Fig. 4.1 Comparator Circuit with Input Voltages and Output Responses

Comparator Circuit Operation

1. The comparator circuit compares the two input voltages V₁ and V₂ and produces an output voltage, whichever is larger or smaller between the two voltages.
2. If the non-inverting input voltage V₁ (or V⁺ ) is greater than the inverting voltage V₂ (V⁻), output voltage becomes or makes transition (jumps) to the maximum positive voltage + V_CC (+V_Sat) in the device.
3. If the non-inverting input voltage V₁ (or V⁺ ) is less than the inverting voltage V₂ (V⁻), output voltage becomes or makes transition (jumps) to the largest negative voltage − V_CC (−V_Sat) in the device.

Comparator Circuit with Input Voltages and Output Responses

Comparator circuit has non-linear behaviour, switching the device between highest positive or negative voltages as shown in Fig. 4.1 (a) and (b). The inputs to comparator circuit are ‘analog signals’ and its output is a ‘digital signal’ whether high or low. The nature of digital output depends upon the magnitudes of comparing voltages at the input port of operational amplifier.

1. Some applications of comparator circuits are (a) analog to digital (A to D) converter and (b) signal conditioning circuits.
2. ‘Precision and low noise operational amplifiers’ are used for conditioning the signals from transducers such as light, heat, and temperature sensors before they are fed to analog to digital conversion circuits.

4.3 OP AMP VOLTAGE COMPARATOR

Comparator circuits basically use operational amplifiers (op amp). One of the two input voltages to op amp is considered as reference voltage V_ref and the second input is considered as input voltage V_in, as shown in Fig. 4.2. Therefore, the expressions for output voltage V_out and voltage gain A_V are as follows:

Fig. 4.2 Voltage Comparator Circuit Using Operational Amplifier

Op amp voltage gain

(4.1)

∴ Comparator output voltage is expressed as in the following:

V_out = A_V (V_in − A_V· V_d(4.2)

where differential input voltage can be given as follows:

V_d = (V_in − V_ref)(4.3)

Input signals are direct coupled to the input terminals of operational amplifier. The op amp is a differential amplifier working with differential input voltage V_d and high voltage gain A_V.

Example 4.1

An operational amplifier in open-loop conFiguration working as a differential amplifier shown in Fig. 4.2 has voltage gain A_V = 10⁴. Supply voltage V_S = ±15 V. Calculate the output voltages for the differential input voltages (a) V_d (1) = 0.25 V and (b) second input V_d (2) = 0.5 mV.

Solution: Voltage gain A_V = 10⁴ and op amp supply voltage V_S = ±15 V.

The saturation level of output voltage V_Sat = ± V_S ± 1 V = ± 15 V ± 1 V = ±14 V

Op amp output voltage V_out (1) = A_V × V_d (1) = 10⁴ × 0.25 V = 2500 V. However, the output voltage will be only at V_Sat = 14 V.

Op amp output voltage V_out (2) = A_V × V_d (2) = 10⁴ × 0.5 × 10⁻³ V = 5 V.

Then, the output voltage will be at V_out = 5 V.

Circuit Analysis and Operation of Electronic Voltage Comparators

An electronic voltage comparator compares two (input) voltages and produces an output voltage, whichever is larger among the two input voltages. The comparator output has two output levels high or low as shown in Fig. 4.4. They are used in analog to digital (A to D) and digital to analog (D to A) converter circuits. Simple comparator circuit uses an operational amplifier, which is shown in Fig. 4.1.

1. It has two input voltages V⁺ and V⁻. These two input voltages are compared with one another and output voltage will be equal to larger of the two inputs.
2. Output voltage assumes only one of the two values (−V_Sat), as shown in Fig. 4.4 and Fig. 4.5. The output signal will be high or low representing digital or binary value.
3. Output voltage will be high (+V_Sat) if V⁺ is larger than V⁻.
4. Output voltage will be low (−V_Sat) if V⁺ is less than V⁻.
5. Polarity of the comparator’s output voltage depends upon the polarity of the difference voltage V_d (differential input voltage), as shown in Fig. 4.1.
6. Transfer characteristic (input−output curve) of the comparator is shown in Fig. 4.2.
7. Input−output curve crosses the origin when V⁺ is equal to V⁻

Simple comparator circuit using op amp is shown in Fig. 4.3.

Fig. 4.3 Voltage Comparator Circuit Using Operational Amplifier

Comparator circuits are of two types (see Fig. 4.4 and Fig. 4.5)

Fig. 4.4 (a) Non-Inverting Comparator Circuit; (b) Transfer Characteristic (Input–Output Curve)

Fig. 4.5 (a) Inverting Comparator Circuit; (b) Transfer Characteristic (Input–Output Curve)

1. Non-inverting comparator.
2. Inverting comparator.

Non-inverting comparator: When the input signal to a comparator circuit is greater than a certain minimum voltage (transition/threshold) level, the output voltage will be in high state for a non-inverting comparator circuit.

