Chapter-1-

Introduction

Introduction:

Virtually every piece of electronic equipment, e.g., computers and their peripherals, calculators, TV and hi-fi equipment, and instruments, is powered from a DC power source, be it a battery or a DC power supply. Most of this equipment requires not only DC voltage but voltage that is also well filtered and regulated. Since power supplies are so widely used in electronic equipment, these devices now comprise a worldwide segment of the electronics market in excess of $5 billion annually.

There are three types of electronic power conversion devices in use today which are classified as follows according to their input and output voltages:1) The AC/DC power supply2) DC/DC converter 3) The DC/AC inverter.

Each has its own area of use but this paper will only deal with the first two, which are the most commonly used.

55

A power supply converting AC line voltage to DC power must perform the following functions at high efficiency and at low cost:

1. Rectification: Convert the incoming AC line voltage to DC voltage.

2. Voltage transformation: Supply the correct DC voltage level(s).

3. Filtering: Smooth the ripple of the rectified voltage.4. Regulation: Control the output voltage level to a

constant value irrespective of line, load and temperature changes.

5. Isolation: Separate electrically the output from the input voltage source.

6. Protection: Prevent damaging voltage surges from reaching the output; provide back-up power or shut down during a brown-out.

An ideal power supply would be characterized by supplying a smooth and constant output voltage regardless of variations in line voltage, load current or ambient temperature at100% conversion efficiency.Figure 1 compares a real power supply to this ideal one and further illustrates some power supply terms.

Ch.3 Multivibrator

55

LINEAR POWER SUPPLIES:

Figure 2 illustrates two common linear power supply circuits in current use. Both circuits employ full-wave rectification to reduce ripple voltage to capacitor C1. The bridge rectifier circuit has a simple transformer but current must flow through two diodes. The center-tapped configuration is preferred for low output voltages since there is just one diode voltage drop. For 5V and 12V outputs and -12V outputs, Schottky barrier diodes are commonly used since they have lower voltage drops than equivalently rated ultra-fast types, which further increase power conversion efficiency. However, each diode must withstand twice the reverse voltage that a diode sees in a full-wave bridge for the same input voltage.

The linear voltage regulator behaves as a variable resistance between the input and the output as it provides the precise output voltage. One of the limitations to the efficiency of this circuit is due to the fact that the linear device must drop the difference in voltage between the input and output.Consequently the power dissipated by the linear device is (Vi–Vo) x Io. While these supplies have many desirable characteristics, such as simplicity, low output ripple, excellent line and load regulation, fast response time to load or line changes and low

Ch.3 Multivibrator

55

EMI, they suffer from low efficiency and occupy large volumes. Switching power supplies are becoming popular because they offer better solutions to these problems.

Block diagram for power supply & transformer:

The first section is the TRANSFORMER steps up or steps down the input line voltage and isolates the power supply from the

power line.

The RECTIFIER section converts the alternating current input signal to a pulsating direct current.

However, as you proceed in this chapter you will learn that pulsating dc is not desirable.For this reason a FILTER section is used to convert pulsating dc to a purer, more desirable form of dc voltage.

Ch.3 Multivibrator

55

The final section, the REGULATOR, does just what the name implies. It maintains the output of the power supply at a constant level in spite of large changes in load current or input line voltages.

Now that you know what each section does, let's trace an ac signal through the power supply. At this point you need to see how this signal is altered within each section of the power supply.The filter section, a network of resistors, capacitors, or inductors, controls the rise and fall time of the varying signal; consequently, the signal remains at a more constant dc level. You will see the filter process more clearly in the discussion of the actual filter circuits.

Transformer only

Ch.3 Multivibrator

55

Transformer + Rectifier

Transformer + Rectifier + Smoothing

Ch.3 Multivibrator

55

Transformer + Rectifier + Smoothing + Regulator

The components used in power supply

Picture

Capacitor 0.1 – 4700 uf

Fuse 0.5mA

Switch

Regulator 7805-7812-7912

Bridge rectifier

Transformer 12V

Ch.3 Multivibrator

55

Circuit of Power supply

Ch.3 Multivibrator

55

Chapter -2-

DC Power supply:

Electric energy is most often delivered in the form of alternating current. The voltage and the current alternate sinusoidal between two extremes. This cycle is repeated 60 times a second (50 times a second in Europe). Very often, and particularly for electronic circuits, direct current is desired.

