voltage Vin is applied to the (+) input of op-amp. When Vin is less than Vref, the output voltage Vout is at Vsat (approximately equal to VEE) as the voltage at () input terminal is higher than that of the (+) input terminal. On the other hand, when the (+) input terminal voltage Vin is greater than Vref, the (+) input terminal becomes positive w.r.t. the () input and the Vout bring switches to +Vsat (approximately equal to +Vcc).

    Comparator (Voltage Level Detector)

    Figure 1

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    Input-Output Waveform Comparator

    Figure 2

    Thus, Vout changes from one saturation level to another whenever Vin = Vref as shown in figure 2 (a). In short comparator is a type of analog-to-digital converter. At any given time the Vout shows whether Vin is greater or less than Vref. This is the reason why it is also called a voltage level detector. In the similar way if the reference voltage is negative w.r.t. ground, with the sinusoidal input applied to the noninverting terminal of op-amp the output will be as shown in figure 2 (b).

    Zero-crossing Detector (Sine wave-to-Square Wave Converter) : The above shown circuit can also be used as a zero crossing detector provided that Vref is set to zero (Vref = 0). As shown in figure 3 (a), which is an inverting comparator used as a zero - crossing detector.

    Figure 3

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    Schmitt Trigger : A Schmitt Trigger is a circuit which converts an irregular shaped waveform to a square wave or pulse. This circuit is also called as a squaring circuit. A Schmitt trigger circuit is as shown in figure 4

    Schmitt Trigger (a) Circuit diagram (b) Input-output waveform

    Figure 4 The input voltage Vin triggers (changes the state of) output Vout every time exceeds certain voltage levels called upper threshold Vut and lower threshold voltage Vlt as shown in figure 5.

    Vout vs. Vin plot of Hysteresis voltage

    Figure 5 These threshold voltages can be obtained by using the voltage divider R1-R2, where the voltage across R1 is fed back to the (+) input. The voltage across R1 is a variable reference threshold voltage that depends on the value and the polarity of the output voltage. When Vout = +Vsat, the voltage across R1 is called the upper threshold voltage, Vut. The input voltage Vin must be slightly more +ve than Vut in order to

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    cause the output voltage Vout to switch from +Vsat to -Vsat. As long as Vin < Vut, Vout is at +Vsat. Using the voltage divider rule,

    )V(RR

    RV sat21

    1ut ++

    = .. (1)

    On the other hand, when Vo = -Vsat, the voltage across R1 is referred to as lower threshold voltage, Vlt. Vin must be slightly more negative than Vlt in order to cause Vout to switch from -Vsat to +Vsat. In other words, for Vin values greater than Vlt, Vout is at -Vsat. Vlt is given by the following equation.

    )V(RR

    RV sat

    21

    1lt +

    = .. (2)

    Thus if the threshold voltages Vut and Vlt are made larger than the input noise voltages, the positive feedback will eliminate the false output transitions. Also, the positive feedback, because of its regenerative action, will make Vout to switch faster between +Vsat and -Vsat. The comparator with positive feedback is said to exhibit hysteresis, a dead zone. That is when the input of the comparator exceeds Vut, its output switches from +Vsat to -Vsat and revert back to its original state +Vsat, when the input goes below Vlt. The hysteresis voltage is, equal to the difference between Vut and Vlt. Therefore, hyV = ltut VV

    hyV = )]V(V[RRR

    satsat21

    1 ++

    .. (3)

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    Experiment 1 Objective : Study of the Operational Amplifier as a Comparator and Zero Crossing Detector Equipments Needed : 1. Analog board of AB45. 2. DC power supplies +12V,-12V and variable +5V and -5V from external source

    or ST2612 Analog Lab. 3. Oscilloscope 4. Function Generator. 5. 2 mm patch cords. Circuit diagram : Circuit used to study Op-amp as Comparator is as shown in figure 6.

    Figure 6

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    Procedure :

    Connect +12V and -12V DC power supplies at their indicated positions on AB45 board from external source or ST2612 Analog Lab.

    1. Connect variable +5V DC signal between points a and g1 i.e. to the inverting input of the Op-amp through 300 Ohms resistance. This DC signal will act as a reference voltage against which the level of input signal will be compared.

    2. Connect a 10Vp-p, 1 KHz signal between points f and g2 i.e. to the non-inverting input of the Op-amp. (Select the inverting and noninverting input terminal resistance values to be equal).

    3. Adjust the variable DC signal to 1V and observe the output waveform between points i and g3 on Ch I of oscilloscope and input signal on Ch II of oscilloscope.

