MODULE IV

Part 1: Analogue & Digital ICs

Analogue Integrated circuits

• Integrated circuit (IC) is an electronic circuit consisting of large number of electronic devices such as resistors, capacitors, diodes, transistors,etc. are fabricated on a single semiconductor substrate.
• Introduction of ICs made electronic devices with very small size combined with a lot of functions in a single device.
• Ics can be divided in to analog and digital.
• Analog Ics are also known as linear Ics. The output signal is a linear function of the input.
• Analog circuits deals with signals free to vary from zero to full power supply voltage.
• The operational amplifier(op-amp) is a common device in these applications. Ex. IC 741
  • Digital Ics operate at only a few defined states, rather than over a continuous range of signal amplitudes.
  • These devices are used in computers, switching circuits, etc.
  • The fundamental building blocks of digital Ics are logic gates, which operate with binary data (logic 0 and logic 1)
  • circuits are deals with signals restricted to the extreme limits of zero and some full amount.

The advantages of ICs are

• Very small size and low weight
• Reduced cost
• High reliability
• Low power requirement
• Easy replacement

Operational Amplifier

• It is a direct coupled high gain differential amplifier followed by a level shifting circuit and an output stage.
• Used for mathematical operations such as summation, subtraction, multiplication, integration and differentiation of electrical signal.
• It is the basic analog integrated circuit.

• • Input stage: the first stage is a Differential amplifier input stage to amplify the difference between input signals (V1-V2).
  • This stage provides high input resistance and voltage gain.
  • Intermediate stage: series of cascaded amplifier stages designed to give maximum amplification to the difference input signal.

Level shifter:

• on cascading, DC level associated with the output waveform rises from stages to stages.
• Level shifter will bring down the DC level to the reference position.
• Output stage: provide large output voltage swing with the help of power amplifiers.

Equivalent circuit of op-amp

• The Equivalent circuit is useful in analysing the basic operating principles of op-amp and in observing the effects of standard feedback arrangements.
• Vo=G (V1- V2)
• This equation indicates that the output voltage Vo is directly proportional to the algebraic difference between two input voltages.

Op-Amp Parameters

• Common-Mode Rejection Ratio (CMRR)
  • The ability of amplifier to reject the common-mode signals (unwanted signals) while amplifying the differential signal (desired signal)
  • Ratio of differential voltage gain, Ad to common-mode gain, Acm
  • CMRR is usually expressed in dB & decreases with frequency

Input Offset Voltage

• • Ideally, output of an op-amp is 0 Volt if the input is 0 Volt.
  • Realistically, a small dc voltage will appear at the output when no input voltage is applied.
  • Voltage that must be applied between two input terminals of an op-amp to nullify the output.
  • This is called the Input Offset Voltage, Vos. Range between 2 mV or less.
• Input Bias Current
  • Ideally should be zero
  • Is the average of both input currents
  • The dc current required by the inputs of the amplifier to properly operate the first stage.

Input Offset Current ( Iio ):

• Is the difference of input bias currents, Iio = |I1 – I2|

Input Impedance

• • Differential input impedance : total resistance between the inverting and non-inverting inputs
  • Common-mode input impedance: total resistance between each input and ground

• • Output Impedance
    • Ideally should be zero
    • It is the resistance viewed from the output terminal of the op-amp and ground.
    • It is 75ohm for the 741 IC op-amp.

• Slew Rate
  • Is the maximum rate of change of the output voltage w.r.t time and is expressed in V/µs.

Ideal op-amp characteristics

Basic op-amp circuits

Inverting amplifier

• An amplifier which provides a phase shift of 180deg. between input and output.
• The input signal Vi is applied to the inverting terminal and the non-inverting terminal is grounded.
• Resistors Ri and Rf determine the gain of the circuit. Rf is known as feedback resistor.The expression for gain is given by

• Negative sign indicates that output is the inversed form of input.

Expression for closed loop voltage gain

• As node B is grounded, node A is also at ground potential from the concept of virtual ground. So VA = 0

II. Non-inverting amplifier

• • The input signal vi is applied to the non-inverting terminal. Resistors R1 and Rf determine the gain of the circuit. The expression for gain is given by

• • Gain, A= 1+ 𝑅𝑓/𝑅1
  • Input and output waveforms are in phase.

