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ELECTRONIC DEVICES AND CIRCUITS

LEARNING OBJECTIVES:

Concept of oscillator.

Study of barkhausen criterion.

About different types of oscillators.

CHAPTER-4(SINOSOIDAL OSCILLATORS)

4.1 POSITIVE FEEDBACK

Feedback is the process of picking up a part of the output signal of an amplifier and feeding it at

the input.

The voltage so fed back to the input is called the feedback voltage.

Positive Feedback: Feedback is called positive (or regenerative) feedback, when the feedback

signal is in phase with the input and adds to the input signal. This results in increase of the gain of

the amplifier. But it has the disadvantage of increased instability of gain, increased distortion and

noise. Hence, positive feedback is rarely used in amplifiers. However positive feedback is used in

oscillators.

4.2 USE OF POSITIVE FEEDBACK

Positive feedback is used in oscillators to keep them running or oscillating. An oscillator

is basically an amplifier which has a feedback path from the output back to the input.

This permits a portion of the output signal to get back to the input to "keep things

going" in stage - to keep things oscillating. The feedback must be positive so that it

will "contribute" to the stage being able to keep oscillating. If the feedback was

negative, it would serve to damp the oscillation and to "kill" the oscillator

4.3 OSCILLATORS

The oscillators are electronic circuits makes a respective electronic signal generally the

sine wave and the square wave. It is very important in other types of the electronic

equipment such as quartz which used as a quartz oscillator. The amplitude

modulation radio transmitters use the oscillation to generate the carrier waveform.

The AM radio receiver uses the special oscillator it is called as a resonator to tune a

station.

4.3.1 Parts of an Oscillator

Most oscillators consist of three basic parts:

4.3.1.1. An amplifier. This will usually be a voltage amplifier and may be biased in class A, B

or C.

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4.3.1.2. A wave shaping network. This consists of passive components such as filter circuits

that are responsible for the shape and frequency of the wave produced.

4.3.1.3. A POSITIVE feedback path. Part of the output signal is fed back to the amplifier input

in such a way that the feed back signal is regenerated, re-amplified and fed back again to

maintain a constant output signal.

Commonly an oscillator is constructed from an amplifier that has part of its output signal fed

back to its input. This is done in such a way as to keep the amplifier producing a signal without

the need for any external signal input as shown in Fig. 1.1.1. It can also be thought of as a way of

converting a DC supply into an AC signal.

4.3.2 Positive feedback.

The feedback in the amplifier section of an oscillator must be POSITIVE FEEDBACK. This is

the condition where a fraction of the amplifier's output signal is fed back to be in phase with the

input, and by adding together the feedback and input signals, the amplitude of the input signal is

increased. For example, a common emitter amplifier creates a phase change of 180° between its

input and output, the positive feedback loop must therefore also produce a 180° phase change in

the signal fed back from output to input for positive feedback to occur.

The result of a small amount of positive feedback in amplifiers is higher gain, though at the cost

of increased noise and distortion. If the amount of positive feedback is large enough however,

the result is oscillation, where the amplifier circuit produces its own signal.

4.3.3 Using Positive Feedback.

When an amplifier is operated without feedback it is operating in "open loop" mode. With

feedback (either positive or negative) it is in "closed loop" mode. In ordinary amplifiers negative

feedback is used to provide advantages in bandwidth, distortion and noise generation, and in

these circuits the closed loop gain of the amplifier is much less than the open loop gain.

However when positive feedback is used in an amplifier system the closed loop gain (with

feedback) will be greater than the open loop gain, the amplifier gain is now increased by the

feedback. Additional effects of positive feedback are reduced bandwidth, (but this does not

matter in an oscillator producing a sine wave having a single freqency), and increased distortion.

However even quite severe distortion in the amplifier is allowed in some sine wave oscillator

designs, where it does not affect the shape of the output wave.

In oscillators using positive feedback it is important that amplitude of the oscillator output

remains stable. Therefore the closed loop gain must be 1 (unity). In other words, the gain within

the loop (provided by the amplifier) should exactly match the losses (caused by the feedback

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circuit) within the loop. In this way there will be no increase or decrease in the amplitude of the

output signal, as illustrated in Fig. 1.1.2.

4.4.4 The conditions for oscillation.

Positive feedback must occur at a frequency where the voltage gain of the amplifier is equal to

the losses (attenuation) occurring in the feedback path. For example if 1/30th of the output signal

is fed back to be in phase with the input at a particular frequency, and the gain of the amplifier

(without feedback) is 30 times or more, oscillation will take place.

The oscillations should take place at one particular frequency.

The amplitude of the oscillations should be constant.

There are many different oscillator designs in use, each design achieving the above criteria in

different ways. Some designs are particularly suited to producing certain wave shapes, or work

best within a certain band of frequencies. Whatever design is used however, the way of

achieving a signal of constant frequency and constant amplitude is by using one or more of three

basic methods

.

Method 1

Make sure that positive feedback occurs only at one frequency, the required frequency of

oscillation. This may be achieved by ensuring that only signals of the required frequency are fed

back, or by ensuring the feedback signal is in the correct phase at only one frequency.

Method 2

Make sure that sufficient amplification for oscillation can take place only at the required

frequency, by using an amplifier that has an extremely narrow bandwidth, extending to the

frequency of oscillation only.

Method 3

Use amplifiers in "switch mode" to switch the output between two set voltage levels, together

with some form of time delay to control the time at which the amplifiers switch on or off, thus

controlling the periodic time of the signal produced.

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Fig. 1.1.2 The Need For Amplitude Stability

Methods 1 and 2 are used extensively in sine wave oscillators, while method 3 is useful in square

wave generators, sometimes called aperiodic (untuned) oscillators. Oscillators using method 3

often use more than one amplifier and timing circuit, and so are called multivibrators (more than

one oscillator).

4.5 TYPES OF OSCILLATORS

There are two types of electronic oscillator’s they are linear and nonlinear oscillators. The linear

oscillators give the sinusoidal input. The linear oscillators consist a mass m and its force in the

linear in equilibrium. By applying the hook’s low the spring creates the force that i9s in linear for

small displacements.

The different types of oscillators are mentioned below and some of them are explained.

Crystal Oscillator

Hartley oscillator

RC Phase Shift Oscillator

Colpitts Oscillators

Phase Shift Oscillator

Wine Bridge Oscillator

4.6 LC OSCILLATORS.

LC oscillator is a type of oscillator where a LC (inductor-capacitor) tank circuit is used for giving

the required positive feedback for sustaining the oscillations. The LC tank circuit is also termed as

LC resonant circuit or LC tuned circuit. According to the Barkhausen criterion for sustained

oscillations, a circuit will sustain stable oscillations only for frequencies at which the loop gain of

the system is equal to or greater than 1 and the phase shift between input and output is 0 or an

integral multiple of  2π. LC oscillators can realized using BJT, FET, MOSFET, opamp etc.