Voltage and current which are stable in time, the degree of stability of the voltage varies according to how the DC is produced. A dry cell, for example, gives very good short term stability. There are no fast fluctuations but the long term stability is poor because the cell runs down. A power supple converting AC to DC may, on the other hand, have a good long term stability but rather poor short term stability. In this case fast but often small variations called ripple originate from the AC input leaking through the AC to DC converter.

The DC Power supply is a device that has the ability to convert a single phase AC Voltage of high amplitude and frequency (such as 220v 50Hz) to DC voltage and providing constant regulated output voltage with almost no fluctuation and noise.

Almost every piece of electronic equipment that makes use of 120v/220v AC as a source of power will have a built in dc power supply.

Ch.3 Multivibrator

55

It's the most important element in the project as it is responsible for feeding the multivibrator circuit; also it can be used to operate in other low power DC devices at the same time.

Overload Protection:

Power supplies should have some types of overload protection. Overload protection is important to protect the electronic equipment hooked up to the power supply and to also prevent overheating. This could potentially lead to an electrical fire. Fuses and circuit breakers are two of the more frequent mechanisms used for overload protection.

Each of the blocks is described in more detail below.

Ch.3 Multivibrator

55

DC Power supply consists of:

1. Transformer: for stepping down the voltage to 24v output with 1A max current.

2. Bridge rectifier: for converting AC signal to pulsating DC signal.

3. Capacitor filter: filters the pulsating DC signal.

4. Voltage regulator: keeps constant output DC voltage.

Ch.3 Multivibrator

55

Transformers:

A Transformer is a device used for stepping the voltage up or down it depends on the transfer of electrical energy from one circuit to another through inductively coupled electrical conductors. Ideally, a transformer shouldn't introduce any change in power factor and should have zero internal power loss. But due to electrical and magnetic losses the maximum efficiency of a transformer can't exceed 80%.

Basic Theory:

The transformer is based on two principles firstly that an electric current can produce a magnetic field (electromagnetism) and secondary: that changing magnetic filed within a coil of wire induces a voltage across the ends of the coil (electromagnetic induction).By changing the current in the primary coil, it changes the strength of its magnetic field since the changing magnetic field extends into the secondary coil and a voltage is induced across the secondary.

Ch.3 Multivibrator

55

Induction law:

The voltage induced across the secondary coil maybe calculated from faraday's law of induction, which states that:

Vs = Ns dQ/dt

Where Vs is the instantaneous voltage, Ns are the number of turns in the secondary coil and Q equals the total magnetic flux through one turn of the coil.

Ch.3 Multivibrator

55

The changing magnetic field induces and electromotive force (EMF) across each winding. And so the voltages Vp and Vs measured at the terminals of the transformer are equal to the corresponding EMF's.

The primary EMF, acting as it does in opposition to the primary voltage, is sometimes termed the "back EMF". This is due to Lenz's law which states that the induction of EMF would always be such that it will oppose development of any such change in magnetic field.

The secondary induced voltage Vs and also the current is scaled from the primary Vp by a factor ideally equal to the ratio if the numbers of turns of wire in their respective windings.

The turn’s ratio determines the ratio of the voltages. A step-down transformer has a large number of turns on its primary (input) coil which is connected to the high voltage mains supply, and a small number of turns on its secondary (output) coil to give a low output voltage.

Ch.3 Multivibrator

55

The second element in the power supply is the bridge rectifier as after the transformer steps down the voltage the rectifier units the direction of the current.

Bridge rectifier:

A diode or bridge rectifier is an arrangement of four diodes connected in a bridge circuit that provides the same polarity of output voltage for any polarity of the input voltage. When used in its most common applications, for conversion of alternating current input into direct current, it’s known as a bridge rectifier. The bridge rectifier provides full wave rectification from a two wire AC input.

Full-Wave rectification:

Full-wave rectification convert both polarity of the input waveform to DC, Each output polarity requiring two rectifiers for example, One for when Ac terminal X is positive and one for when AC terminal Y is positive. The other requires exactly the same, resulting in for individual junctions. Four rectifiers arranging this way are called a diode bridge or bridge rectifier.

Ch.3 Multivibrator

55Ch.3 Multivibrator

55

Basic operation:

When the input connected at the left corner is positive with respect to the one connected at the right hand corner, current flows to the right along the upper colored pass to the output, and returns to the input supply via the lower one.