Expression for closed loop voltage gain

• • Applying KCL at node A

Comparator

• It is a circuit used to compare two input signals.
• Signal Vin is compared with a known voltage called, reference voltage Vref and produces a high or low output depending on difference of the inputs.
• An op-amp in open-loop configuration can be used as a comparator.
• In an op-amp comparator if the voltage at non inverting terminal is more than that of the inverting terminal, then the output is high.
  • The voltage at which a comparator changes from one level to another is called the crossover (threshold) voltage.
  • Since the open loop gain of the op-amp is very high, it will be going into saturation giving an output voltage of either +Vsat or –Vsat depending on the magnitude of input voltage.

Non-inverting Comparator

• The output voltage of an op-amp is expressed as Vo = A(V⁺ − V⁻)
• Where A= open-loop gain, V⁺ & V⁻ voltages at the non-inverting and inverting terminals respectively.
• Here V⁺ = Vin & V⁻ = Vref
• If Vin>Vref, the output voltage is at positive saturation (+Vsat)
• If Vin<Vref, the output is at negative saturation (-Vsat)
• Vsat= saturation voltage
• If the reference voltage Vref=0, then the circuit is known as zero crossing detector.

Non-inverting comparator with Vref >0

• • When the input voltage is greater than Vref, the output is high, otherwise it is low.

Non-inverting comparator with Vref <0

Inverting comparator

• Input is applied at inverting terminal of op-amp.
• Inverting comparator saturates at negative voltage when the input goes greater than Vref
• similarly, it will saturate at positive voltage when the input voltage is lower than Vref

Inverting comparator with Vref >0

• When the input voltage increases than a positive reference voltage, output saturates at the negative voltage. Otherwise output remains in positive saturation.

Inverting comparator with Vref <0

• When the input voltage goes lower than reference voltage, output saturates at the positive voltage. Otherwise output remains in negative saturation.

LOGIC GATES

• Logic gates are fundamental building blocks of digital circuitry.
• A logic gate is an element that takes binary input signals and produces an appropriate binary outputs.
• The types of gates are NOT, AND, OR, NAND, NOR, XOR, and XNOR
• Truthtable: A table listing all the possible combinations of input variables and corresponding outputs
• Basic Gates: AND,OR & NOT
• Universal Gates: NAND & NOR

Inverter Gate

• It has only one input.
• It also called NOT gate.
• It outputs the opposite logic state of its input.

OR Gate

• • It has two or more than two inputs.
  • Its output is 0 only when all its inputs are 0.
  • Q = A+B.

AND Gate

• It has two or more than two inputs.
• Output is 1 only when all its inputs are 1.
• Q = A.B

NAND Gate

• It has two or more than two inputs.
• Its output is 0 only when all its inputs are 1.
• Q =

NOR Gate

• • It has two or more than two inputs.
  • Its output is 1 only when all its inputs are 0.

XOR Gate

• • • It has two or more than two inputs.
    • Its output is 1 only when exactly one of its inputs is 1.

XNOR Gate

• • • It has two or more than two inputs.
    • Its output is 1 only when all its inputs are set to the same logic state.

• • • Realise the Logic functions

1. X= AB + CD
2. X= AB + Bbar.C

De Morgan’s Theorems

Universal gates

• It is possible to implement any Boolean expression with NAND & NOR gate. Therefore NAND & NOR gates are called universal gates.

Electronic Measurements and

Measuring Equipments.

• Measurement is the process of determining the amount, degree or capacity by comparison with the accepted standards of the systems being used.
• Electronic instrument is a device for determining the value or magnitude of a quantity or variable such as voltage, current or resistance.

PERFORMANCE PARAMETERS

• Accuracy : It is the degree in closeness of actual measurement compared to the expected value of measured quantity.
• Precision: It is a measure of consistency or repeatability of measurement.That means, when a quantity is measured repeatedly the instrument should give the same value.
• Resolution: The smallest change in a measurement variable to which an instrument will respond.

• Sensitivity: It is the ratio of change in the output of the instrument to a change in the input of the instrument.
• Expected value: It is the desired value of measured quantity or the most probable value that is expected to obtain.

• Error: It is the deviation of true value from the expected value.

TYPES OF ERRORS IN MEASUREMENT

• Gross errors : These errors occur due to human mistakes in reading instruments and calculating results of measurements. Observation errors arise due to carelessness of operators is another source of gross error.

– These errors can be minimized by taking care in using and reading measurements.