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Typical applications of LC oscillators include RF signal generators, frequency mixers, tuners, sine

wave generators, RF modulators etc. Before going into the LC oscillators in detail let’s have a look

at the LC tank circuit.

LC tank circuit.

Though the original tank circuit means a capacitor and

inductor connected in parallel, the switch and a voltage source are included in the circuit for the

ease of explanation. Initially the switch S is assumed to be in position 1. The cacpacitor will be

charged to a voltage V which is the voltage source. Assume the switch is moved to position 2 as

shown in the figure below.

The capacitor C will start discharging through inductor

L. The voltage across capacitor will start to decrease and the current through the inductor starts

increasing. The increasing current creates an electromagnetic field around the coil and when the

capacitor is fully discharged the electrostatic energy stored in the capacitor will be fully transferred

into the coil as electro-magnetic field. With no more energy in the capacitor to sustain the current

through the coil, the field around the coil starts to fall and the current through the coil tends to

decrease. Due to electromagnetic induction the inductor will generate a back emf equal to L(di/dt)

in order oppose the change in current. This back emf will start to charge the capacitor again.

When the capacitor is fully charged , the energy once stored in the inductor as elecro-magnetic

field will be moved  to the capacitor as electrostatic field. Then the capacitor starts discharging

again and the cycle is repeated. This cyclic transfer of of energy between the capacitor and inductor

is the reason behind the production of oscillations in the tank circuit.

If  an  ideal capacitor and inductor are used, these oscillation will sustain until the end of time.

But in  practical case  the inductor will have some ohmic resistance and the capacitor will have

some amount of leakage. These imperfections will waste some amount of energy in between the

cycles  resulting in the loss of amplitude step by step and eventually the oscillations will die out.

This gradual decay in amplitude which tends to the death of an oscillation is called damping. The

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oscillations produced in a damped  LC tank circuit  will look like what shown in the figure

below.

In a practical LC oscillator, in addition to the Barkahusen criterion there must be some means to

compensate for the energy lost in the tank circuit. Application of active elements like BJT, FET,

opamp etc in the LC oscillator  is a way for meeting all these requirements. The active element in

an LC oscillator circuit has three essential jobs.

To give necessary gain.

Help in attaining the required positive feedback conditions.

Compensate the energy lost in the tank circuit.

4.6.1. LC oscillators and types.

4.6.1.1. Tuned collector oscillator.

Tuned collector oscillator can be said to be the basic type of LC oscillators. Here a transformer and

a capacitor connected in parallel across the collector circuit of the oscillator. Primary of the

transformer and the capacitor forms the essential tank circuit. The secondary of the transformer

feeds back a fraction of the oscillations produced in the tank circuit to the base of the transistor.

The circuit diagram of a typical tuned collector oscillator is shown in the figure below.

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4.6.1.2. Hartley Oscillator

The Hartley oscillator is an electronic oscillator. The frequency of this oscillation is determined

by the tuned circuit. The tuned circuit consists of the capacitor and inductor, hence it is an LC

oscillator. In 1915 by American engineer Ralph Hartley has invented this oscillator. The features of

the Hartley circuit are the tuned circuit consists of a single capacitor in parallel with the two

inductors which are in series. From the center connection of the two inductors for oscillation

purpose, the feedback signal is taken. Follow the below link to know more about Hartley Oscillator

Circuit and Its Working

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Hartley Oscillator

The Hartley oscillator is parallel to the Colpitts apart from that it uses a pair of tapping coils as an

alternate of two tapped capacitors. From the below circuit the output voltage is developed across

the inductor L1 and the feedback voltages are across the inductor L2. The feedback network is

given in the mathematical expression which is given below

Feedback network = XL2 / XL1 = L 2 / L 1

Applications

This oscillation will produce a desired range of frequencies

The Hartley oscillators are used in the radio frequency in a range of the 30Mhz

In radio receiver, this oscillator is used and it has a wide range of frequency

4.6.1.3. Colpitts Oscillator

The Colpitts Oscillator was by American engineering by Edwin H. Colpitts in the year of 1918.

This oscillator is a combination of both inductors and capacitor. The features of the Colpitts

Oscillator are the feedback for the active devices and they are taken from the voltage divider and

made up of two capacitors which are in series across the inductor. Follow the below link to know

more about Collpits Oscillator Working and Its Applications

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Colpitts Oscillator

The Colpitts circuits consist of gain devices such as the bipolar junction, field effect transistor,

operational amplifier and vacuum tubes. The output is connected to an input in a feedback loop it

has a parallel tuned circuit and it functioned as a band-pass filter is used as a frequency of the

oscillator. This oscillator is an electrically dual of the Hartley oscillator hence the feedback signal

is taken from the inductive voltage divider it has two coils in the series.

The following circuit diagram shows the common base Colpitts circuit. The inductor L and the

both the capacitors C1 & C2 are in series with the parallel resonant tank circuit and it gives the

frequency of the oscillator. The voltage across the C2 terminal is applied to the base-emitter

junction of the transistor to create the feedback oscillations.

Applications

It is used to generate the sinusoidal output signals with a very high frequency

Very wide range of frequencies is involved

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It is used in the radio and mobile communications

In commercial purpose, many applications are used

4.6.2. RC OSCILLATORS

4.6.2.1 PHASE SHIFT OSCILLATOR

What is a phase shift oscillator?

"Phase shift oscillator" is the term given to a particular oscillator circuit topology that uses

an RC network in the feedback loop of a tube, transistor, or opamp to generate the required

phase shift at a particular frequency to sustain oscillations. They are moderately stable in

frequency and amplitude, and very easy to design and construct.

Where are they used?

Phase shift oscillators are most commonly used in tremolo circuits in guitar amplifiers.

They are used as the low-frequency oscillator (LFO) that generates the sinusoidal waveform

which amplitude modulates the guitar signal to produce the characteristic tremolo amplitude

variations.

How do they work?

In order to create and sustain an oscillation at a particular frequency, a circuit must have a

gain higher than unity, and a total phase shift around the loop of 360 degrees (which is

equivalent to 0 degrees, or positive feedback). When used with a single-stage inverting

amplification element, such as a tube, transistor, or inverting opamp configuration, the

amplifier itself provides 180 degrees of phase shift (a gain of -A, where A is the gain of the

amplification stage). The remaining 180 degrees of phase shift necessary to provide a total

of 360 degrees is provided by an external network of resistors and capacitors.

Following is a schematic diagram of a typical phase shift oscillator:

Phase Shift Oscillator

The triode is configured as an inverting amplifier to provide the necessary gain, and the

feedback network is connected from the plate to the grid.