When the right hand corner is positive relative to the left hand corner, currnet flows along the upper colored pass and returns to the supply via the lower colored pass.

Ch.3 Multivibrator

55

In each case the upper right output remains positive with respect to the lower right one. Since this is true whether the input is ac or dc, this circuit not only produces dc power when supplied with ac power, it also can provide what is sometimes called "reverse polarity protection".

It permits normal functioning when batteries are installed backwards or dc input power supply wiring.

Then after rectification the unidirectional signal is filtered using a filter capacitor to be smoothed.

Ch.3 Multivibrator

55

The capacitor filter:

Ch.3 Multivibrator

55

The simple capacitor filter is the most basic type of power supply filter. the capacitor filter is also used where the power supply ripple frequency is not critical, this frequency can be relatively high .the capacitor c1 show in figure is a simple filter connected across the output of the rectifier in parallel with the load.

When this filter is used, the RC charge time of the filter capacitor c1 must be short and the RC discharge time must be long to eliminate ripple action. In other words, the capacitor must charge up fast, with no discharge at all. Better filtering also results when the input frequency is high.

The output wave forms in the figure above represent the unfiltered and filtered outputs of the full wave rectified circuit. Current pulses flow through the load resistance (Rl) each time a diode conductens. With no capacitor connected across the output of the rectifier circuit the wave form in figure has a large pulsating component (ripple) compared with the average or dc component. When a capacitor is connected across the output, the average value of output voltage is increased due to the filtering reaction C.

The value of the capacitor must be fairly large (4700 microfarads as used), it presents a relatively low reactance to the pulsating current and it stores a substantial charge.

The rate of charge for the capacitors is limited only by the resistance of the conducting diode which is relative low. There for, the RC charge time of the circuit is relatively short. As a result, when the pulsating voltage is first applied to the circuit, the capacitor charges rapidly and almost reaches the peak value of the rectified voltage within the first few cycles.

Ch.3 Multivibrator

55

The capacitor attempts to charge to the peak value of the rectified voltage any time a diode is conducting, and tends to return its charge when the rectifier output falls to zero (the capacitor cannot discharge immediately ).The capacitor slowly discharge through the load resistance Rl during the time rectifier is non-conducting .

The rate of charge of the capacitor is determined by the value of the capacitance and the value of the load resistance. If the capacitance and load resistance values are large, the RC discharge time foe the circuit long.

A voltage regulator is added to ensure the stability of the output as the input changes:

Voltage regulators:

A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components. Depending on the design, it may be used to regulate one or more AC or DC voltages.

Electronic voltage regulators are found in devices such as computer power supplies where they stabilize the DC voltages used by the processor and other elements. In automobile alternators and central power station generator plants, voltage regulators control the output of the plant. In an electric power distribution system, voltage regulators may be installed at a substation or along distribution lines so that all customers receive steady voltage independent of how much power is drawn from the line.

Ch.3 Multivibrator

55

Ic voltage regulators are three terminal devices that provide a constant DC output voltage that is independent the input voltage, output load current, and temperature. There are three types of Ic voltage regulators: IC linear voltage regulators, IC switching voltage regulators, and DC/DC converter chips. IC linear voltage regulators use an active pass element to reduce the input voltage to a regulated output voltage. By contrast, IC switching voltage regulators store energy in an inductor, transformer or capacitor and then use this storage device to transfer energy from the input to the output in discrete packets over a low resistance switch. DC/DC converter chips, a third type of IC voltage regulators, also provide a regulated dc voltage output from a different, unregulated input voltage. In addition, dc/dc converters are provide noise isolation regulate power busses.

Ch.3 Multivibrator

55

For each type of IC voltage regulator, the output voltage can be fixed or adjusted to a value within a specified range.

Ch.3 Multivibrator

55

Chapter -3-

Multivibrator

Introduction:

A Multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices (transistors, electron tubes or other devices) cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics. The following list is terms associated with a timing pulse or waveform.

Active HIGH - if the state changes occur at the clock's rising edge or during the clock width.

Active LOW - if the state changes occur at the clock's falling edge.

Duty Cycle - is the ratio of clock width and clock period.

Ch.3 Multivibrator

55

Clock Width - this is the time during which the value of the clock signal is equal to one.

Clock Period - this is the time between successive transitions in the same direction, i.e., between two rising and two falling edges.