4.3.1 Non-inverting Comparator

Basic non-inverting comparator circuit with its transfer characteristic is shown in Fig. 4.4 (a) and 4.4 (b). Assume an op amp in open-loop conFiguration, as shown in Fig. 4.4 (a), to understand its operation as a comparator.

Output response: Output response of comparator circuit is shown as transfer characteristic relating the input and output voltages. It has two horizontal lines. The first level represents high output corresponding to (−V_Sat) as shown in Fig. 4.4 (b). It is a non-linear characteristic. Expressions for voltage gain and input voltage are as follows:

Amplifier gain is expressed as follows:

(4.4)

Input voltage is given as in the following:

(4.5)

Consider voltage gain of op amp A_V = 10⁴ (voltage gains A_V will be of the order of 10³−10⁵) and maximum value of V_out = 14 V, when the supply voltage V_CC = 15 V.

From the data, the input voltage

Thus, the difference between non-inverting voltage V⁺ and inverting voltage V⁻ is 1.4 mV.

1. If a non-inverting input signal larger than 1.4 mV is applied to the op amp, output voltage V_out becomes 14 V (+V_Sat) by comparing with the existing level of other input voltage with the applied input signal.
2. In Fig. 4.4 (b), the values for −V_Sat are given as 15 V to indicate that the maximum level of saturation voltage the op amp output can reach under ideal conditions.

4.3.2 Inverting Comparator Circuit

Basic inverting comparator circuit with its transfer characteristic is shown in Fig. 4.5 (a) and (b). If an inverting input signal larger than 1.4 mV is applied to the op amp, output voltage V_out becomes −14 (− V_Sat) by comparing the existing level of input voltage with the applied input signal.

These transitions among output voltages (+V_Sat and −V_Sat) for input voltages (see Fig. 4.5) suggest that an operational amplifier works as ‘comparator’ circuit.

The definition of comparator can be framed from the previous discussions:

1. Voltage at one of the inputs of comparator op amp compares with the voltage at the second input terminal and produces high voltage if it is larger than the second voltage.
2. The second voltage used as reference is also known as ‘reference voltage’.
3. If reference voltage V_ref is zero, the transfer characteristic passes through origin as shown in Fig. 4.5(b).
4. If the reference voltage has a finite voltage larger than the minimum required level for transitions, the transfer curve will not pass through origin but shifts away from the origin, which is equal to V_ref. It is also known as ‘threshold voltage V_T’.
5. Voltage at one of the inputs of comparator op amp compares with the voltage at second input terminal and produces low voltage if it is less than the second voltage.
6. Saturation voltages +V_Sat and −V_Sat are shown as 15 V at which the output response can attain its maximum saturation under ideal conditions. However, practical situation has slight deviations.

4.4 DIFFERENT TYPES OF COMPARATOR ICS

Single voltage comparator ICs: LM 311, LM 111-N, LM211-N.

Some specifications and features as follows:

1. Supply voltage to ICs = 5 V.
2. Differential input voltage: ± 30 V.
3. Power consumption: 135 mW at ± 15 V.
4. Less susceptible to spurious oscillations because of stable power supply.
5. Less speed of operation.

The applications of LM 311 (see Fig. 4.6) are as follows:

Fig. 4.6 Single Voltage Comparator IC LM 311

1. Driver for lamps or LEDs and relays. It can drive TTL or DTL ICS.
2. Zero crossing detector for magnetic transducer.
3. Low-level photodiode detector.

Dual Comparator IC LM393 (see Fig. 4.7)

Fig. 4.7 Dual Comparator IC LM 393

Quad Voltage Comparator IC LM 339 (see Fig. 4.8)

Fig. 4.8 Quad Voltage Comparator IC LM 339

1. Comparator ICs such as LM 339, LM 393, and LM 311 comparator ICs work on single or dual power supply voltages. Maximum supply voltage is 32 V.
2. Four comparators are embedded in a single pack with low cost. Therefore, this IC is preferred in industrial applications with low budget applications.

Definition of Hysteresis: Hysteresis means deficiency or lagging behind. It was adopted from a Greek word by Alfred Ewing to describe the behaviour of magnetic materials. Hysteresis is the characteristic feature of materials, whose behaviour depends upon its present and previous environments. The concept is extended to electronic systems. The current state of a system depends upon its internal or previous state and history in the system. Further, the future state of the system can be predicted from its present history in the system, depending upon the nature of inputs.