The phase shift elements are C1/R1, C2/R2, and C3/R3. Three of these phase

lead1 networks contribute a total of 180 degrees of phase shift at the oscillation

frequency. Note that a phase shift oscillator could also be built using four or more phase

shift elements, with each element contributing less overall phase shift at the oscillation

frequency. Normally, there is no need to do this, as it takes extra components. A minimum

of three phase shift networks is required, however, because the maximum theoretical phase

shift available from any one RC network is 90 degrees, and the actual phase shift

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approaches this value asymptotically.

A phase shift oscillator can also be made using three phase lag networks, which are

obtained by swapping the positions of the R and C value components in the above

schematic. The lag network would require one additional coupling cap to block the DC on

the plate voltage from the grid, and one additional resistor to provide the grid bias ground

reference for V1A, so it is not normally used.

Following is an example of both a phase lead and a phase lag network, designed for a 45

degree phase shift at the -3dB point of f = 1/(2*Pi*R*C) = 1/(2*Pi*1Meg*.01uF) = 15.9Hz:

Phase Lead Network Phase Lag Network

4.6.2.2. WEIN BRIDGE OSCILLATOR

The Wien bridge oscillator is an electronic oscillator and produces the sine waves. It is a

two stage RC circuit amplifier circuit and it has high quality of resonant frequency,

low distortion, and also in the tuning. Consider the very simple sine wave oscillator

used by the RC circuit and place in the conventional LC circuit, construct the output

of sinusoidal waveform is called as an Wien bridge oscillator. The Wien bridge

oscillator is also called as a Wheatstone bridge circuit.

Wein Bridge Oscillator Circuit

The Wien bridge oscillator is used to find unknown values of components. In most of the cases

this oscillator is used in the audios. The oscillators are designed simply, size is compressed and it

has stable in frequency output. Hence the maximum output frequency of the Wien bridge oscillator

is 1MHz and this frequency is from the phase shift oscillator. The total phase shift of the

oscillator is from the 360° or 0°.

It is a two stage amplifier with RC bridge circuit and the circuit has the lead lag networks. The

lags at the phase shift are increasing the frequency and the leads are decreasing the frequency. In

additional by adding the Wien Bridge oscillator at a particular frequency it becomes sensitive. At

this frequency the Wien Bridge is balance the phase shift of 0°. The following diagram shows the

circuit diagram of the Wienbridge oscillator. The diagram shows R1 is series with the C1, R3, R4

and R2 are parallel with the C2 to from the four arms.

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Wien Bridge

Oscillator Circuit

From the above diagram we can see the two transistors are used for the phase shift of 360°and also

for the positive feedback. The negative feedback is connected to the circuit of the output with a

range of frequencies. This has been taken through the R4 resistor to from the temperature sensitive

lamp and the resistor is directly proportional to the increasing current. If the output of the

amplitude is increased then the more current is offered more negative feedback.

Wien Bridge Oscillator Operation

The circuit is in the oscillation mode and the base current of the first transistor is changed randomly

because it is due to the difference in voltage of DC supply. The base current is applied to the

collector terminal of the first transistor and the phase shift is about the 180°. The output of the first

transistor is given to the base terminal of the second transistor Q2 with the help of the capacitor C4.

Further, this process is amplified and from the second transistor of collector terminal the phase

reversed signal is collected.

The output signal is connected to the phase with the help of the first transistor to the base terminal.

The input point of the bridge circuit is from the point A to point C the feedback of this circuit is the

output signal at the second transistor. The feedback signal is given to the resistor R4 which gives

the negative feedback. In this same way the feedback signal is given to the base bias resistor R4

and it produces the positive feedback signal.

By using the two capacitors C1 and C2 in this oscillator, it can behave continuous frequency

variation. These capacitors are the air gang capacitors and we can also change the values of the

frequency range of the oscillator.

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4.7. CRYSTAL OSCILLATOR

A crystal oscillator is an electronic oscillator circuit which is used for the mechanical resonance of

a vibrating crystal of piezoelectric material. It will create an electrical signal with a given

frequency. This frequency is commonly used to keep track of time for example: wrist watches are

used in digital integrated circuits to provide a stable clock signal and also used to stabilize

frequencies for radio transmitters and receivers.Quartz crystal is mainly used in radio-frequency

(RF) oscillators. Quartz crystal is the most common type of piezoelectric resonator, in oscillator

circuits we are using them so it became known as crystal oscillators. Crystal oscillators must be

designed to provide a load capacitance.

There are different types of oscillator electronic circuits which are in use they are namely: Linear

oscillators – Hartley oscillator, Phase-shift oscillator, Armstrong oscillator, Clapp

oscillator, Colpitts oscillator. Relaxation oscillators – Royer oscillator, Ring oscillator,

Multivibrator and Voltage Controlled Oscillator (VCO). Soon we are going to discuss in detail

about crystal oscillator like, working and applications of a crystal oscillator.

What is a Quartz Crystal?

A quartz crystal exhibits a very important property known as the piezoelectric effect. When a

mechanical pressure is applied across the faces of the crystal, a voltage which is proportional to

mechanical pressure appears across the crystal. That voltage causes distortion in the crystal.

Distorted amount will be proportional to the applied voltage and also an alternate voltage applied to

a crystal it causes to vibrate at its natural frequency.

Quartz Crystal Circuit

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The below figure represents the electronic symbol of a piezoelectric crystal resonator and also

quartz crystal in an electronic oscillator that consists of resistor, inductor, and capacitors.

Crystal Oscillator Circuit Diagram

The above figure is a 20psc New 16MHz Quartz Crystal Oscillator and it is a one kind of crystal

oscillators, that works with 16MHz frequency.

Crystal Oscillator

Generally bipolar transistors or FETs are used in construction of Crystal oscillator circuits. This is

because operational amplifiers can be used in different low frequency oscillator circuits which are

below 100KHz but operational amplifiers do not have the bandwidth to operate. It will be a

problem at the higher frequencies that are matched to crystals which are above 1MHz.

To overcome this problem Colpitts crystal oscillator is designed. It will work at higher

Frequencies. In this Oscillator, the LC tank circuit that provides the feedback oscillations has been

replaced by a quartz crystal.

Crystal Oscillator Circuit

Diagram

Crystal Oscillator Working

Crystal oscillator circuit usually works on the principle of the inverse piezoelectric effect. The

applied electric field will produce a mechanical deformation across some materials. Thus, it utilizes

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the vibrating crystal’s mechanical resonance, that is made with a piezoelectric material for

generating an electrical signal of a particular frequency.

Usually quartz crystal oscillators are highly stable, consists of good quality factor(Q), they are

small in size, and are economically related. Hence, quartz crystal oscillator circuits are more

superior compared to other resonators like LC circuits, turning forks. Generally in Microprocessors

and Micro controllers we are using an 8MHz crystal oscillator.