Clock Frequency - the clock frequency is the reciprocal of the clock period, frequency = 1/clock period

Clock pulse generation circuits can be a combination of analogue and digital circuits that produce a continuous series of pulses (these are called Astable multivibrators) or a pulse of a specific duration (these are called Monostable multivibrators). Combining two or more of multivibrators provides generation of a desired pattern of pulses (including pulse width, time between pulses and frequency of pulses).

We can divide multivibrator by what it's made of to three types:

A.multivibrator using transistor.B.multivibrator using ic555.C.multivibrator using op-amp.

Ch.3 Multivibrator

55

A- Multivibrator using transistor:

There are three types of multivibrator using transistor circuits depending on the circuit operation:

(Astable) , in which the circuit is not stable in either state it continually switches from one state to the other. It does not require an input such as a clock pulse it's also

called a free-running multivibrator that has no stable states but switches continuously between two states this action produces a train of square wave pulses at a fixed frequency.

(Monostable) , in which one of the states is stable, but the other state is unstable (transient). A trigger causes the circuit to enter the unstable state it's also called a one-shot multivibrator that has only one stable state and is triggered

externally with it returning back to its first stable state after entering the unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful for creating a timing period of fixed duration in response to some external event.

Ch.3 Multivibrator

55

(Bistable) , in which the circuit is stable in either state. The circuit can be flipped from one state to the other by an external event or trigger it's also called a flip-flop that has two stable states that produces a single pulse either positive or negative in value.

Multivibrators find applications in a variety of systems where square waves or timed intervals are required.

For example, before the advent of low-cost integrated circuits, chains of multivibrators found use as frequency dividers. A free-running multivibrator with a frequency of one-half to one-tenth of the reference frequency would accurately lock to the reference frequency.

This technique was used in early electronic organs, to keep notes of different octaves accurately in tune. Other applications included early television systems, where the various line and frame frequencies were kept synchronized by pulses included in the video signal.

Ch.3 Multivibrator

55Ch.3 Multivibrator

55

b- multivibrator using ic555:

(Astable MTV using IC555) :

Simple Monostable or Astable timing circuits can now be easily made using standard waveform generator ICs as relaxation oscillators by connecting a few passive components to their inputs and the most commonly used waveform generator type IC is the classic 555 timer.

The 555 timer is a very versatile low cost timing IC that produces very accurate timing periods with good stability of around 1% and which can be varied from a few micro-seconds to many hours with the timing period being controlled by a single RC network connected to a single positive supply, The 555 connected as an Astable oscillator is given below.

Ch.3 Multivibrator

55

C -multivibrator using op-amp:

(Astable MTV using op-amp) :

Circuits for generating square and triangular waves are always of great use to the electronics designer. A useful astable multivibrator (triangular wave generator) is shown in Fig.1.

Therefore we are not able to specify an originator of inaccurate (optimistic) multivibrator properties description.

One way of producing a very simple clock signal is by the interconnection of logic gates. As NAND gates contains amplification, they can also be used to provide a clock signal or timing pulse with the aid of a single Capacitor, C and Resistor, R which provide the feedback and timing function.

Ch.3 Multivibrator

55

These timing circuits are often used because of there simplicity and are also useful if a logic circuit is designed that has un-used gates which can be utilized to create the Monostable or Astable oscillator. This simple type of RC Oscillator network is sometimes called a "Relaxation Oscillator .So we can describe these types of multivibrator by NAND gates and Capacitor, C and Resistor, R as shown below.

Simple NAND Gate Monostable Circuit:

NAND Gate Astable Multivibrators:

Ch.3 Multivibrator

55

NAND Gate Bistable Multivibrator:

Components of multivibrator

picture

Op amp 741

Variable resistor

ResistorsCapacitors

Zener diode 1n914

Ch.3 Multivibrator

55

Circuit of A stable Multivibrator

Ch.3 Multivibrator

55Ch.3 Multivibrator

55

Astable multivibrator using operational amplifier:

A multivibrator is an electronic circuit used to implement a variety of simple two-state systems such as oscillators, timers and flip-flops. It is characterized by two amplifying devices cross-coupled by resistors or capacitors. The name "multivibrator" was initially applied to the free-running oscillator version of the circuit because its output waveform was rich in harmonics.

A stable multivibrator, in which the circuit is not stable in either state, it continually switches from one state to the other. It does not require an input such as a clock pulse.

astable, in which the circuit is not stable in either state —it continually switches from one state to the other. It does not require an input such as a clock pulse.