• • Also the instrument should be calibrated before using it.
• Systematic errors : These errors occur due to the faults of measuring instruments. It can be caused by effects such as sensitivity shift, zero-offset and non-linearity. It is again divided into two :

• • Instrumental error , which arise due to misuse of the instruments.
  • Environmental errors arise due to external conditions to the measuring device ( eg: effect of pressure, temperature etc.)
• Random errors : The cause of random errors are not exactly known, so they can not be eliminated. They can be only reduced. These errors are accidental, small and independent.
• Absolute errors : It is the amount of physical error in a measurement. If a resistor is said to have 100Ώ with a possible error of 50Ώ, then ±50Ώ is the absolute error.
• Relative error : If the absolute error is expressed as percentage or as a fraction of total resistance, it is known as relative error. For example resistance is expressed as 100Ώ ± 50%.

Relative error = (Measured value – Actual value)/Actual value

General performance parameters of electronic equipment are

• Accuracy
• Precision
• Resolution
• Sensitivity
• Expected value
• Error

Types of errors in measurement

• Gross errors
• Systematic errors
• Random errors
• Absolute errors and relative errors

Multimeter

• Multimeter is an electronic instrument that combines several measurement functions in one unit.
• it is used to measure voltages(ac & dc), currents (ac & dc) capacitance and resistance.

Digital multimeter(DMM)

• Digital multimeter displays the measured quantities in digits.
• Digital meters offer higher accuracy, reduction of reading error, elimination of parallax error, unambiguous readings at greater viewing distances, smaller size

• • The main part of digital multimeters is the Analog to Digital converter which converts an analog input signal to a digital signal.
  • DMM is a voltage sensing meter, so for current measurement, current is converted to volts by passing it through a resistance.
  • For the measurement of low value current, a precision low resistance shunt is used.
  • The value of current is obtained by the voltage developed across the resistor divided by the value of resistance.
  • Attenuator scales down the voltage to be measured.
• For the measurement of ac quantities, ac is first converted to dc with the help of rectifiers and filters.
• Finally, this dc level is applied to ADC to obtain the digital information.
• For resistance measurement, the meter includes a precision low current source that is applied across the unknown resistor.
• Then the dc voltage drop across the resistor, which is proportional to the value of the unknown resistor, is measured.
• For ac measurements, the digital multimeter is a true rms instrument which measures true rms value of any periodic signal.

Digital Storage Oscilloscope (DSO)

• • Analog oscilloscopes fail to perform well for displaying high frequency signals.
  • Higher input frequency causes the electron beam to move fast across the screen and hence only a faint trace is obtained.
  • The signals to be observed is first applied to amplifier/attenuator.
  • Low amplitude signals are amplified and high amplitude signals are attenuated to desired value.
  • This signal is digitized using sampling circuit and an ADC. Sampling rate is decided by the time-base generator.
  • Time-base provides gating pulses to sampling circuit.
  • The discrete values of the waveform, called waveform points, are further converted to digital values by the ADC.
  • The digital values are stored in memory.

• • • Digital values stored in memory is to be converted to analog form for displaying it.
    • The DAC triggered by the pulses fom time-base, converts each digital sample back to analog form and passes it to the vertical deflection amplifier.
    • The time-base also generates a staircase waveform that is to be applied to horizontal deflection amplifier.
      • Nm

Function Generator

• Various waveforms such as sine, triangular, square etc. are required in many applications of electronics at laboratories and industries.
• For such purposes signal generators are widely used.
• A function generator consists of an integrator, squaring circuit, sine wave converter and attenuator in addition to amplitude and frequency control.
• Integrator generates a linearly varying ramp waveform from the applied dc supply.
• The integrator output is fed into squaring circuit and the sine wave converter.

• Integrator converts the squaring wave from the squaring circuit to a triangular wave.
• Sine wave converter converts the signal from integrator to sine wave.
• A switch is used for selection of sine, triangular or squaring waves.
• The attenuator is used for reducing amplitude to desired value.
• It provides low output impedance and control on output amplitude.

Generated Waveforms

• Sine wave :This is the standard waveform that oscillates between two levels with a standard sinusoidal shape.

• Square wave : A square wave is a signal which takes one high level and a low level. It is characterized by the duty cycle, which is the ratio of time the signal posses high level to its time period.

• Pulse wave : It is similar to square wave, but with either positive or negative voltage

levels.

• Triangular wave : This form of signal produced by the function generator linearly moves between a high and low point.

• Sawtooth wave : This is similar to triangular waveform, but the fall time is negligibly small.