The equivalent electrical circuit is also describes the crystal action of the crystal. Just look at the

equivalent electrical circuit diagram shown in the above. The basic components used in the

circuit, inductance L represents crystal mass, capacitance C2 represents compliance, and C1 is used

to represent the capacitance that is formed because of crystal’s mechanical moulding, resistance R

represents the crystal’s internal structure friction, The quartz crystal oscillator circuit diagram

consists of two resonances such as series and parallel resonance, i.e., two resonant frequencies.

Crystal Oscillator Working

The series resonance occurs when the reactance produced by capacitance C1is equal and opposite to the

reactance produced by inductance L. The fr and fp represents series and parallel resonant frequencies

respectively, and the values of ‘fr’ and ‘fp’ can be determined by using the following equations shown in

the figure below.

The above diagram describes an equivalent circuit, plot graph for Resonant frequency, Formulae for

Resonant frequencies.

Uses of Crystal Oscillator

In general, we know that, in the design of microprocessors and microcontrollers, crystal oscillators are

used for the sake of providing the clock signals. For instance, let us consider 8051 microcontroller, in this

particular controller an external crystal oscillator circuit will work with 12MHz that is essential, even

though this 8051 microcontroller (based on model) is capable to work at 40 MHz (max) have to provide

12MHz in most of the cases because for a machine cycle 8051 requires 12 clock cycles, so that to give

effective cycle rate at 1MHz (taking 12MHz clock) to 3.33MHz (taking the maximum 40MHz clock). This

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particular crystal oscillator which is having cycle rate at 1MHz to 3.33MHz is used to generate clock pulses

which are required for the synchronization of all the internal operations.

QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

1)which of following is oscillator:

(a) colptts (b) nagative

(c) voltage (d)all

2) which of following is LC oscillator

(a) hartley (b) ) collpits

(c) tuned collector (d) none of the above

SHORT ANSWER TYPES QUESTIONS:

1) What is positive feedback?

2) LC stands for ……………..

3) What do you mean by tuned circuit?

4) What is function of crystal?

5) Write three types oscillators.

LONG ANSWER TYPES QUESTIONS:

1) Explain in detail crystal oscillator.

2) Explain in detail wein bridge oscillator.

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ELECTRONIC DEVICES AND CIRCUITS

LEARNING OBJECTIVES:

Concept of tuned voltage amplifier.

Study of resonance.

About single and double tuned ckt..

CHAPTER-5(TUNED VOLTAGE AMPLIFIER)

6.1. TUNED VOLTAGE AMPLIFIER:

6.1.1 RESONANCE

Resonance in AC circuits implies a special frequency determined by the values of

the resistance , capacitance , and inductance . For series resonance the condition of resonance is

straightforward and it is characterized by minimum impedance and zero phase. Parallel resonance ,

which is more common in electronic practice, requires a more careful definition.

This is an active graphic. Click on either for more detail.

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6.2. SERIES RESONANCE

The resonance of a series RLC circuit occurs when the inductive and capacitive reactances are

equal in magnitude but cancel each other because they are 180 degrees apart in phase. The

sharp minimum in impedance which occurs is useful in tuning applications. The sharpness of

the minimum depends on the value of R and is characterized by the "Q" of the circuit.

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6.2.1 Selectivity and Q of a Circuit

Resonant circuits are used to respond selectively to signals of a given frequency while

discriminating against signals of different frequencies. If the response of the circuit is more

narrowly peaked around the chosen frequency, we say that the circuit has higher "selectivity".

A "quality factor" Q, as described below, is a measure of that selectivity, and we speak of a

circuit having a "high Q" if it is more narrowly selective.

An example of the application

of resonant circuits is the

selection of AM radio stations

by the radio receiver. The

selectivity of the tuning must

be high enough to

discriminate strongly against

stations above and below in

carrier frequency, but not so

high as to discriminate

against the "sidebands"

created by the imposition of

the signal by amplitude

modulation.

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The selectivity of a circuit is

dependent upon the amount

of resistance in the circuit.

The variations on a series

resonant circuit at right

follow an example in Serway

& Beichner. The smaller the

resistance, the higher the "Q"

for given values of L and C.

The parallel resonant

circuit is more commonly

used in electronics, but the

algebra necessary to

characterize the resonance is

much more involved.

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Using the same circuit

parameters, the illustration at

left shows the power

dissipated in the circuit as a

function of frequency. Since

this power depends upon the

square of the current, these

resonant curves appear steeper

and narrower than the

resonance peaks for current

above.

The quality factor Q is

defined by

where Δω is the width of the

resonant power curve at half

maximum.

Since that width turns out to be Δω =R/L, the value of Q can also be expressed as

The Q is a commonly used parameter in electronics, with values usually in the range of Q=10

to 100 for circuit applications.

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6.2.2 Power in a Series Resonant Circuit

The average power dissipated in a series resonant circuit can be expressed in terms of the rms voltage

and current as follows:

Using the forms of the inductive reactance and capacitive reactance, the term involving them can be

expressed in terms of the frequency.

where use has been made of the resonant frequency expression

Substitution now gives the expression for average power as a function of frequency.

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This power distribution is plotted at left

using the same circuit parameters as

were used in the example on the Q

factor of the series resonant circuit

The average power at resonance is just

since at the resonant frequency ω0 the

reactive parts cancel so that the circuit

appears as just the resista

1

6.3 PARALLEL RESONANCE CIRCUIT

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1 Parallel resonance occurs when the supply frequency creates zero phase difference

between the supply voltage and current producing a resistive circuit

2 In many ways a parallel resonance circuit is exactly the same as the series

resonance circuit we looked at in the previous tutorial. Both are 3-element

networks that contain two reactive components making them a second-order

circuit, both are influenced by variations in the supply frequency and both have a

frequency point where their two reactive components cancel each other out

influencing the characteristics of the circuit. Both circuits have a resonant

frequency point.

3 The difference this time however, is that a parallel resonance circuit is influenced

by the currents flowing through each parallel branch within the parallel LC tank

circuit. A tank circuit is a parallel combination of L and C that is used in filter

networks to either select or reject AC frequencies. Consider the parallel RLC

circuit below.

6.3.1 Parallel RLC Circuit

2

3 Let us define what we already know about parallel RLC circuits.

4

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5 A parallel circuit containing a resistance, R, an inductance, L and a capacitance, C will

produce a parallel resonance (also called anti-resonance) circuit when the resultant

current through the parallel combination is in phase with the supply voltage. At

resonance there will be a large circulating current between the inductor and the capacitor

due to the energy of the oscillations, then parallel circuits produce current resonance.