One of the latest applications is the introduction of ‘Hysteresis’ in computer operations during the ‘Graphical User Interface’ (GUI) to simplify the system operations in most efficient way by system users. Computer system working should adapt to the user operations. Visual effects in the system should match to the user operations in simple ‘user-friendly’ functionality.

The desired amount of hysteresis is added in electronic circuits to reduce noise due to bouncing operations between switching operations by introducing delay in electronic switches.

4.5 COMPARATOR APPLICATIONS

Comparator is normally used in applications where some level of a varying signal is compared to a fixed magnitude of voltage reference V_ref.

1. Platinum RTD thermometer to measure 0°C to 500°C.
2. Exponential voltage to frequency converter for electronic music synthesizers.
3. Window comparators are used in temperature control circuits.

4.5.1 Window Detector

Basic principle: A simple comparator determines whether one of its input voltages is higher than the other voltage. However, the method of identification of a region between two threshold levels of unknown (input) signal voltages can be done by a circuit known as window detector.

Window detector determines whether a signal lies within a desired range of values.

Window Detector (Comparator Circuit Application)

Window detector is a special type of comparator circuit, which identifies the desired range of voltages at its input. It consists of two operational amplifiers (operational amplifier-1) and (operational amplifier-2) operating together and two diodes (D₁ and D₂) connected as shown in Fig. 4.10. There are three input terminals. Two input terminals are meant for reference voltages and third input terminal is for input signal.

There are two reference voltages V_ref (high) and V_ref (low) and the input signal voltage V_input whose range of signal has to be estimated. One output voltage is considered for the prediction of the input signal status whether it lies or stays in the specified range (window) or outside the desired range.

There are three voltage input points:

1. V_ref (high)
2. V_ref (low)
3. Actual input signal V_input

Reference voltages Vref (high) and Vref (low): The two reference voltages correspond to two threshold voltage levels. They define the two voltage levels of the desired range (window) of voltages of a signal. Voltage V_ref (high) corresponds to upper level (threshold) of the signal. It is similar to ‘upper trigger level’ (UTP) of Schmitt trigger circuit. Similarly, voltage V_ref (low) corresponds to low level (threshold) of the signal. It is similar to ‘low trigger level’ (LTP).

If input voltage V_input is lower than the higher reference voltage V_ref (high), output voltage V_output will be ‘positive voltage’. Similarly, if the input voltage is higher than lower reference voltage V_ref (low), output voltage will be ‘positive’. Thus, the output of window detector will be positive within a range of ‘two threshold voltages’ in a signal waveform. Thus, a window detector identifies the known range of input signal within ‘two threshold levels’.

If the input signal (voltage) is below the lower reference voltage V_ref (low) output voltage will be zero. If the input voltage is above the higher reference voltage V_ref (high), output voltage will be zero. Such points of operation can easily be understood from input and output signal conditions shown in Fig. 4.9 and Fig. 4.10.

Fig. 4.9 Window Detector (Input and Output Signal Waveforms)

Fig. 4.10 Window Detector (Comparator) Using Two Op Amps and Two Diodes

Tripping points (lower and upper): From the discussion, we can identify a signal within a range of two voltages. The two voltages are considered as two threshold voltages, which in turn correspond to lower tripping point (LTP) and upper tripping point (UTP) considered in the operation of Schmitt trigger circuit.

Comparator output voltages: For input signal voltages, within the specified two threshold voltages, the window comparator output has one type of voltage say ‘X’. Then, for input signal voltages that lie outside the specified range, the window detector output will be different from ‘X’.

Circuit description: Window detector circuit has two operational amplifiers (741 op amps) (op amp-1 and op amp-2) and two diodes connected as shown in the circuit of Fig. 4.11. In this circuit, V_ref (high) has been interchanged as V_ref (low) and V_ref (low) has been interchanged as V_ref (high). Then, the circuit produces opposite type of output signal to the previous circuit. It means that the output voltage will be the negative voltage.

Fig. 4.11 Window Detector Using Two 741 IC Op Amps and Two Diodes

4.5.2 Zero Crossing Detector

Comparator circuits can be used with zero threshold voltage, that is, V_R = 0. The comparator detects and marks voltage level transitions at zero reference crossings of input signal voltage. Then, the comparator circuit is known as ‘zero crossing detector’.

1. Sine wave signal is represented with voltage (on Y-axis) and time (on X-axis).
2. Zero voltage point crossings occur, when the sine wave function changes from positive to negative voltages and at the times of negative to positive voltage transitions across the time axis. Signal crosses through zero voltage points two times in a cycle of sine waveform. Such points are called as ‘zero crossing points’, as shown in Fig. 4.13 (a).
3. ‘Zero crossing detector’ detects the points of crossing zero voltage, while any signal makes transitions from (a) positive to negative voltage or (b) negative to positive voltage. Thus, in a sinusoidal signal, there will be two ‘zero crossing points’ as shown in Fig. 4.13(a).
4. Simple circuits to understand zero crossing detector or comparator circuits are shown in figs 4.12 and 4.13.

