RECTIFIERS

INTRODUCTION
The electrical power is almost exclusively generated, transmitted and distributed in the form of ac economical consideration but for operation of most of the electronic devices and circuits, dc supply is required.Dry cells and batteries can be used for this purpose.No doubt they have the advantages of begin portable and ripple free but their voltage are low,they need frequent replacement and are expensive in comparison to conventional dc power supplies. Now a days ,almost all electronic equipment include a circuit that converts ac into dc supply.The part of equipment that converts ac into dc is called the dc power supply.In general,at the input of the power supply ,there is a power transformer.It is followed by a rectifier ( a diode circuit), a smoothing filter and then by a voltage regulator circuits.
the basic power supply  is constituted by four elements viz.. a transformer, a rectifier, a filter and a regulator put together.
The output of the dc power supply is used to provide a constant dc voltage across the load . let us briefly outline the function of each of the elements of the dc power supply.
Transformer is used to step up or step down(usually to step down) the supply voltage as per need of the solid-state electronic devices and circuits to be supplied by the dc power supply. It  can provide isolation from the supply line -an  important safety consideration. It may also include internal shielding to prevent unwanted electrical noise signal on the power line from getting into the power supply and possibly distributing the load.

 Rectifier is a device which converts the sinusoidal ac voltage into either positive or negative publishing dc. P-N junction diode,which conducts when forward biased and practically does not conduct when reverse biased,can be used for rectification i.e., for conversion of ac into dc.The rectifier typically needs one, two or four diodes.Rectifiers may be either half-wave (center-tap or bridge) type.
the output voltage from a rectifier circuits unwanted ac components sating character i.e it contains unwanted ac components (components of supply frequency f and its harmonics) along with dc components.For most supply purposes,constant direct voltage is required than that furnished by a rectifier.To reduce ac components from the rectifier output voltage a filter circuit is required.Thus filter is a device which passes dc components to the load and blocks ac components of the rectifier output.Filter is typically formed from reactive circuits elements such as capacitors and/or inductor and resistors.
 The magnitude of output dc voltage may very with the variation of either the input ac voltage or the magnitude of load current. so at the output of a rectifier-filter combination a voltage regulator is required as shown in figure.
to provide an almost constant dc voltage ath output of the regulator.the voltage regulator may be constructed from a Zener diode, and/or discrete transistors,and/or integrated circuits(ICs).Its main function is to maintain  a constant  dc output voltage.However,it also rejects any ac ripple voltage that is not removed by the filter.The regulator may also include protective devices such as short-circuit protection,current limiting,thermal shutdown,or over voltage protection..

RECTIFIER

FILTER

VOLTAGE REGULATOR

REGULATED DC OUTPUT VOLTAGE

this figure is show how to convert ac into dc

CONSTRUCTION OF SINGLE-PHASE TRANSFORMERS

A single-phase transformer consists of primary and secondary winding put on a magnetic core. Magnetic core is used to confine flux to a definite path.Transformer cores are made from thin sheet(called lamination) of high-grade silicon steel.The lamination reduce eddy-current loss and the silicon steel reduced hysteresis loss. the lamination are insulated from one another by heat resistant enamel insulation coating. L-type and E-type lamination are used. The limitations are built up into stack and the joints in the lamination are staggered to minimize air gaps(which require large exciting currents).The lamination are tightly clamped.

STEP-UP AND STEP-DOWN TRANSFORMERS

step-up transformer:

A transformer in which  the output(secondary)voltage is greater than its input(primary)voltage is called a step-up transformer.

step-down transformer:

A transformer in which the output(secondary) voltage is less than its input (primary) voltage is called a step-down transformer.

The same transformer can be used as a step-up transformer or a step -down transformer depending on the way it is connected in the circuit.when the transformer is used as a step-up transformer,the low voltage winding is the primary. in a step-down transformer,the high-voltage winding is the primary.