6 A parallel resonant circuit stores the circuit energy in the magnetic field of the inductor

and the electric field of the capacitor. This energy is constantly being transferred back

and forth between the inductor and the capacitor which results in zero current and

energy being drawn from the supply.

7 This is because the corresponding instantaneous values of IL and IC will always be equal

and opposite and therefore the current drawn from the supply is the vector addition of

these two currents and the current flowing in IR.

8 In the solution of AC parallel resonance circuits we know that the supply voltage is

common for all branches, so this can be taken as our reference vector. Each parallel

branch must be treated separately as with series circuits so that the total supply current

taken by the parallel circuit is the vector addition of the individual branch currents.

9 Then there are two methods available to us in the analysis of parallel resonance circuits.

We can calculate the current in each branch and then add together or calculate the

admittance of each branch to find the total current.

10 We know from the previous series resonance tutorial that resonance takes place

when VL= -VC and this situation occurs when the two reactances are equal, XL = XC.

The admittance of a parallel circuit is given as:

11 Resonance occurs when XL = XC and the imaginary parts of Y become zero. Then:

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12 Notice that at resonance the parallel circuit produces the same equation as for the series

resonance circuit. Therefore, it makes no difference if the inductor or capacitor are

connected in parallel or series.

13 Also at resonance the parallel LC tank circuit acts like an open circuit with the circuit

current being determined by the resistor, R only. So the total impedance of a parallel

resonance circuit at resonance becomes just the value of the resistance in the circuit

and Z = R as shown.

14

15 Thus at resonance, the impedance of the parallel circuit is at its maximum value and

equal to the resistance of the circuit creating a circuit condition of high resistance and

low current. Also at resonance, as the impedance of the circuit is now that of resistance

only, the total circuit current, I will be “in-phase” with the supply voltage, VS.

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16 We can change the circuit’s frequency response by changing the value of this resistance.

Changing the value of R affects the amount of current that flows through the circuit at

resonance, if both L and C remain constant. Then the impedance of the circuit at

resonance Z = RMAX is called the “dynamic impedance” of the circuit.

6.3.2 Impedance in a Parallel Resonance Circuit

17 Note that if the parallel circuits impedance is at its maximum at resonance then

consequently, the circuits admittance must be at its minimum and one of the

characteristics of a parallel resonance circuit is that admittance is very low limiting the

circuits current. Unlike the series resonance circuit, the resistor in a parallel resonance

circuit has a damping effect on the circuits bandwidth making the circuit less selective.

18 Also, since the circuit current is constant for any value of impedance, Z, the voltage

across a parallel resonance circuit will have the same shape as the total impedance and

for a parallel circuit the voltage waveform is generally taken from across the capacitor.

19 We now know that at the resonant frequency, ƒr the admittance of the circuit is at its

minimum and is equal to the conductance, G given by 1/R because in a parallel

resonance circuit the imaginary part of admittance, i.e. the susceptance, B is zero

because BL = BC as shown.

6.4. SINGLE TUNED AMPLIFIERS

6.4.1 DEFINITION: Single Tuned Amplifiers are multistage amplifier circuit that employs a

parallel tuned circuit as a load. However, the tuned circuit in each stage is required to be tuned to

similar frequencies. A common emitter configuration amplifier can be used as a single tuned

amplifier which includes the parallel tuned circuit.

During wireless communication, the radio frequency stage requires a tuned voltage amplifier in

order to select the desired carrier frequency and amplify the allowed passband signal.

6.4.2 CONSTRUCTIONAL DETAILS OF SINGLE TUNED AMPLIFIER

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The figure below shows the circuit arrangement of a single tuned amplifier with capacitive

coupling.

It is noteworthy here that for a tuned circuit, capacitance and inductance value must be so selected

that the frequency of resonance must be equivalent to the frequency of the applied signal.

We can get the output of the circuit either by capacitive or inductive coupling. However, here we

have used capacitive coupling.

The capacitor CE employed in the circuit is a bypass capacitor whereas biasing and stabilization

circuits are followed by R1, R2 and RE.

Tuned LC circuit employed in the collector region acts as the load. In order to have a variable

resonant frequency, the capacitor is variable. Large signal amplification can be obtained if the

frequency of input signal is similar to the frequency of resonance of the LC circuit.

6.4.3 OPERATION OF SINGLE TUNED AMPLIFIER

The circuit operation of single tuned amplifiers begins with the application of the high-frequency

signal that is to be amplified at the base-emitter terminal of the transistor, shown in

the figure above.

By varying the capacitor employed in the tuned circuit, the resonant frequency of the circuit can be

made equivalent to the frequency of the applied input signal.

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Here, the high impedance is offered to the signal frequency by the tuned circuit. Thus, a large

output is achieved. For an input signal with multiple frequencies, only the frequency that

corresponds to resonant frequency will get amplified. While all other frequencies are rejected the

LC circuit.

Hence, only the desired frequency signal gets selected and thus amplified by the circuit.

6.4.4 VOLTAGE GAIN AND FREQUENCY RESPONSE

For a tuned amplifier, voltage gain is given by

: Rac= impedance of tuned circuit

z = L/CR

rin= input impedance

Let us now move further and have a look at the frequency response curve shown below:

As we know that the impedance of the circuit is very high and is entirely resistive in nature at the

frequency of resonance.

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Thus, the maximum voltage is obtained across RL for a circuit tuned at the resonant frequency.

The bandwidth for a tuned amplifier is given as

Any frequency within this range will be amplified by the amplifier.

Cascading effect on bandwidth

6.5 DOUBLE- TUNED AMPLIFIER

A double-tuned amplifier is a tuned amplifier with transformer coupling between the amplifier

stages in which the inductances of both the primary and secondary windings are tuned separately

with a capacitor across each. The scheme results in a wider bandwidth and steeper skirts than a

single tuned circuit would achieve.

There is a critical value of transformer coupling coefficient at which the frequency response of the

amplifier is maximally flat in the passband and the gain is maximum at the resonant frequency.

Designs frequently use a coupling greater than this (over-coupling) in order to achieve an even

wider bandwidth at the expense of a small loss of gain in the centre of the passband.

Cascading multiple stages of double-tuned amplifiers results in a reduction of the bandwidth of the

overall amplifier. Two stages of double-tuned amplifier have 80% of the bandwidth of a single

stage. An alternative to double tuning that avoids this loss of bandwidth is staggered tuning.

Stagger-tuned amplifiers can be designed to a prescribed bandwidth that is greater than the

bandwidth of any single stage. However, staggered tuning requires more stages and has lower gain

than double tuning.

6.5.1 Typical circuit

A typical 2-stage double-tuned amplifier

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The circuit shown consists of two stages of amplifier in common emitter topology.