Fig. 4.12 (a) Zero Crossing Detector with Positive Input Voltage; (b) Zero Crossing Detector with Negative Input Voltage

Fig. 4.13 (a) Input Sine Waveform; (b) Output Waveform

Sine wave input signal is applied to non-inverting (NIV) input terminal in Fig. 4.12 (a) and zero reference voltage is maintained at (−) inverting terminal (INV). Then, the output waveform with the detection of zero crossings for NINV input voltages is shown in Fig. 4.13(b).

Zero crossing is the point where the voltage is zero on a waveform. Zero crossing points in a sine wave are shown in Fig. 4.13 (a), points 1 and 2 for one (sine wave) cycle.

Zero crossing detector wave forms for inverting input voltages are shown in Fig. 4.14 (a) and (b).

Fig. 4.14 (a) Input Sine Waveform to Zero Crossing Detector; (b) Output Square Waveform

Sine wave input signal is applied to inverting (INV) input terminal in Fig. 4.12 (b) and zero reference voltage is maintained at (+) non-inverting terminal (NIV). Then, the output waveform with the detection of zero crossings is shown in Fig. 4.14.

Zero crossing is the point where the voltage is zero on a waveform. The zero crossing points in a sine wave are shown in Fig. 4.14 (a).

Applications

1. The fundamental frequency of speech is detected by counting the number of ‘zero crossings’ in a speech signal in speech recognition systems. The fundamental frequency of adult male speech ranges from 85 to 180 Hz, whereas the fundamental frequency of adult female speech ranges from 165 to 255 HZ.

   Typical values of fundamental frequencies are 120 Hz for men and 210 Hz for females. With age, these values change. Doctors may advise the patients suitably for the reduction in speech sound based on fundamental frequency measurements.

2. During the transmission of digital signals on analog channel (AC) in modem circuits.
3. The detection of amplitude and zero crossings during the conversion of digital values into binary data in digital communication systems.
4. Image processing techniques, where digital signal of an image passes through a set of preset-zero values.
5. Digital audio.
6. Comparators are mostly analog ICs after operational amplifiers.

Some important specifications on data sheets of comparator ICs for circuit design are as follows:

1. Propagation delay (speed) T_PD: The voltage level comparisons of analog signals need a minimum amount of time between time T₁, when the input signal crosses the reference voltage V_ref and time T₂ when the output voltage crosses 50% of the supply voltage V_CC.
2. Slew rate (rise and fall times of signal responses defining the speed of the device).
3. Bandwidth: It relates to the maximum frequency range of input signals that can be operated with the system. For high frequency signal operation, operational amplifier with large bandwidth has to be considered.
4. Voltage gain: It is the total device amplification between input and output signals.
5. Current consumption for power dissipation considerations.
6. Input off-set voltage V₁₀: It is differential input voltage for device operation without going to false switching between voltage levels.
7. Input common mode voltage range V_ICM: The range of input voltages to be maintained by both the inputs for regular function of the comparator.
8. Common mode rejection ratio (CMRR).
9. Supply voltage rejection ratio (SVR).

4.6 SCHMITT TRIGGER (REGENERATIVE COMPARATOR)

Schmitt trigger name has two words: (a) Schmitt and (b) trigger.

Schmitt is the name of the inventor and trigger means that the output voltage remains constant until the input voltage changes sufficiently to trigger changes in voltage levels.

Otto Herbert Schmitt, American Scientist and Engineer (1913−1998) invented the following circuits: (a) Schmitt trigger (b) cathode follower (c) differential amplifier, and (d) chopper stabilized amplifier.

Schmitt trigger circuit is generally used in signal conditioning applications:

1. To remove noise from signals.
2. To convert sine waves into square waves.
3. To obtain digital output (signal wave shape) by suitable selection of two ‘threshold voltage levels’ for any type of analog input signal waveform.
4. Digital circuits.
5. Squaring circuit.
6. Amplitude comparator circuit.

Schmitt trigger circuit uses positive feedback for its operation. It has two levels of transitions in its output voltage with suitable threshold voltage levels in its input voltages. They are known as follows:

1. Upper threshold point (UTP), where the output voltage makes transition from low to high voltage level for a suitable level of ‘trigger’ initiated by input voltage. For UTP, the input voltage should be higher than the chosen ‘threshold voltage’.
2. Lower threshold point (LTP), where the output voltage makes transition from high to low voltage level for a suitable level of ‘trigger’ initiated by input voltage. For LTP, the input voltage should be lower than the chosen ‘threshold voltage’.
3. During the two transition levels, the output voltage remains constant.