The astable multivibrator may be created directly with transistors or with use of integrated circuits such as operational amplifiers (op amps) or the 555 timer. Most operational amplifiers are powered by a positive and negative rail voltage, the output never able to exceed these rail voltages. Depending upon initial conditions, the op amp’s output will drive to either positive or negative rail. Upon this occurance, the capacitor will either charge or discharge through the resistor R2, its voltage slowly rising or falling.

As soon as the voltage at the op amp’s inverting terminal reaches that at the noninverting terminal (the op amp’s output voltage divided by R1 and R2), the output will drive to the opposing rail and this process will repeat with the

Ch.3 Multivibrator

55

capacitor discharging if it had previously charged and vice versa.

Ch.3 Multivibrator

55

Once the inverting terminal reaches the voltage of the non-inverting terminal the output again drives to the opposing rail voltage and the cycle begins again. Thus, the astable multivibrator creates a square wave with no inputs.

What is operational amplifier?

An Amplifier is made of: 

1) A Gain "Block" (ideally possessing infinite gain) 

2) Feedback 

3) A Network that sets the amount of feedback (e.g., resistors)

Ch.3 Multivibrator

55

Operational Amplifier:

The "Op Amp" is one of the most valuable and versatile integrated circuits to ever come down-the-pike. If you want to spend a Month -of-Sundays abusing yourself, and you've run out of sharp sticks: build an Op Amp, from scratch--one that works as well as the monolithic version; or for that matter, one that works period! There is almost no circuit design that can't benefit from the use of the Op Amp.  From the 741--the first internally compensated Op Amp, and son of the 709--to the super-fast near GHz Op Amps; these little "boogers" are easy to use: If you bypass, decouple, and use common sense... Ha! There I said it. Ha!  "How does this Damn thing work?" "Very well thank you."  Let’s keep this simple: The Op Amp is basically three amplifiers or stages. The input differential stage; the gain stage, and the output stage.   

.. Input Stage Gain Stage Output Stage

Ch.3 Multivibrator

55

Op-Amp-Stages

Contents:

1- Operational amplifier ( comparator,amplifier and integrator types )

2- Resistors, capacitors and diodes

OP AMPs as Amplifiers:

An op amp can also easily amplify a signal such as audio. The diagram below shows the circuit for an op amp that would give an output signal twice as large as the input. Op amps don't like errors. To get amplification, you induce an error in the signal going back to the negative input of the op amp. An op amp will do everything in it's power to get the signal on the negative input to match the signal on its positive input. To get an output that's twice as large as the input, you use 2 equal value resistors as a voltage divider to reduce the return (feedback) signal at the negative input by half. If the return signal doesn't match the input signal, the op amp will increase the output until the signal returned to the negative input is the same as the input to the positive input. Since the voltage divider cuts the signal in half, the signal at the output must be doubled. You can create any amount of gain needed by changing the value of ONE of the resistors in the 'feedback' path. The actual limit of gain will be determined by the op amp design. When using an op amp as a non-inverting amplifier, the gain will always be greater than or equal to 1. To get a gain of less than 1, you need to use a voltage divider on the input signal.

Ch.3 Multivibrator

55

Calculating Voltage Gain (non-inverting input):

By knowing the value of the feedback, inverting input resistor and input voltage, we can calculate the output voltage. The formula is:

Vout = Vin*((Rf/Ri)+1)

Ch.3 Multivibrator

55

Comparitor operational amplifier:

The comparator provides a large change in signal when the input signal only changes slightly and converts an analogue signal into a digital signal. It ‘compares’ the voltage input signal and the voltage from a potentiometer.

The comparator subsystem provides an output signal that stays high while the input signal is higher than the reference signal orthreshold from a potentiometer. The output signal remains low otherwise.

Ch.3 Multivibrator

55

Integrator operational amplifier:

Figure 2: Integrator circuit.

An integrator circuit is shown in Figure 2

1. Show that the output signal of the amplifier is 

2. Build the circuit with   k ,    F and use square and sinusoidal wave forms to test the predicted behavior. Also place a   M  resistor in parallel with the capacitor. This resistor drains charge to avoid saturation due to very low frequency or DC signals.

Ch.3 Multivibrator

55

Operation:

Our multivibrator is used to give three different types of output pulses with variable frequencies.