 A transformer may receive energy at one voltage and deliver it at the same voltage.such a transformer is called a one-to-one(1:1)transformer. for a 1:1 transformer T1=T2 and |E1|=|E2|.such a transformer is used to isolate two circuits.

TRANSFORMERS

PRINCIPLE OF TRANSFORMER OPERATION
A transformer is a static device which consists of two or more stationary electric circuits interlinked by a common magnetic circuit for the purpose of transferring electrical energy between them. the transfer of energy from one circuit to another takes place without a change in frequency.

 consider two coils 1 and coils 2 wound on a simple magnetic circuit as shown in fig . these two coils are insulated from each other and there is no electrical connection between them .Let T1 and T2 be the number of turns in coils 1 and 2 respectively. when a source of alternating current produces an alternating flux 0m in the magnetic circuit.the mean path of this flux is shown in figure  by the dotted line. this alternating flux links the turns T1 of coil 1 and induces in them an alternating voltage E1 by self-induction. let us make the following simplifying assumptions for an ideal transformer:
(a) there are no losses either in the electric circuits or in the magnetic circuit.
(b) the whole of the magnetic flux o is confined to the magnetic circuit,so that there is no leakage flux.
(c)the permeability of the core is infinite.
 Thus, all the flux produced by coil 1 also linkes T2 turns of coils 2 and induced in them a voltage E2 by mutual induction . If coil 2 is connected to a load then an alternating current will flow through it and energy will be delivered to the load. Thus, electrical energy is transferred from coil 1 to coil 2 y a common magnetic circuit.since there is no  relative motion between the coils,the frequency of the induced voltage in coil 2 is exactly the same as the frequency of the applied voltage to coil1

coil 1 which receives energy from the source energy from the source of a.c supply is called the primary  coil or primary winding or simply the primary,coil 2,which is connected to load and delivers energy to the load, is called the secondary. the circuit symbol for a two-winding or simply the secondary. the circuit symbol for a two - winding transformer is shown in fig the vertical bars are used to signify tight magnetic coupling between the windings.

sensitivity and selectivity

sensitivity:
The sensitivity of a radio receiver may be defined as its  ability to amplify weak signals.
It is generally, defined in terms of the voltage which must be applied at receiver input terminals to provide a standard output power measured at the output terminals .
for AM broadcast receivers,several relevant quantities have been standardized. A signal modulated by a 400 Hz sine wave and  modulation index of 30% is applied through standard coupling network known as a dummy antenna.
In addition to this,the loud-speaker is replaced by an equivalent load resistance and it must be equal to be equal to the standard value of 50 mW.

Sensitivity is also expressed in microvolts or in decibels below 1 volt and is measured at three points along the tuning range when a production receiver is lined up.
the sensitivity is generally quoted in terms of signal power required to produced a minimum acceptable output signal with a minimum acceptable output signal with a minimum acceptable output noise level.Few factors determining the sensitivity of a superheterodyne receiver are as under:

(i) The gain of the IF amplifiers.
(ii) The gain of the RF amplifiers.
(iii) The noise figure of the receiver.
It may be noted that the typical values of sensitivity are 150μ volt for small broadcast band receivers,and 1μV or below for high quality communication receiver in the HF band
selectivity:
The selectivity of a receiver may be defined as the ability to reject unwanted signals. It also expresses the attenuation that the receiver offers to signal at frequencies adjacent to the one to which it is tuned.In selectivity measurement,the frequency of the generators is varied to either side of the frequency to which the receiver is tuned.Naturally,the output of the receiver falls since the input frequency is not incorrect.Thus the input voltage must be increased until the output is the same as it was originally.The ratio of the voltage required of resonance to the voltage required. when the generator is tuned to the receiver's frequency it is calculated at a number of point and then plotted in decibels.

(i)Selectivity varies with receiving frequency and becomes somewhat worse when the receiving frequency is raised.