The bias resistors all serve their usual functions. The input of the first stage is coupled in the

conventional way with a series capacitor to avoid affecting the bias. However, the collector load

consists of a transformer which serves as the inter-stage coupling instead of capacitors. The

windings of the transformer have inductance. Capacitors placed across the transformer windings

form resonant circuits which provide the tuning of the amplifier.

A further detail that may be seen in this kind of amplifier is the presence of taps on the transformer

windings. These are used for the input and output connections of the transformer rather than the top

of the windings. This is done for impedance matching purposes; bipolar junction

transistor amplifiers (the kind shown in the circuit) have a quite high output impedance and a quite

low input impedance. This problem can be avoided by using MOSFETs which have a very high

input impedance.[1]

The capacitors connected between the bottom of the transformer secondary windings and ground

do not form part of the tuning. Rather, their purpose is to decouple the transistor bias resistors from

the AC circuit.

QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

1)which of following is tuned circuit:

(a) RC (b) LC

(c) CC (d)all

2) which of following is a part of tuned amplifier.

(a)Resistor (b) bandwidth

(c) capacitor (d) none of the above

SHORT ANSWER TYPES QUESTIONS:

1) What is bandwidth?

2) LC stands for ……………..

3) What do you mean by resonance?

4) What is function of impedence?

5) Write formula for Q.

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LONG ANSWER TYPES QUESTIONS:

1) Explain in detail tuned voltage amplifier.

2) Explain in detail series and parallel resonance.

P a g e | 17

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ELECTRONIC DEVICES AND CIRCUITS

LEARNING OBJECTIVES:

Concept of multivibrator.

Study of transistor as switch.

About IC555.

CHAPTER-6(MULTIVIBRATOR CIRCUITS)

6.1 TRANSISTOR AS A SWITCH

Most of microcontrollers work within 5 volt environment and the I/O port can only handle current

up to 20mA; therefore if we want to attach the microcontroller’s I/O port to different voltage level

circuit or to drive devices with more than 20mA; we need to use the interface circuit. One of the

popular method is to use the Bipolar Junction Transistor (BJT) or we just called it transistor in this

tutorial. I have to make clear on this BJT type to differentiate among the other types of transistors

family such as FET (Field Effect Transistor), MOSFET (Metal Oxide Semiconductor FET), VMOS

(Vertical MOSFET) and UJT (Uni-Junction Transistor).

A. The Switch

The transistor actually works as a current gainer; any current applied to the base terminal will be

multiplied by the current gain factor of the transistor which known as hFE. Therefore transistor can

be used as amplifier; any small signal (very small current) applied to the base terminal will be

amplified by the factor of hFE and reflected as a collector current on the collector terminal side.

All the transistors have three state of operation:

1. Off state: in this state there is no base current applied or IB = 0.

2. On active state: in this state any changes in IB will cause changes in IC as well or IC = IB x

hFE. This type of state is suitable when we use transistor as a signal amplifier because

transistor is said is in the linear state. For example if we have a transistor with gain of 100

and we increase the IBfrom 10uA to 100uA; this will cause the IC to swing from 1000uA to

10000uA (1 mA to 10 mA).

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3. On saturate state: in this state any changes in IB will not cause changes in IC anymore (not

linear) or we could say IC is nearly constant. We never use this state to run the transistor as

a signal amplifier (class A amplifier) because the output signal will be clamped when the

transistor is saturate. This is the type of state that we are looking for on this tutorial.

From the picture above we could see the voltage and current condition of transistor on each state; if

you notice when transistor is in off state the voltage across collector and emitter terminal is equal to

the supplied voltage, this is equivalent to the open circuit and when transistor is in saturate state the

collector to emitter voltage is equal or less then 0.2 Volt which is equivalent to the close circuit.

Therefore to use transistor as a switch we have to make transistor OFF which equivalent to the

logical “0” and SATURATE which is equivalent to the logical “1“.

6.2 MULTIVIBRATOR

A multivibrator is an electronic circuit used to implement a variety of simple two-

state[1][2][3] devices such as relaxation oscillators, timers and flip-flops. It consists of two amplifying

devices (transistors, vacuum tubes or other devices) cross-coupled by resistors or capacitors.[not in

citation given] The first multivibrator circuit, the astable multivibrator oscillator, was invented by Henri

Abraham and Eugene Bloch during World War I.[4][5] They called their circuit a "multivibrator"

because its output waveform was rich in harmonics.[6]

The three types of multivibrator circuits are:

Original vacuum tube Abraham-Bloch multivibrator oscillator, from their 1919 paper

Astable multivibrator, in which the circuit is not stable in either state —it continually

switches from one state to the other. It functions as a relaxation oscillator.

Monostable multivibrator, in which one of the states is stable, but the other state is unstable

(transient). A trigger pulse causes the circuit to enter the unstable state. After entering the

unstable state, the circuit will return to the stable state after a set time. Such a circuit is useful

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for creating a timing period of fixed duration in response to some external event. This circuit is

also known as a one shot.

Bistable multivibrator, in which the circuit is stable in either state. It can be flipped from one

state to the other by an external trigger pulse. This circuit is also known as a flip-flop. It can

store one bit of information, and is widely used in digital logic and computer memory.

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.

6.3 IC 555

The 555 timer IC was introduced in the year 1970 by Signetic Corporation and gave the

name SE/NE 555 timer. It is basically a monolithic timing circuit that produces accurate and

highly stable time delays or oscillation. When compared to the applications of an op-amp in the

same areas, the 555IC is also equally reliable and is cheap in cost. Apart from its applications as

a monostable multivibrator and astable multivibrator, a 555 timer can also be used in dc-dc

converters, digital logic probes, waveform generators, analog frequency meters and

tachometers, temperature measurement and control devices, voltage regulators etc. The timer

IC is set up to work in either of the two modes – one-shot or monostable or as a free-running or

astable multivibrator. The SE 555 can be used for temperature ranges between – 55°C to 125

°. The NE 555 can be used for a temperature range between 0° to 70°C.

6.3.1 The important features of the 555 timer are :

It operates from a wide range of power supplies ranging from + 5 Volts to + 18 Volts supply

voltage.

Sinking or sourcing 200 mA of load current.

The external components should be selected properly so that the timing intervals can be made into

several minutes along with the frequencies exceeding several hundred kilohertz.

The output of a 555 timer can drive a transistor-transistor logic (TTL) due to its high current

output.

It has a temperature stability of 50 parts per million (ppm) per degree Celsius change in

temperature, or equivalently 0.005 %/ °C.

The duty cycle of the timer is adjustable.

The maximum power dissipation per package is 600 mW and its trigger and reset inputs has logic

compatibility. More features are listed in the datasheet.

6.3.2 IC Pin Configuration

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555 Timer IC Pin Configuration

The 555 Timer IC is available as an 8-pin metal can, an 8-pin mini DIP (dual-in-package) or a 14-

pin DIP. The pin configuration is shown in the figures.

