# Schmitt Trigger

Prior to the Lab session:

1.    Study the operation and working principle of Schmitt Trigger.

2.   Study the procedure for conducting the experiment in the lab.

Objectives:

1. To design the circuit of Schmitt trigger with UTP=2.2V and LTP=1V.

2. To obtain square wave from sine wave.

3. To obtain UTP and LTP values practically

Apparatus:

1.         CRO 0-20 MHz (Dual channel)                                               -           1No.

2.         Function generator 0- 1MHz                                                    -           1No.

3.         Capacitor (0.1µF)                                                                    -           1 No.

4.         Resistors (1kohms, 2.2Kohms, 330ohms                                                            -           1 No. each

5.         Resistor (1.2 Kohms                                                                    -           3 No.

6.         Transistor (BC 107)                                                                 -           2 No.

7.         Regulated Power supply 0-30 V (dual )                                               -           1 No.

8.         Connecting wires

Circuit diagram:

Theory:

In digital circuits fast waveforms are required  i.e, the circuit remain in the active region for a very short time (of the order of nano seconds) to eliminate the effects of noise or undesired parasitic oscillations  causing  malfunctions  of  the circuit.  Also if the rise  time  of  the input  waveform is long,  it requires  a large  coupling  capacitor.  Therefore  circuits  which can convert  a slow changing  waveform (long  rise  time)  in to  a  fast  changing  waveform (small  rise  time)  are required.  The circuit which performs  this operation is known as “Schmitt Trigger”.

In Schmitt  trigger  circuit, the  output  is in one of  the two levels  namely  low  or  high.  When the  input  voltage  is raising  above the UTP (upper threshold point) i.e. V1, the output changes to high level.  Similarly  when  a  falling output  voltage  passes through a voltage  Vknown  as  lower  threshold point (LTP), the output changes to low.  The level  of  the output changes V1   is  always greater than V2.The differences of these two voltages is known as “Hysteresis”.

Design Procedure:

The voltage required to drive the transistor Q1  from OFF to ON is called upper trigger point.

UTP = V1  = V —  0.1

Where  V1 = V = ( VCCR2)/   (R1+R2+RC1)

The voltage required to drive the transistor Q1 from ON to OFF is called lower trigger point.

LTP = V2= VBE(active)+ (V1-V?2) .Re/(aRcth+Re)

Where Rcth =(RC1(R1+R2))/( R1+R2+RC1)                       a =R2/(R1+R2)

Choose BC107 transistor with hfe= 200

Let Vcc=12V, R1=2.2Kohms ,  R2=1.2Kohms

Set UTP = V —  0.1  then       2.2= V —  0.1    then      V1= 2.3V

But V1= (12X15)/(2.2+1.5+RC1)                      RC1= 4.12Kohms = 4Kohms

Rcth = (4X 103(2.2+1.5))/(2.1+1.2+4.4K) = 1.97Kohms

a= 0.3529

LTP =V2= VBE(active)+ (V1-V?2) .Re/(aRcth+Re)

= 0.7+(2.3-0.6).330/(0.35X1.97K+330)

= 1.21V

VE=V1-VBE= 2.3-0.7 = 1.6V

IB2= VE/RE(1+hfe) = 1.6/330(1+200) = 0.0030A

IC2 = hfeIB2 =0.6V

When Vin < V2, output =1

(VC-output)/IC2 = RC2        --------->                  (12-1)/0.6 = RC2= 18.33%u2126

Procedure:

1. Connect the circuit diagram as shown in circuit diagram.

2. Apply a sine wave input   of 15 Vp-p   amplitude and 1 KHz frequency to the circuit

3. Observe the output voltage on CRO.

4. Obtain the output voltage at which LOW to HIGH transition occurs and measure the corresponding

input voltage. This input voltage is called UTP (Upper threshold point)

5. Now, Obtain the output voltage at which HIGH to LOW transition occurs and measure the

corresponding input voltage. This input voltage is called LTP (Lower threshold point).

6. Compare these practical values with theoretical values.

Expected Waveforms:

Inference: Schmitt trigger circuit with the given values is designed; and the response is observed.

Viva Questions:

1.      What is Schmitt Trigger?

Ans:A Schmitt trigger is an emitter coupled binary.

2.      What are the applications of Schmitt Trigger?

Ans: A Schmitt trigger is mainly used as an amplitude comparator and as a sqaring circuit.

3.      Define hysteresis action?

Ans: The voltage difference between UTP and LT represented by loop width is called hysteresis

Voltage

4.      Why Schmitt Trigger is called a squaring circuit?

Ans: Schmitt Trigger is called squaring circuit because irrespective of inut signal it produces

square wave .

5.     Define UTP?  Write its expression.

Ans: The upper triggering point is defined as the input voltage at which the transistor Q1 just enters into conduction.

UTP(v1)=V1 –VBE2 +V%u02641=V1-0.1 where V1=VccR2/(R2+Rc+R1)

6.    Define LTP? Write its expression.

Ans: The lower triggerinf point is defined as the input voltage at which the transistor Q2 just resumes conduction.

LTP =V2= VBE(active)+ (V1-V?2) .Re/(aRcth+Re)

7.      What is the difference between a Binary and Schmitt Trigger?

Ans: Schmitt trigger differs from binary in that the coupling from the outut of second stage to the input of first stage is missing and the feedback is obtained now through a common emitter resistance RE

8.      How noise can be eliminated on a given signal using Schmitt Trigger?

Ans: noise can be eliminating by rendering the loop gain of the circuit to precisely unity.

9.      Explain how a Schmitt Trigger converts a sine wave to a square wave?

Ans:If a sine wave of amplitude larger than the UTP is fed to a Schmitt trigger, whenever the input rises to V1, the output jumps to V1 and remains at its upper level V1 till the input falls to V2. The output remains at its lower level V2 till the input rises to V1.

10.     A Schmitt trigger exhibits hysteresis when loop gain is ___.

Ans: Greater than one.

Design Projects

1.Design Schmitt trigger circuit to get UTP=5V and LTP=7V for VCC=15V.

Outcomes: After finishing this experiment students are able to Design Schmitt trigger circuit using transistor and they are able to find UTP and LTP.

• Updated
Mar 05, 2020
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