(ii)In general,it is mainly determiine by the response of the IF section,with the mixer and RF amplifier input circuits playing a small but significant part.

(iii)Selectivity is the main factor which determines the adjacent channel rejection of a receiver 

TRF and Drawback of TRF Receiver

Tuned radio frequency (TRF) receiver is the simplest radio receiver.in TRF  signal is passed to a RF stage.This stage generally contains two or three RF amplifier. Actually,these RF(radio frequency)amplifiers are tuned RF amplifiers i.e. they have variable tuned circuit at the input and output sides.At the input of the receiver,there is a receiving antenna.this antenna signals  from different source are present. However,with the help of input variable tuned circuits of RF amplifier the desired signal is selected .but this selected signal is usually very weak of the order of μV. this selected weak signal is amplified by the RF amplifier.

thus the function of RF stage is to select the desired signal and amplify it.
After this,the amplified incoming modulated signal is applied to the demodulator. the demodulator or detector demodulated the modulated signal and thus at the output of the demodulator,we get modulating or baseband signal(i.e. audio signal).this audio signal is amplified by audio amplifier.After that,this audio signal is further amplified by a power amplifier upto desired power level to drive the loudspeaker.The last stage of this receiver is the loudspeaker.A loudspeaker is a transducer which changes electrical signal into sound signal.

(i)The TRF receiver suffers from a tendency to oscillate at higher frequencies from the multistage RF amplifiers with high gain and operating at same frequency.If such an amplifier has gain of 20,000 then if a small portion of the output leaked back to the input of the RF stage,then positive feed back and oscillation will result.this type of leakage could result from power supply coupling, stray capacitance coupling,radiation coupling or coupling  through any other element common to the input and output stages.Definitely,this type of condition is undesirable for a good receiver.

(ii)The selectivity of a receiver is its ability to distinguish between a desired signal and an undesired signal.The selectivity of TRF receiver is poor.In fact,it is difficult to achieve sufficient selectivity at high frequencies due to the enforced use of single-tuned circuits.

(iii)Another problem associated with the TRF receiver is the bandwidth variation over the tuning range. for example,in AM broadcast system,let us consider that a tuned circuit is According to the definition,the Quality factor Q of this tuned circuit must be 

Q=resonance frequency/bandwidth

 =540/10

now, at the order end of this AM broadcast band(i.e.1640 KHz),the Quality factor Q of the coil, according to above equation,must increase by a factor of 1640/535(i.e.3) to a value of 164. however,in practice due to several losses dependent upon frequency would prevent such a large increase.Thus, practically,the quality factor Q of this tuned circuit is unlikely to exceed 120 and hence providing a bandwidth of the tuned circuit equal to

Δf=fr/Q=(1640/120)=13.8kHz

PIN Diode and applications of PIN diode

PIN DIODE: In a PIN(p-intrinsic-n) diode, the semiconductor wafer has heavily doped narrow layer of p type material separated from an equally heavily doped narrow layer of n type material by a thicker layer of high resistivity material that is intrinsic(or very lightly doped material).
silicon is widely used because of its power handling capacity and high resistivity in the intrinsic region and easy fabrication(although GaAs can also be used but without these advantages).
Electrical contacts are taken from the two heavily doped regions.
PIN diode  in forward bias mode, offers a variable resistance-decreasing with the increasing in forward current. For larger dc current it will appear like a short. In reverse bias mode, it offers infinite resistance.
 when there is no bias applied to the diode, there will be diffusion of charge carrier due to concentration gradient across the junction. Let us consider NI junction.  Diffusion of holes and electrons across NI junction form a very thin depletion layer in N-region and a relatively thicker depletion layer in I-region containing equal but opposite fixed charges. when the reverse bias is applied and gradually increased, the thickness of the depletion layers increases until the entire I-region is swept free of mobile carriers. the reverse bias required to sweep out mobile carriers from I-region is known as the swept-out voltage.in most microwave applications, sweeping-out of I-region is necessary, otherwise the current flow due to mobile charge carriers in the high resistivity I-region will be a source of signal loss. so PIN diodes, when used as microwave switches, are reverse biased. the bias is well beyond the swept out voltage(usually-2V).For larger reverse bias, a thin depletion layer appears in the P-region. At zero bias, most of the I layer has mobile charge carrier and diode has high resistance. more and more charge carrier inject into I-region from P- and N-regions and forward resistance is reduced.

thus when a PIN diode is forward biased it acts like a variable resistance.