This IC consists of 23 transistors, 2 diodes and 16 resistors. The use of each pin in the IC is

explained below. The pin numbers used below refers to the 8-pin DIP and 8-pin metal can

packages. These pins are explained in detail, and you will get a better idea after going through the

entire post.

P a g e | 5

Pin 1: Grounded Terminal: All the voltages are measured with respect to the Ground terminal.

Pin 2: Trigger Terminal:Â The trigger pin is used to feed the trigger input hen the 555 IC is set

up as a monostable multivibrator. This pin is an inverting input of a comparator and is

responsible for the transition of flip-flop from set to reset. The output of the timer depends on the

amplitude of the external trigger pulse applied to this pin. A negative pulse with a dc level greater

than Vcc/3 is applied to this terminal. In the negative edge, as the trigger passes through Vcc/3, the

output of the lower comparator becomes high and the complimentary of Q becomes zero. Thus the

555 IC output gets a high voltage, and thus a quasi stable state.

Pin 3: Output Terminal: Output of the timer is availÂable at this pin. There are two ways in

which a load can be connected to the output terminal. One way is to connect between output pin

(pin 3) and ground pin (pin 1) or between pin 3 and supply pin (pin 8). The load connected between

output and ground supply pin is called the normally on load and that connected between output

and ground pin is called the normally off load.

Pin 4: Reset Terminal: Whenever the timer IC is to be reset or disabled, a negative pulse is

applied to pin 4, and thus is named as reset terminal. The output is reset irrespective of the input

condition. When this pin is not to be used for reset purpose, it should be connected to + VCC to

avoid any possibility of false triggering.

Pin 5: Control Voltage Terminal: The threshold and trigger levels are controlled using this pin.

The pulse width of the output waveform  is determined by connecting a POT or bringing in an

external voltage to this pin. Â The external voltage applied to this pin can also be used to modulate

the output waveform. Thus, the amount of voltage applied in this terminal will decide when the

P a g e | 6

comparator is to be switched, and thus changes the pulse width of the output. When this pin is not

used, it should be bypassed to ground through a 0.01 micro Farad to avoid any noise problem.

Pin 6: Threshold Terminal: This is the non-inverting input terminal of comparator 1, which

compares the voltage applied to the terminal with a reference voltage of 2/3 VCC. The amplitude of

voltage applied to this terminal is responsible for the set state of flip-flop. When the voltage applied

in this terminal is greater than 2/3Vcc, the upper comparator switches to +Vsat and the output gets

reset.

Pin 7 : Discharge Terminal: This pin is connected internally to the collector of transistor and

mostly a capacitor is connected between this terminal and ground. It is called discharge terminal

because when transistor saturates, capacitor discharges through the transistor. When the transistor

is cut-off, the capacitor charges at a rate determined by the external resistor and capacitor.

Pin 8: Supply Terminal: A supply voltage of + 5 V to + 18 V is applied to this terminal with

respect to ground (pin 1).

6.3.3 555 Timer Basics

The 555 timer combines a relaxation oscillator, two comparators, an R-S flip-flop, and a discharge

capacitor.

S-R-Flip Flop

As shown in the figure, two transistors T1 and T2 are cross-coupled. The collector of transistor T1

drives the base of transistor T2 through the resistor Rb2. The collector of transistor T2 drives the

P a g e | 7

base of transistor T1 through resistor Rb1. When one of the transistors is in the saturated state, the

other transistor will be in the cut-off state. If we consider the transistor T1 to be saturated, then the

collector voltage will be almost zero. Thus there will be a zero base drive for transistor T2 and will

go into cut-off state and its collector voltage approaches +Vcc. This voltage is applied to the base

of T1 and thus will keep it in saturation.

S-R Flip Flop Symbol

Now, if we consider the transistor T1 to be in the cut-off state, then the collector voltage of T1 will

be equal to +Vcc. This voltage will drive the base of the transistor T2 to saturation. Thus, the

saturated collector output of transistor T2 will be almost zero. This value when fed back to the base

of the transistor T1 will drive it to cut-off. Thus, the saturation and cut-off value of any one of the

transistors decides the high and low value of Q and its complement. By adding more components to

the circuit, an R-S flip-flop is obtained. R-S flip-flop is a circuit that can set the Q output to high or

reset it low. Incidentally, a complementary (opposite)Â output Q is available from the collector of

the other transistor. The schematic symbol for a S-R flip flop is also shown above. The circuit

latches in either the Q state or its complimentary state. A high value of S input sets the value of Q

to go high. A high value of R input resets the value of Q to low. Output Q remains in a given state

until it is triggered into the opposite state.

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555 IC Timing Circuit

6.3.4 Basic Timing Concept

From the figure above, assuming the output of the S-R flip flop, Q to be high. This high value is

passed on to the base of the transistor, and the transistor gets saturated, thus producing a zero

voltage at the collector. The capacitor voltage is clamped at ground, that is, the capacitor C is

shorted and cannot charge.

The inverting input of the comparator is fed with a control voltage, and the non-inverting input is

fed with a threshold voltage. With R-S flip flop set, the saturated transistor holds the threshold

voltage at zero. The control voltage, however, is fixed at 2/3 VCC, that is, at 10 volts, because of

the voltage divider.

Suppose that a high voltage is applied to the R input. This resets the flip-flop R-Output Q goes low

and the transistor is cut-off. Capacitor C is now free to charge. As this capacitor C charges, the

threshold voltage rises. Eventually, the threshold voltage becomes slightly greater than (+ 10 V).

The output of the comparator then goes high, forcing the R S flip-flop to set. The high Q output

saturates the transistor, and this quickly discharges the capacitor. An exponential rise is across the

capacitor C, and a positive going pulse appears at the output Q. Thus capacitor voltage VC is

exponential while the output is rectangular. This is shown in the figure above.

6.3.5 555 IC Timer Block Diagram

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555 IC Timer Block Diagram

The block diagram of a 555 timer is shown in the above figure. A 555 timer has two comparators,

which are basically 2 op-amps), an R-S flip-flop, two transistors and a resistive network.

Resistive network consists of three equal resistors and acts as a voltage divider.

Comparator 1 compares threshold voltage with a reference voltage + 2/3 VCC volts.

Comparator 2 compares the trigger voltage with a reference voltage + 1/3 VCC volts.

Output of both the comparators is supplied to the flip-flop. Flip-flop assumes its state according to

the output of the two compaÂrators. One of the two transistors is a discharge transistor of which

collector is connected to pin 7. This tranÂsistor saturates or cuts-off according to the output state

of the flip-flop. The saturated transisÂtor provides a discharge path to a capacitor conÂnected

externally. Base of another transistor is connected to a reset terminal. A pulse applied to this

terminal resets the whole timer irrespective of any input.

