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SUBJECT:
ANALOG ELECTRONICS
(2130902)
EXPERIMENT NO. 08
TITLE:
TO DESIGN SCHMITT TRIGGER CIRCUIT USING OP-AMP
AND TAKE MEASUREMENTS.
DATE :
DOC. CODE : DIET/EE/3rd SEM
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
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Figure.1: Schmitt Trigger [a] Circuit diagram [b] (U) Input waveform (A) comparator output
waveform (B) Schmitt Trigger output waveform
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 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.
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On the other hand, when Vout = - Vsat, the voltage across R1 is referred to as lower threshold voltage, Vlt.
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.
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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
Darshan Institute of Engineering And Technologies, Rajkot
H.O.D
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