SUBJECT: TITLE: ANALOG ELECTRONICS TO DESIGN SCHMITT TRIGGER CIRCUIT USING OP-AMP (2130902) AND TAKE MEASUREMENTS. DOC. CODE : DIET/EE/3 rd SEM EXPERIMENT NO. 08 DATE : REV. NO. : 1.00/JUNE-2015 AIM: To design Schmitt trigger circuit using op-amp and take measurements. APPARATUS: ST2612 Analog Lab, AB45. DC power supplies +12V, -12V and variable +5V and -5V from external source Oscilloscope Function Generator. 2 mm patch cords. 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. Schmitt Trigger: In electronics, a Schmitt trigger is a comparator circuit with hysteresis, implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. It is an active circuit which converts an analog input signal to a digital output signal. The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration, when the input is higher than a certain chosen threshold, the output is high. When the input is below a different (lower) chosen threshold, the output is low, and when the input is between the two levels, the output retains its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bistable circuit (latch or flip-flop). 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 1. Darshan Institute of Engineering And Technologies, Rajkot Page 26 Figure.1: Schmitt Trigger [a] Circuit diagram [b] (U) Input waveform (A) comparator output waveform (B) Schmitt Trigger output waveform The input voltage V in triggers (changes the state of) output V out every time exceeds certain voltage levels called upper threshold V ut and lower threshold voltage V lt as shown in figure 2. Figure. 2: Vout vs. Vin plot of Hysteresis voltage 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. Darshan Institute of Engineering And Technologies, Rajkot Page 27 On the other hand, when Vout = - Vsat, the voltage across R1 is referred to as lower threshold voltage, V lt . The hysteresis voltage is, equal to the difference between Vut and Vlt. Therefore, Procedure : 1. Connect +12V& -12V DC power supplies at their indicated positions on AB45 board from external source or ST2612 Analog Lab. 2. Connect the point g and h using a 2mm patch cord. This will activate the positive feedback to the op-amp circuit. 3. Connect point d with g2 using a 2mm patch cord. Darshan Institute of Engineering And Technologies, Rajkot Page 28 4. 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 1a) 5. Observe the output waveform between points i and g3 on Ch I of oscilloscope and input signal on Ch II of oscilloscope. 6. 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. 7. 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. 8. 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). 9. 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. 10. 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. 11. Calculate the Hysteresis voltage for the above three cases using Equation.3 and plot the Hysteresis voltage on graph paper. CALCULATION: CONCLUSION: LAB-INCHARGE H.O.D Darshan Institute of Engineering And Technologies, Rajkot Page 29 .