The three types of outputs are:

1- Square pulse from o/p 1 terminal2- Sawtooth pulse from o/p2 terminal3- Rectangular pulse from o/p3 terminal

Ch.3 Multivibrator

55

1- The square pulse output operation:

The terminal o/p 1 generates a square pulse with :Max dutycycle = 1 msMin frequency = 1 KHzMin dutycycle = 12 usMax frequency = 83.3 KHz

The non-sinusoidal waveform generators are also called relaxation oscillators. The op-amp relaxation oscillator shown in figure is a square wave generator. In general, square waves are relatively easy to produce.

Ch.3 Multivibrator

55

Like the UJT relaxation oscillator, the circuit’s frequency of oscillation is dependent on the charge and discharge of a capacitor C through feedback resistor R, The “heart” of the oscillator is an inverting op-amp comparatorThe comparator uses positive feedback that increases the gain of the amplifier. In a comparator circuit these offer two advantages. First, the high gain causes the op-amp’s output to switch very quickly from one state to another and vice-versa. Second, the use of positive feedback gives the circuit hysteresis. In the op-amp square-wave generator circuit given in figure, the output voltage vout is shunted to ground by two Zener diodes Z1 and Z2 connected back-to-back and is limited to either  VZ 2 or –VZ 1. A fraction of the output is fedback to the non-inverting (+) input terminal. Combination of IL and C acting as a low-pass R-C circuit is used to integrate the output voltage vout and the capacitor voltage vc is applied to the inverting input terminal in place of external signal. The differential input voltage is given as vin = vc - β vout

Ch.3 Multivibrator

55

When vin is positive, vout = – Vz1 and when vin is negative vout = + Vz 2. Consider an instant of time when vin < 0. At this instant vout = + Vz 2 , and the voltage at the non-inverting (+) input terminal is  β Vz 2 , the capacitor C charges exponentially towards Vz 2, with a time constant Rf C. The output voltage remains constant at Vz 2 until vc equal β Vz 2.When it happens, comparator output reverses to – Vz 1. Now vc changes exponentially towards -Vz1with  the  same  time  constant  and  again  the  output  makes  a  transition  from -Vz1 to + Vz 2. when vc equals -βVz 1

Let    Vz1 = Vz 2

The time period, T, of the output square wave is determined using the charging and discharging phenomena of the capacitor C. The voltage across the capacitor, vc when it is charging from – B Vz to + Vz is given byVc = [1-(1+β)]e-T/2τ

Where τ = RfCThe waveforms of the capacitor voltage vc and output voltage vout (or vz) are shown in figure.When t = t/2Vc = +β Vz or + β Vout

Therefore β Vz = Vz [1-(1+β)e-T/2τ]Or e-T/2τ = 1- β/1+ βOr T = 2τ loge 1+β/1- β = 2Rf C loge [1+ (2R3/R2)]

Ch.3 Multivibrator

55

The frequency, f = 1/T , of the square-wave is independent of output voltage Vout. This circuit is also known as free-running or astable multivibrator because it has two quasi-stable states. The output remains in one state for time T1 and then makes an abrupt transition to the second state and remains in that state for time T2. The cycle repeats itself after time T = (T1 + T2) where T is the time period of the square-wave.The op-amp square-wave generator is useful in the frequency range of about 10 Hz -10 kHz. At higher frequencies, the op-amp’s slew rate limits the slope of the output square wave. The symmetry of the output waveform depends on the matching of two Zener diodes Z1 and Z2. The unsymmetrical square-wave (T1 not equal to t2) can be had by using different constants for charging the capacitor C to +Vout and -Vout.

Ch.3 Multivibrator

55

2- Sawtooth pulse output operation

The terminal o/p 2 generates sawtooth pulse with:Max dutycycle = 2 msMin frequency = 500 HzMin dutycycle = 0.12 msMax frequency = 6.67 KHz

Ch.3 Multivibrator

55

The sawtooth pulse generator depends on op-amp integrator circuit:

Op-amp Integrator Circuit:

As its name implies, the Op-amp Integrator is an operational amplifier circuit that performs the mathematical operation of Integration, which is we can cause the output to respond to changes in the input voltage over time. The integrator amplifier acts like a storage element that "produces a voltage output which is proportional to the integral of its input voltage with respect to time". In other words the magnitude of the output signal is determined by the length of time a voltage is present at its input as the current through the feedback loop charges or discharges the capacitor as the required negative feedback occurs through the capacitor.