PIN diodes are widely used for microwave power switching, limiting and modulation.

The PIN diode acts as a low frequency rectifier that could rectify more power than an ordinary p-n junction diode. upto about 100MHz, the operation is similar to an ordinary p-n junction diode.

at higher frequencies the PIN diode acts like a variable resistance.Rectification ceases at higher frequencies due to carrier storage junction and the transit time across the large intrinsic region.

APPLICATIONS OF PIN DIODE: 

1.PIN diode as a switch :
It can be used either in series or in shunt.In the first case when the diode is reverse biased,switch is 'off' or open and when it is forward biased it is closed.the bias is changed by a suitable control system.
In the second case when the diode is forward biased,it offers a shot circuit.Hence energy is fully reflected back and no power flows to the load.Hence switch is open.If reverse biased the diode is open.Hence the load receives the power i.e.,,switch is closed.
2.PIN diode as an Amplitude Modulator: 
the diode is kept at low reverse bias and in series with the low frequency modulating signal. the modulating signal amplitude is kept smaller than the  RF carrier signal.the modulating signal changes the RF resistance of diode so that varying amount of mismatch results. hence the amount of carrier power reflected back and hence the amount of carrier passed beyond diode circuit towards the output varies as the input value of modulating signal.

3.PIN diode as a phase shifter.
 The circular coupled phase shifter  the input signal travels a distance equal to L or L+l depending on whether S is closed or open respectively.Therefore between the two conditions we have a phase shift of 2 θ (the signal has to travel to permanent short and return). By adjusting 'l' phase shift can be adjusted.'L' will be far greater in magnitude than 'l' which will be fraction of a wavelength. It finds applictions in phased array radars.

4.PIN Diode as a limiter. when the input power is moderate,output follows inut power and for larger input,the diode absorbs power and output will be limited i.e., will not vary much with the input power.

FREQUENCY MODULATED CW RADAR

FREQUENCY MODULATED CW RADAR:

The transmitter signal in a CW radar is not modulated. therefore it can neither provide range of the target nor sense which particular cycle of oscillation is being received at any instant. this major drawback can be eliminated by frequency modulation of the transmitted signal even though it increases the bandwidth.

a saw tooth generator frequency modulates the CW carrier. thus transmitted signal frequency increases linearly with the increasing amplitude of the modulating signal.

Earth acts as a target which is stationary with respect to the aircraft.

for a given altitude of aircraft, a known time will be required for the signal to travel from earth to aircraft and hence during this time a definite change in single frequency would take place.If this frequency change can be measured,an indication of the height of the aircraft is obtained.the frequency counter is switched on first when the modulating signal has zero frequency(i.e. the  transmitter frequency equals fc ) and this signal frequency is picked up after its reflection from the earth and used to switch off the counter. the final reading of the counter will give an indication of the change in frequency and hence the altitude of the aircraft,which is displayed on the indicator.

If the relative velocity of the aircraft and earth is not zero, there will be a Doppler frequency shift in addition to the frequency change.

This frequency shift provides a measure of the relative velocity of the aircraft.however the time difference between the transmission and reception of a particular cycle of the signal will be constant and hence the average frequency difference will also be constant. therefore accurate altitude measurement can still be made on the basis of average frequency difference.