Thus, the Schmitt trigger has dual thresholds. Dual threshold action is considered as possessing ‘Hysteresis’. The two stable states in the output voltage suggest that the behaviour of Schmitt trigger circuit is similar to bistable multivibrator or flip-flop.

Schmitt Trigger circuit can be understood as a comparator (differential amplifier) with positive feedback (loop gain greater than one). Positive feedback is introduced by using two resistors R₁ (R_F) and R₂ as shown in Fig. 4.17. A part of output voltage is feedback to input voltage using the two resistors R₁ (R_F) and R₂ (voltage divider circuit).

Schmitt Trigger uses the following concepts of (UTP) and (LTP):

1. Upper Trigger or Threshold point (UTP)
2. Lower Trigger or Threshold point (LTP)

Unsymmetrical or symmetrical output signal waveforms can be obtained using suitable design and selection of UTP and LTP. Block diagram is shown in Fig. 4.15.

Fig. 4.15 Conversion of Sine Wave (Input) into Square Wave (Output Signal) by Schmitt Trigger Circuit

The layout of transistor Schmitt trigger circuit is shown in Fig. 4.16. Circuit has the following components:

Fig. 4.16 Schmitt Trigger Circuit Using Transistors to Generate Square Waveform from a Sine Waveform

1. Two transistors.
2. Biasing resistors R_C1, R₁, and R_2/1
3. Timing circuit R₁ and C₁.
4. Feed back and DC bias stabilization providing resistor R_E.
5. Output voltage of the first transistor T₁ is connected to the base terminal of T₂.
6. DC power supply voltage V_CC.

There are two conditions of operations for bistable multitype of operation using UTP and LTP concepts in the Schmitt trigger circuit:

1. Transistor T₂ is in ON state, while the transistor T₁ is in the OFF state.
2. Transistor T₁ is in ON state, while the transistor T₂ is in the OFF state.
3. Upper threshold voltage
4. Lower threshold voltage

4.7 INVERTING SCHMITT TRIGGER

Schmitt trigger circuit using operational amplifier and positive (regenerative) feedback.

Inverting Schmitt trigger circuit using op amp is shown in Fig. 4.17.

Fig. 4.17 Inverting Schmitt Trigger Circuit Using Op Amp and Regenerative (Positive) Feedback

Circuit description: Inverting Schmitt trigger circuit consists of the following components:

1. Operational amplifier (op amp).
2. Dual power supply to provide + V and − V voltages to op amp terminals on IC.
3. Input signal source (function generator) to provide sine wave of desired amplitude and frequency (function generator with negligibly low internal resistance).
4. Cathode ray oscilloscope (CRO) to observe and measure input and output signals.
5. Resistors R₁ and R₂ provide positive (regenerative) feedback to fix the upper threshold point and lower threshold points to determine output wave shape.

Input voltage V_in (sine wave) from a signal (function) generator or beat frequency oscillator (BFO) is applied to negative (−) (inverting) input terminal of operational amplifier preferably μA 741. Feedback voltage V_F (V_Y) is applied to the non-inverting input (+) terminal of op amp through the resistor R₁ (R_F). The feedback voltage V_F = βV_out, where . Then

Between voltage levels defined by upper threshold point (UTP) and lower threshold point (LTP), points of input sine wave signal amplitudes, clamping of output signal waveform occurs, so that output is a square waveform.

Thus, it works as an excellent voltage level detector with fast switching actions.

Positive feedback in Schmitt trigger circuits: Consider small magnitude of positive voltage V_in at the inverting input terminal (1) of the op amp. It is amplified and produces increased negative output voltage at the output terminal (3).The voltage divider R₁ (R_F) and R₂ feeds a part of negative output voltage to the non-inverting input terminal (2).

The feedback voltage V_F = β V_out where . It is amplified and feedback to the input port. This regenerative action continues till the output voltage V_out reaches V_out (max) = V_CC = − V_Sat (negative saturation voltage). Output voltage will be held in the ‘low state’ of value V_CC = − V_Sat.

Thus, output voltage level − V (negative saturation voltage) is obtained through regenerative action (positive feedback) in comparator. Such action suggests that the ‘Schmitt trigger works as regenerative comparator’.

On similar lines, it can be understood that for small values of negative input voltages, regenerative comparator action due to positive feedback causes output voltage to be held at V_CC = + V_Sat. Finally, output voltage will be held at ‘high state of value V_CC = + V_Sat’.