    4. Vary the DC signal gradually from 1V to 5V and observe the output voltage waveform with respect to input signal.

    5. Disconnect the +5V variable supply and connect variable -5V DC signal between points a and g1 i.e. to the inverting input of the Op-amp through 300 Ohms resistance. This DC signal will act as a reference voltage against which the level of input signal will be compared.

    6. Vary the DC signal gradually from -1V to -5V and observe the output voltage waveform with respect to input signal.

    7. Plot the output waveforms on graph paper for both the above cases. (Refer figure 1 and figure 2).

    8. Connect the point a to the point g1 of AB45 board to analyze Op-amp as Zero Crossing Detector. This connection will make reference voltage to be equal to 0V (Vref = 0V).

    9. Observe the output waveform between points i and g3 on Ch I of oscilloscope and input signal on Ch II.

    10. To make sure that the output waveform is crossing zero level at the same instant to that of the input signal, adjust the offset pot.

    11. Plot the output waveforms on graph paper for the Vref = 0V. (Refer figure 3a and 3b).

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    Experiment 2 Objective : Study of Operational Amplifier as a Schmitt Trigger Equipments Needed : 1. Analog board of AB45. 2. DC power supplies +12V, -12V from external source or ST2612 Analog Lab. 3. Oscilloscope. 4. Function Generator. 5. Digital Voltmeter. 6. 2 mm patch cords. Circuit diagram : Circuit used to study Op-amp as Schmitt Trigger is as shown in figure 7.

    Figure 7

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    Procedure : Connect +12V& -12V DC power supplies at their indicated positions on AB45

    board from external source or ST2612 Analog Lab. 1. Connect the point g and h using a 2mm patch cord. This will activate the

    positive feedback to the op-amp circuit. 2. Connect point d with g2 using a 2mm patch cord. 3. Connect a 10Vp-p, 1 KHz sine wave signal between points a and g1 i.e. to the

    inverting input of the Op-amp (R1 = R4 || RF). You can also connect points e and g2 or points f and g2 and signal will be applied between point b and g1 or c and g1 respectively. (Refer to figure 4a)

    4. Observe the output waveform between points i and g3 on Ch I of oscilloscope and input signal on Ch II of oscilloscope.

    5. Calculate the amplitude of the square wave (The square wave amplitude will vary between +Vsat = +12V and -Vsat = -12V). You can check this by applying some other value of DC signals, say +5V and -5V or + 15V and -15V, and check the amplitude.

    6. You can also check the above point by varying the input signal amplitude and observing whether the output signal amplitude varies with the input signal amplitude variations or not.

    7. Connect a 10Vp-p, 1KHz triangular wave signal between points a and g1 i.e. to the inverting input of the Op-amp (R1 = R4 || RF).

    8. Calculate Vut and Vlt using Equation (1) and (2) respectively for the following three cases: a. Signal applied between points a and g1 and points d and g2 are connected

    using a 2mm patch cord. b. Signal applied between points b and gl and points e and g2 are connected

    using a 2mm patch cord. c. Signal applied between points c and g1 and points f and g2 are connected

    using a 2mm patch cord. 9. Check the voltage drop across R4, R5 and R6 respectively for the above three

    cases using DMM and check the results against the theoretically calculated values in step 8.

    10. Calculate the Hysteresis voltage for the above three cases using Equation.3 and plot the Hysteresis voltage on graph paper (Refer to figure 5).

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    Data Sheet

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    Warranty 1. We guarantee the product against all manufacturing defects for 24 months from

    the date of sale by us or through our dealers. Consumables like dry cell etc. are not covered under warranty.

    2. The guarantee will become void, if a) The product is not operated as per the instruction given in the operating

    manual. b) The agreed payment terms and other conditions of sale are not followed. c) The customer resells the instrument to another party. d) Any attempt is made to service and modify the instrument.

    3. The non-working of the product is to be communicated to us immediately giving full details of the complaints and defects noticed specifically mentioning the type, serial number of the product and date of purchase etc.

    4. The repair work will be carried out, provided the product is dispatched securely packed and insured. The transportation charges shall be borne by the customer.

    For any Technical Problem Please Contact us at [email protected]

    List of Accessories

    1. 2 mm Patch Cords (Red) ...................................................................... 2 Nos. 2. 2 mm Patch Cord (Blue) ....................................................................... 3 Nos. 3. 2 mm Patch Cord (Black) ..................................................................... 3 Nos. 4. e-Manual.................................................................................................1 No.

    Updated 25-03-2009

    mailto:[email protected]