 If 2Δf is the range of frequency and T is the modulation period, rate of change of frequency is given by

df/dt=2Δf/(T/2)

APPLICATION:

FMCW radar is mostly used as altimeter in aircraft due to shorter ranges and lower power requirements as compared to pulsed radars,smaller size for aircraft installation and smaller transmitter powers.
typical equipment developed for FMCW radar has a power less than 1 watt and operates at carrier frequency of 440MHz with modulating frequency of 120 hz and frequency deviation of either 2 or 20 MHz.a

what is a function generator?

FUNCTION GENERATOR:

A function generator is a signal source that has the capability of producing different types of waveform as its output signal. the most common output waveforms are sine waves, triangular waves,square waves,and sawtooth waves. the frequencies of such waveforms may be adjusted from a fraction of a hertz to several hundred kHz.

   Actually the function generators are very versatile instruments as they are capable of producing a wide variety of waveforms and frequencies. in fact,each of the waveform they generate are particularly suitable for a different group of applications. the uses of sinusoidal outputs and square wave outputs have already been described in the earlier arts. the tringular wave and sawtooth wave outputs of function generators are commonly used for those applications which need a signal that increases specific linear rate. they are also used in driving sweep oscillators in oscilloscopes and the X-axis of X-Y recorders.

Many function generators are also capable of generating two different waveforms simultaneously(from different output terminals,of course). this can be a useful feature when two generated signals are required for particular application. for instance,by providing a square wave for linearity measurements in an audio system, a simultaneous sawtooth output may be used to drive the horizontal deflection amplifier of an oscilloscope,providing a visual display of the measurement results.

P-N junction diode work

P-N junction diode:
Semiconductor materials was to join a P-type material with an N-type to form a simple but impotant semiconductor device called a diode.
the term diode denotes a two-electrode device. A semiconductor diode is simply a P-N junction with connecting leads or terminals on the two sides of the P-N junction. P-N junction  is grown out off crystal. A diode is a unidirectional devices permitting the easy flow of current in one direction but restraining the flow in opposite direction.

A major application of diodes is in rectification i.e., in conversion of ac into dc.semiconductor diode is becoming more and more popular these days due to its smaller size,cheapness,robustness and higher operating efficiency.

 gray color is denoted a cathode terminal.
The P-type material of the diode is the positive electrode and is called the anode whereas the N-type material is the negative electrode and is called the cathode. The arrow symbol used for the anode represents the direction of flow of conventional current. the electrode current flow in a direction opposite to the arrow.

The symbol for semiconductor diode is shown in figure Diode have polarity and, therefore,must be connected in circuits correct direction. small diodes are marked with a band to indicate the cathode end. larger diodes use the diode symbol to indicate their polarity. Diodes are available in many package configuration,depending on their ratings.Small diodes,used for low current application in circuits up to 1A of average current,are generally packaged in a simple glass or  plastic tube.Diode for handling larger currents are packaged in metal plates to dissipate heat.

THERMISTORS

THERMISTORS:

The thermistors is a combination of a thermal and resistor. A thermistor is a resistor with definite thermal characteristics.
most thermistors have a negative temperature coefficient,but positive temperature devices are also available. the reason of negative temperature coefficient of resistance of a semiconductor is that when its temperature increased, the concentration of charge carriers increases resulting in a decrease in resistance.

Thermistors are essentially semiconductor devices, that behave as resistors with high negative temperature coefficient. Thermistor has a very nonlinear resistance-temperature relation is

                                  β/T
                   R=Roα e

symbol of thermistor

 where,
 R is a resistance of a thermistor.
T is a temperature.
α and β are constants depending upon the material and manufacturing technique employed.

positive temperature coefficient (PTC) Thermistors.
such thermistor are resistors with a high positive temperature coefficient of resistance. They differ from NTC  thermistors is the following aspects.
1. the temperature coefficient of a PTC thermistor is positive  only between a certain temperature  range,outside this range the temperature coefficient is either zero or negative.
2.the absolute value of the temperature coefficient of PTC thermistor is usually much higher than of NTC thermistors.