Schmitt trigger is similar to bistable multivibrator circuit: It is used for converting analog signals into digital signals using the concepts of UTP and LTP.

The analysis of input and output voltage levels to determine UTP and LTP Schmitt trigger is as follows:

1. When the input voltage to inverting input terminal V_in = 0 V, assume the maximum level of output voltage V_out (max) = V_CC = + V_Sat.
2. Then, the non-inverting input voltage .
3. As the input voltage goes on increasing, at a trigger (threshold) voltage level of magnitude , the output voltage makes a sudden transition or jump to −V_Sat. Then, the transition is defined to take place (occur) at upper threshold (trigger) point (UTP) at voltage V₁.
4. .
5. Then, the output voltage V_out (max) = V_CC = − V_Sat.
6. Second (another) transition occurs when the input voltage V_in reduces below
7. Voltage V₂ at LTP = .
8. Then, the output voltage changes (switches) from − V_Sat to + V_Sat/1

Fig. 4.18 Input and Output Waveforms of Operational Amplifier

Change in voltage levels from UTP to LTP is due to hysteresis in the transfer curve for Schmitt trigger circuit performance. Hysteresis voltage .

Transfer Characteristic to Explain Hysteresis in Schmitt Trigger Circuit

Transfer characteristic of Schmitt trigger is a graph between the input voltage and its output voltage response. The analysis of voltages for input and output signals are also shown using hysteresis loops. Figure 4.19 (a) shows the sudden transition of output voltage V_out from + V_Sat to V_Sat, when the input voltage (V_in) exceeds V₁ (V_UTP). This shows input and output transitions at UTP.

Fig. 4.19 (a) UTP (b) LTP (c) Total Hysteresis Loop of Inverting Schmitt trigger

Figure 4.19 (b) shows the sudden transition of output voltage V_out from − V_Sat to + V_Sat, when the input voltage (V_in) exceeds V₂ (V_LTP). This shows the input and output signal transitions at LTP.

Figure 4.19 (c) shows the transitions of output voltage V_out from − V_Sat to + V_Sat and vice versa at both UTP and LTP points. It shows the composite or total hysteresis loop characteristics during UTP and LTP transitions over a cycle of variations.

4.8 WAVE SHAPING OF OUTPUT SIGNAL USING ADJUSTABLE LTP AND UTP

Irrespective of the nature of repetitive input (analog) signal, Schmitt trigger converts them into square wave signals. It is achieved with adjustable LTP and UTP points of the device operation.

With the previous circuit in Fig. 4.17, UTP and LTP voltages are symmetrical about the origin ‘O’ on the transfer characteristic of the hysteresis loop. Using two semiconductor diodes and adjustable (variable) resistors, (Fig. 4.20) UTP and LTP points can be shifted to desired levels to modify the points of transitions in the output waveforms.

Fig. 4.20 (a) UTP (b) Adjustable LTP and (c) Composite Hysteresis Loop of Inverting Schmitt Trigger

Assuming initial output voltage level as +V_Sat, output voltage in the output waveform can be seen as .

4.9 NON-INVERTING SCHMITT TRIGGER

Non-inverting Schmitt trigger circuit is shown in Fig. 4.21. Input current I_in through op amp input port will be negligible, that is, I_in ≅ 0. Practically, I_in ≅ I_B ≅ 200 nA (max).

Fig. 4.21 (a) Non-Inverting Schmitt Trigger Circuit (b) Hysteresis Loop

However, the current through R₂ will be equal to current I₁ through R₁. Then, V_R1 = V_R2

which means

Therefore,

As the input terminal (2) shown in Fig. 4.21 is at virtual ground potential, (voltage across R₂) = V_in (input voltage at non-inverting input terminal).

If the op amp is in lower state (voltage across R₁), V_R1 will be equal to the output voltage, − V_Sat at the initial state of transition. Therefore, initially V_R1 = −V_Sat.

At lower threshold point (LTP) .

Therefore, .

As the input voltage increases, sudden jump or transition takes place from −V_Sat to +V_Sat.

Then, at UTP.

V_Sat to −V_Sat at LTP, the voltage level is

at LTP.

Hysteresis voltage

4.10 ADJUSTABLE UTP/LTP SCHMITT TRIGGER

UTP and LTP can be individually adjusted using the non-inverting Schmitt trigger circuit shown in Fig. 4.22 (a) and (b).

Fig. 4.22 (a) Non-inverting Schmitt Trigger Circuit Using Adjustable UTP and LTP (b) Hysteresis Loop

Adjustable Schmitt trigger circuit consists of the following electronic components in the circuit:

1. Input signal source.
2. Operational amplifier.
3. Two diodes (rectifier devices) D₁ and D₂.
4. Three resistors R₁, R₂, and adjustable resistor R_A.
5. Cathode ray oscilloscope (CRO).