6.3.6 Working Principle

Refer Block Diagram of 555 timer IC given above:

The internal resistors act as a voltage divider network, providing (2/3)Vcc at the non-inverting

terminal of the upper comparator and (1/3)Vcc at the inverting terminal of the lower comparator. In

most applications, the control input is not used, so that the control voltage equals +(2/3) VCC.

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Upper comparator has a threshold input (pin 6) and a control input (pin 5). Output of the upper

comparator is applied to set (S) input of the flip-flop. Whenever the threshold voltage exceeds the

control voltage, the upper comparator will set the flip-flop and its output is high. A high output

from the flip-flop when given to the base of the discharge transistor saturates it and thus discharges

the transistor that is connected externally to the discharge pin 7. The complementary signal out of

the flip-flop goes to pin 3, the output. The output available at pin 3 is low. These conditions will

prevail until lower comparator triggers the flip-flop. Even if the voltage at the threshold input falls

below (2/3) VCC, that is upper comparator cannot cause the flip-flop to change again. It means that

the upper comparator can only force the flip-flop’s output high.

To change the output of flip-flop to low, the voltage at the trigger input must fall below + (1/3)

Vcc. When this occurs, lower comparator triggers the flip-flop, forcing its output low. The low

output from the flip-flop turns the discharge transistor off and forces the power amplifier to output

a high. These conditions will continue independent of the voltage on the trigger input. Lower

comparator can only cause the flip-flop to output low.

From the above discussion, it is concluded that for the having low output from the timer 555, the

voltage on the threshold input must exceed the control voltage or + (2/3) VCC. This also turns the

discharge transistor on. To force the output from the timer high, the voltage on the trigger input

must drop below +(1/3) VCC. This turns the discharge transistor off.

A voltage may be applied to the control input to change the levels at which the switching occurs.

When not in use, a 0.01 nano Farad capacitor should be connected between pin 5 and ground to

prevent noise coupled onto this pin from causing false triggering.

Connecting the reset (pin 4) to a logic low will place a high on the output of flip-flop. The

discharge transistor will go on and the power amplifier will output a low. This condition will

continue until reset is taken high. This allows the synchronization or resetting of the circuit’s

operation. When not in use, reset should be tied to +VCC.

6.3.7 Applications of 555 Timer circuits

To know more about applications of 555 Timer IC take a look at the following posts:

555 TIMER AS AN ASTABLE MULTIVIRATOR

555 TIMER AS A MONOSTABLE MULTIVIBRATOR

555 TIMER OSCILLATOR

555 TIMER – RAMP GENERATOR

555 TIMER PROJECTS

6.4 IC 555 AS MONOSTABLE MULTIVIBRATOR

A monostable multivibrator (MMV) often called a one-shot multivibrator, is a pulse generator

circuit in which the duration of the pulse is determined by the R-C network,connected externally to

the 555 timer. In such a vibrator, one state of output is stable while the other is quasi-stable

(unstable). For auto-triggering of output from quasi-stable state to stable state energy is stored by

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an externally connected capaciÂtor C to a reference level. The time taken in storage determines the

pulse width. The transition of output from stable state to quasi-stable state is accomÂplished by

external triggering. The schematic of a 555 timer in monostable mode of operation is shown in

figure.

555-timer-monostable-multivibrator

6.4.1 Monostable Multivibrator Circuit details

Pin 1 is grounded. Trigger input is applied to pin 2. In quiescent condition of output this input is

kept at + VCC. To obtain transition of output from stable state to quasi-stable state, a negative-going

pulse of narrow width (a width smaller than expected pulse width of output waveform)Â and

 amplitude of greater than + 2/3 VCC is applied to pin 2. Output is taken from pin 3. Pin 4 is

usually connected to + VCC to avoid accidental reset. Pin 5 is grounded through a 0.01 u F capacitor

to avoid noise problem. Pin 6 (threshold) is shorted to pin 7. A resistor RA is connected between

pins 6 and 8. At pins 7 a discharge capacitor is connected while pin 8 is connected to supply VCC.

6.4.2 555 IC Monostable Multivibrator Operation.

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555 monostable-multivibrator-operation

The operation of the circuit is explained below:

Initially, when the output at pin 3 is low i.e. the circuit is in a stable state, the transistor is on and

capacitor- C is shorted to ground. When a negative pulse is applied to pin 2, the trigger input falls

below +1/3 VCC, the output of comparator goes high which resets the flip-flop and consequently the

transistor turns off and the output at pin 3 goes high. This is the transition of the output from stable

to quasi-stable state, as shown in figure. As the discharge transistor is cutÂoff, the capacitor C

begins charging toward +VCC through resistance RA with a time constant equal to RAC. When the

increasing capacitor voltage becomes slightly greater than +2/3 VCC, the output of comparator 1

goes high, which sets the flip-flop. The transistor goes to saturation, thereby discharging the

capacitor C and the output of the timer goes low, as illustrated in figure.

Thus the output returns back to stable state from quasi-stable state.

The output of the Monostable Multivibrator remains low until a trigger pulse is again applied. Then

the cycle repeats. Trigger input, output voltage and capacitor voltage waveforms are shown in

figure.

6.4.3 Monostable Multivibrator Design Using 555 timer IC

The capacitor C has to charge through resistance RA. The larger the time constant RAC, the longer

it takes for the capacitor voltage to reach +2/3VCC.

In other words, the RC time constant controls the width of the output pulse. The time during which

the timer output remains high is given as

tp = 1.0986 RAC where RA is in ohms and C is in farads. The above relation is derived as below. Voltage across the

capacitor at any instant during charging period is given as

vc = VCC (1- e-t/RAC)

Substituting vc = 2/3 VCC in above equation we get the time taken by the capacitor to charge from 0

to +2/3VCC.

So +2/3VCC. = VCC. (1 – e–t/RAC)  or  t – RAC loge 3 = 1.0986 RAC

So pulse width, tP = 1.0986 RAC s 1.1 RAC

The pulse width of the circuit may range from micro-seconds to many seconds. This circuit is

widely used in industry for many different timing applications.

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QUESTIONS:

MULTIPLE CHOICE QUESTIONS:

1)which of following is multivibrator:

(a) astable (b) bistable

(c) monostable (d)all

2) which of following is timer?

(a) 555 (b) ) 741

(c) 8085 (d) none of the above

SHORT ANSWER TYPES QUESTIONS:

1) What is switch?

2) IC 555works as……………..

3) What do you mean by multivibrator?

4) What is function of timer?

5) Write three types of multivibrator.

LONG ANSWER TYPES QUESTIONS:

1) Explain in detail IC 555 as monostable multivibrator.

2) Explain in detail transistor as a switch.

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