 PTC thermistors are employed as current limiters, temperature  sensors, and protectors again ts overheating in equipment such as electric motors.They are also employed as level indicators,time delay devices,thermostats,compensation resistors etc.
PTC thermistors are prepared from BaTiO3  and  SrTiO3 in a similar way as  NTC thermistors.If prepared in the absence of Oxygen, these semiconductors have a weak negative temperature coefficient.A strong positive temperature coefficient is obtained by firing the sample in an oxygen-rich atmosphere. this is  caused by the penetration of oxygen along pores and crystal boundaries during cooling after the firing process.
The oxygen atoms, absorbed on the crystal surface attract electrons from a thin Zone of the semiconducting crystals.In this way  electrical potential barriers are formed consisting of a negative surface charge with,on both sides,thin layers having a positive space charge resulting from the now uncompensated foreign ions.These 
                                                                                                                                                  barriers to because an extra resistance of the thermistor. At very high temperatures,i.e.,above 160 degree c to 200 degree c the electrons captured at the boundaries  are gradually liberated causing the potential barriers to decrease in strength.This means  that the PTC loses its properties and may eventually act as an NTC if the temperature becomes too thigh.Therefore the applications of PTC thermistors are restricted by a certain temperature .

Zener diode

Zener diode:

Zener diode,also sometime called the breakdown diode,is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition. the diode may use either zener breakdown mechanism or avalanche breakdown mechanism.
the breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating tempature and current capability.

symbol of a zener diode

Zener diode are connected into circuits with their cathodes as the positive terminal because they operate in the reverse-bias mode.small zener diodes are marked the same way as conventional diodes with a cathode band.The larger diodes are marked with the zener diode.

The symbol of zener diode is given figure this is similar to that of a normal diode except that the line representing the cathode is bent at both ends i.e the bar is turned into  Z-shape; stand for zener.

construction of a zener diode:

Zener diode is like an ordinary P-N junction diode except that it properly doped so as to have a sharp breakdown voltage. There are several methods used in the manufacture of zener diodes, such as diffused structure ,diffused and passivated structure and alloy diffused structure.
 Diffused structure zener diode consists of two N and P substrates diffused together and has metallic layers deposited on both sides to connect anode and cathode terminals as shown in figure. the drawback of such a construction is that the edge of the junction are exposed to contamination.
In passivated structure of a zener diode,the edges of the junction are covered by layer of sio2, as shown in fig 3.
similarly,all junctions are protected in alloy diffused structures as shown in figure 4.
 In general, the alloy diffused structure gives better zener characteristics at lower zener voltages.on the other hand, diffused and passivated zener diode is used at higher zener voltages 

what is a Active and passive components?

Active components: 

Active components or devices used in electronic circuits are numerous but all of these active components or devices can be broadly classified into two categories-tube type and semiconductor type devices. Tube devices can be further divided into two categories -vacuum tubes(such as vacuum diode,vacuum triode,vacuum tetrode and vacuum pentode)
and gas tubes.
semiconductor devices include junction diode,bipolar junction transistor(BJT),field effect transistor(FET),uni junction

example of a active component:picture of  a BJT

transistor(UJT),silicon controlled rectifier(SCR)tunnel diode.

Passive components:

passive components include resistors,inductor and capacitors,these components are split into two categories:those which dissipate energy and those which store it. in the latter category is the inductor,which stores energy by virtue of a current passing through it, and the capacitor,which stores energy by virtue of a voltage exiting across it. The third components, the resistor ,is the only one of the virtue of a voltage exitsing across it.

example of a passive components:picture of a capacitor

Zener Diode application

Zener diode:

Zener diode,also sometime called the breakdown diode,is a P-N junction diode specially designed for operation in the breakdown region in reverse bias condition. the diode may use either zener breakdown mechanism or avalanche breakdown mechanism.

the breakdown diode may be silicon or germanium one but silicon is preferred over germanium because of higher operating tempature and current capability.