In the non-inverting Schmitt trigger circuit in Fig. 4.22 (a), there are two types of signal feedbacks from the output to the non-inverting input terminal through two diodes D₁ and D_A For positive and negative outputs, diode D₁ conducts during negative output (− V_Sat) and diode D_A conducts for positive output (+ V_Sat).

At UTP, required input voltage

At LTP, required input voltage

Threshold voltage

Example 4.2

Draw the output signal waveform of non-inverting Schmitt trigger circuit using op amp with the following data:

1. Supply voltage V_CC = ± 15 V.
2. Saturation voltage levels (−V_Sat) = −14 V.
3. Input signal V_JN = V_m sin (ω t) 4 sin (ω t)
4. R₁ = 22 K and R₂ = 2.2 K

Solution: Upper threshold voltage V₁ at UTP =

Lower threshold voltage V₂ at LTP

1. Output voltage makes transition to +V_Sat = −14 V, when the input voltage exceeds + 1.4 V at UTP.
2. Output voltage stays at +14 V till the input signal reduces to V₂ = −1.4 V at LTP. Then, it suddenly jumps or drops to voltage −V_Sat = −14 V.
3. Output voltage stays at +14 V till the next transition to + V_Sat again at second UTP at second cycle of sine wave input signal.

Fig. 4.23 Hysteresis Loop

Fig. 4.24 (a) Block Diagram Concept of Schmitt Trigger Circuit Using 555 Timer IC. (b) Input and Output Waveforms Using the Concept of UTP and LTP Threshold Voltage Levels

Schmitt trigger circuit operation: Input signal V_in, for example, a sine wave is applied between the input pair of terminals (5) and (1) of 555 IC, as shown in Fig. 4.24. When the input signal voltage rises slightly above the UTP shown as voltage V₁ in Fig. 4.24 (b), the trigger action begins and the comparator output becomes high as shown in Fig. 4.24 (b) across the terminals (3) and (1) of 555IC. It is shown as point (1) on the output square wave.

is the UTP voltage level. At a later stage, if input voltage V_in goes on decreasing, the 555 IC timer output remains constant (without undergoing any change from the previous high value), until the input voltage reduces to a value below V₁ that corresponds to LTP. It is shown as point V₂ on the output waveform in Fig. 4.24 (b). It is marked as point (2), where output is triggered to change (makes transition) to a low value.

is the LTP voltage level.

Fig. 4.25 Typical Schmitt Trigger Circuit Using IC 555

Such output function of 555 IC will be similar to Schmitt trigger behaviour. Output remains constant between UTP and LTP threshold voltage levels. Hysteresis is the voltage range between the two voltage levels V₁ and V₂. Maintaining V₁ always higher than V₂, adjustable hysteresis with desired points of UTP and LTP can be realized in the Schmitt trigger behaviour.

1. Name of 555 IC is suggested due to the use of three resistors (5 kΩ) inside IC 555.
2. 556 IC is 14-pin dual package containing two 555 ICs.

4.11 MULTIVIBRATORS

There are three types of multivibrator circuits. They are as follows:

1. Astable multivibrator: It is known as free running oscillator or square wave generator.
2. Monostable multivibrator: It is referred as ‘one shot’ multivibrator.
3. Bistable multivibrator: It is popularly known as ‘flip-flop’ circuit.

4.11.1 Astable Multivibrator

Astable Multivibrator Circuit Using Operational Amplifier

Introduction:

1. Astable multivibrator produces square-wave output signal. It is also known as free running oscillator, because the multivibrator does not require any external trigger input signals to change states in output square wave signal waveform.
2. Astable multivibrator output has two quasi-stable states. The output switches between positive and negative saturation level voltages ±V_Sat. No external trigger signal is required to produce changes in levels ±V_Sat of output voltage waveform.
3. The concepts of positive feedback, hysteresis, UTP, and LTP are used in the circuit with an addition of a capacitor at the input port of the operational amplifier shown in Fig. 4.26. Charging and discharging cycles of ‘capacitor C’ through the ‘feedback resistor R_F’ forces the output to swing between the charging positive voltage level − V_Sat (equal to supply voltage − V_CC or V_S). Such voltage swings produces free running square wave output without any external inputs. The frequency of output waveform will be decided by ‘RC time constant’ (R_F · C).
4. Time period of the output signal waveform .
5. Frequency of output signal

   

Astable Multivibrator Circuit

Fig. 4.26 Astable Multivibrator Circuit Using Operational Amplifier

Op Amp Multivibrator Circuit:

1. Op amp multivibrator circuit are very simple to design, construct and use them in a user friendly way. By simply turning ‘ON’ the Astable Multivibrator, the circuit functions as free running oscillator generating square-wave output signal. It can be used when a simple square wave is required.
2. Op amp Astable Multivibrator circuit consists of the following components:
   1. one operational amplifier
   2. three resistors R_F, R₁, and R₂, and
   3. one capacitor ‘C’.
3. It has two operations embedded in the circuit function:
   1. Feedback from output terminal to capacitor ‘C’ is provided by the feedback resistor R_F.
   2. Presence of hysteresis (to maintain the two threshold states of operation of LTP and UTP) is provided by the two resistors R₁ and R₂.
4. Circuit design:
   1. Assume required frequency ‘f’ of square wave to get from ‘Astable Multivibrator’.
   2. Calculate the time period sec.
   3. Choose the practical values of R_F, C · R₁, and R₂ to satisfy following equation.
   4. (d) Frequency of output signal

      f =

   5. (e) Observe the waveform using CRO to verify the design with estimated values.
5. In computer circuits, logic gates function as decision making devices. Binary addition and subtraction are performed by logic gates. However, the presence of decision making circuits is not enough. During the logical addition and subtraction processes, memory devices are needed to do the total arithmetical functions. Such memory elements are realized by using multivibrator circuits known as flip-flop circuits.
6. Op amp multi (multivibrator) has the advantage of providing higher output voltages equal to V_CC or V_S ranging from 12 to 15 V. While, multivibrator circuits using ‘logic gates’ provide a maximum output voltage equal to its supply voltage of 5 V.

4.11.2 Monostable Multivibrator

Monostable Multivibrator Circuit Using Operational Amplifier

Monostable Multivibrator is known as one-shot multivibrator. Output has one stable state and one quasi-stable state. Output will remain in stable state (V_out = + V_S), (where V_S = supply voltage to op amp) till the multivibrator receives a trigger input. Trigger input pulses are shaped into sharp (narrow) pulses by the R–C differentiator circuit consisting of capacitor C₁ and resistor R₃. The time constant R₃ · C₁ should be much less than the time period ‘T’ to produce narrow triggering pulses and avoid false transitions in states.

Fig. 4.27 Monostable Multivibrator Using Operational Amplifier

Once the sharp trigger pulse is applied at non-inverting input terminal (3) of op-amp, the output makes a transition to quasi-stable (temporary) state (V_out = − V_S) for a specified (designed) time period ‘T’. The time period is decided by the R−C combination of C₂ and R_F. The diode D₁ limits the positive amplitude of the trigger pulses. After the expiry of the time period, the output returns to the previous stable state, as shown in the output waveform in Fig. 4.27.

Op amp monostable multivibrator circuit in Fig. 4.27 has the following electronic components:

1. Operational amplifier.
2. Feedback resistor R_F and timing capacitor C₂ to determine the time period ‘T’ of output signal and its frequency.
3. Diode D₂ is connected in parallel to capacitor C₂.
4. Trigger pulse wave shaping circuit consisting of differentiator with elements C₁, R₃, and diode D₁.
5. Potential divider network consists of R₁ and R₂ to provide threshold voltage levels.
6. Output waveform appears across terminal ‘6’ and common terminal.

4.12 PRECISION RECTIFIERS

4.12.1 Series Diode Half-Wave Rectifier (HWR) Circuit

Half-wave rectifier circuit has AC input signal with alternating half sinusoids. It allows only positive half sinusoids into the output signal. The negative half sinusoids are blocked by rectifier device.

Circuit components are as follows:

1. Input signal source: sinusoidal signal (function generator/mains AC with step-down transformer): sinusoidal signal.
2. Ideal diode (functions as rectifier).
3. Resistor.
4. Cathode ray oscilloscope (CRO to observe waveforms and measure amplitudes).

Working Principle:

1. During the positive half cycle of input sine waveform, semiconductor diode is forward biased. However, the diode conducts and behaves as a closed switch with negligibly small forward resistance. Therefore, the output wave is similar to input signal shown in Fig. 4.29.
2. During the negative half cycle of input sine waveform, diode is reverse biased. Diode does not conduct and behaves as open switch with zero I_R ≅ 0 current in the circuit. Thus, the output voltage will be zero as shown in Fig. 4.29.
3. In Fig. 4.28, the situation is explained for one cycle of input signal. The feature repeats and the output waveform consists of positive half sinusoids for each cycle of input signal waveform.
4. Thus, the alternating input sine waveform appears at the output with half sinusoids in one (positive) direction only.
5. Rectification is one of the important process in signal condition of electronic systems.

Fig. 4.28 Series Diode Half-Wave Rectifier (HWR) with Input and Output Signal Waveforms