When a voltage, Vin is firstly applied to the input of an integrating amplifier, the uncharged capacitor Chas very little resistance and acts a bit like a short circuit (voltage follower circuit) giving an overall gain of less than one. No current flows into the amplifiers input and point X is a virtual earth resulting in zero output.

Ch.3 Multivibrator

55

As the feedback capacitor C begins to charge up, its reactance Xc decreases this results in the ratio of Xc/Rin increasing producing an output voltage that continues to increase until the capacitor is fully charged.

At this point the capacitor acts as an open circuit, blocking anymore flow of DC current. The ratio of feedback capacitor to input resistor (Xc/Rin) is now infinite resulting in infinite gain. The result of this high gain (similar to the op-amps open-loop gain), is that the output of the amplifier goes into saturation as shown below. (Saturation occurs when the output voltage of the amplifier swings heavily to one voltage supply rail or the other with little or no control in between).

The rate at which the output voltage increases (the rate of change) is determined by the value of the resistor and the capacitor, "RC time constant". By changing this RC time constant value, either by changing the value of the Capacitor, C or the Resistor, R, the time in which it takes the output voltage to reach saturation can also be changed for example.

Ch.3 Multivibrator

55

If we apply a constantly changing input signal such as a square wave to the input of an Integrator Amplifier then the capacitor will charge and discharge in response to changes in the input signal. This results in the output signal being that of a sawtooth waveform whose frequency is dependant upon theRC time constant of the resistor/capacitor combination. This type of circuit is also known as a Ramp Generator and the transfer function is given below

Ramp Generator:

We know from first principals that the voltage on the plates of a capacitor is equal to the charge on the capacitor divided by its capacitance giving Q/C. Then the voltage across the capacitor is output Vouttherefore: -Vout = Q/C. If the capacitor is charging and discharging, the rate of charge of voltage across the capacitor is given As:

Ch.3 Multivibrator

55

But dQ/dt is electric current and since the node voltage of the integrating op-amp at its inverting input terminal is zero, X = 0, the input current I(in) flowing through the input resistor, Rin is given as:

The current flowing through the feedback capacitor C is given as:

Assuming that the input impedance of the op-amp is infinite (ideal op-amp), no current flows into the op-amp terminal. Therefore, the nodal equation at the inverting input terminal is given as:

Ch.3 Multivibrator

55

From which we derive an ideal voltage output for the OP-amp Integrator as:

To simplify the math's a little, this can also be re-written as:

Where jω = 2πƒ and the output voltage Vout is a constant 1/RC times the integral of the input voltageVin with respect to time. The minus sign (-) indicates a 180o phase shift because the input signal is connected directly to the inverting input terminal of the op-amp.

Ch.3 Multivibrator

55

3-The rectangular pulse output operation:

The terminal o/p 3 generates rectangular pulse with:Max dutycycle = 0.78 msMin frequency = 1.28 KHzMin dutycycle = 0.32 msMax frequency = 3.125 KHz

Ch.3 Multivibrator

55

The rectangular pulse output depends on op-amp differentiator circuit:

Op-amp Differentiator Circuit

The input signal to the differentiator is applied to the capacitor. The capacitor blocks any DC content so there is no current flow to the amplifier summing point, X resulting in zero output voltage. The capacitor only allows AC type input voltage changes to pass through and whose frequency is dependent on the rate of change of the input signal. At low frequencies the reactance of the capacitor is "High" resulting in a low gain (Rf/Xc) and low output voltage from the op-amp. At higher frequencies the reactance of the capacitor is much lower resulting in a higher gain and higher output voltage from the differentiator amplifier.

However, at high frequencies an op-amp differentiator circuit becomes unstable and will start to oscillate.

This is due mainly to the first-order effect, which determines the frequency response of the op-amp circuit causing a

Ch.3 Multivibrator

55

second-order response which, at high frequencies gives an output voltage far higher than what would be expected. To avoid this the high frequency gain of the circuit needs to be reduced by adding an additional small value capacitor across the feedback resistor Rf.

Since the node voltage of the operational amplifier at its inverting input terminal is zero, the current, i flowing through the capacitor will be given as:

The charge on the capacitor equals Capacitance x Voltage across the capacitor

The rate of change of this charge is

but dQ/dt is the capacitor current i

Ch.3 Multivibrator

55

From which we have an ideal voltage output for the op-amp differentiator is given as:

Ch.3 Multivibrator