Zener Diode have numerous applications in transistor circuitry. some of their common applications are explained below. 

       1. zener Diode shunt Regulator(or voltage regulation).

 voltage regulation is a measure of a circuit's ability to maintain a constant output voltage even when either input voltage or load current varies.

electronic symbol of zener diode

Zener diode can be use a voltage regulator to provide a constant voltage from a source whose voltage may very  appreciably. 

2.Meter protection:

 Zener diodes can be used as meter protection. a zener diode protect a meter movement or circuit against damage from an accidental application of excessive voltage. in this arrangement zener diode is connected in parallel with meter.

3.Switching Operation:

Zener diode can produce a sudden change from low current to high current,so it is useful for switching operations. the use of zener diode in switching operation has made possible an extremely fast performance in computer applications.

OR GATE,AND GATE and NOT GATE.

OR GATE:
The OR gate performs logical addition, more commonly known as the OR function. an OR gate has two or more inputs signals with only one output signal.
In OR gate, output voltage is very high if any or all of the input voltages are high. Diodes may be used to build and OR gate.
TRUTH TABLE of OR gate

INPUT          OUTPUT
a       b               c
0       0               0
0       1               1
1       0               1
1       1               1

where 1 is represents  a high signal and 0 is represents a low signal
a and b is inputs of gate and c is a output.
 1->high signal(given voltage/current)
 0->low signal(not given any signal voltage/current)

AND GATE:

The AND gate performs logical multiplication,more commonly known as AND function.
 the AND gate has two or more inputs and a single output. The AND gate provides high output only when all inputs are high.
TRUTH TABLE of AND gate
INPUT                 OUTPUT

a       b                    c
0       0                   0
0       1                   0
1       0                   0
1       1                   1

where 1 is represents  a high signal and 0 is represents a low signal
a and b is inputs of gate and c is a output.
 1->high signal(given voltage/current)
 0->low signal(not given any signal voltage/current)

NOT GATE OR INVERTER:

The inverter (NOT circuit)performs a basic logic function called inversion or complementation. the purpose  of the inverter is to change one logic level to the opposite level. in terms of bits, it changes a 1 to a 0 and vice versa.

  

This gate has only one input and one output. NOT gate is called the inverter because output state is always opposite to the input state,so when the input is low signal, output signal is high and vice versa. it is called NOT gate because the output state is always opposite to that of input state.

1----->0(using NOT gate)

0----->1(using NOT gate)

fig: AND GATE,NAND GATE,OR GATE,NOR GATE and XOR GATE,XNOR GATE. .

figures show that:

 row 1: AND gate then use NOT gate it's make NAND gate.

 row 2: OR gate then use NOT gate it's make NOR gate.

 row 3: NOR gate then use NOT gate it's make XNOR gate.

TRUTH-TABLE

TRUTH-TABLE:

A TRUTH-TABLE as suggested is a tabular or graphic technique for listing all possible combinations of input variables, arguments,or whatever they may be called,in a vertical order,listing each input combination one row at a input combination one row at a time .when every possible combination is recorded,each  combination can be studied to determine whether the "output"or "combined interaction" of that combination should be ASSERTED or NOT-ASSERTED.

the TRUTH-TABLE came into existence during the early year of logic study when logician used them to validate their thinking. it is somewhat unfortunate that is TRUE/FALSE connotation has been carried over into digital hardware design . in digital design, the TRUTH-TABLE is used primarily to specify an INPUT/OUTPUT relation for a digital network,not to determine the truth value of an argument.

example of a truth table:

---

INPUTS          OUTPUTS
 a      b                  c
 0      0                  0
 0      1                  1 
 1      0                  1
 1      1                  1

---

this is a example of a OR gate truth table where a and b is a input and c is a output