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1. Explain the working of zener diode.

Zener diodes are a form of semiconductor diode that are widely used in electronics circuits as

voltage references. Zener diodes provide a stable and defined voltage and as a result Zener diodecircuits are often used in power supplies when regulated outputs are needed. Zener diodes are

cheap and they are also easy to use and as a result they are used in many applications and manycircuits.

Zener diode basics

Zener diodes are sometimes referred to as reference diodes as they are able to provide a stable

reference voltage for many electronics circuits. The diodes themselves are cheap and plentiful

and can be purchased in virtually every electronics components store.

Zener diodes have many of the same basic properties of ordinary semiconductor diodes. They

conduct in the forward direction and have the same turn on voltage as ordinary diodes. For

silicon this is about 0.6 volts.

In the reverse direction, the operation of a Zener diode is quite different to an ordinary diode. Forlow voltages the diodes do not conduct as would be expected. However, once a certain voltage is

reached the diode "breaks down" and current flows. Looking at the curves for a Zener diode, it

can be seen that the voltage is almost constant regardless of the current carried. This means that aZener diode provides a stable and known reference voltage.

Zener diode markings

There are many styles of package for a Zener diode. Some are used for high levels of power

dissipation and others are contained within surface mount formats. For home construction, themost common type is contained within a small glass encapsulation. It has a band around one end

and this marks the cathode.

It can be seen that the band around the package corresponds to the line on the diode circuit

symbol and this can be an easy way of remembering which end is which. For a Zener diode

operating in its reverse bias condition the band is the more positive terminal in the circuit.

Zener diode markings, symbol and package outlines



How a Zener diode works

The Zener diode is particularly interesting in the way that it operates. There are actually two

mechanisms that can cause the breakdown effect that is used to provide the voltage reference

effect:

1. Zener breakdown: Although the physics behind the effect is quite complicated, it can beconsidered that this effect occurs when the electric field within the semiconductor crystal

lattice is sufficiently high to pull electrons out of the lattice to create a hole and electron.

The electron then moves under the influence of the field to provide an electric current.

2. Impaction ionisation: Again this effect occurs when there is a high level of electric

field. Electrons are strongly attracted and move towards the positive potential. In view of

the high electric field their velocity increases, and often these high energy electrons willcollide with the semiconductor lattice. When this occurs an electron may be released,

leaving a hole. This newly freed electron then moves towards the positive voltage and is

accelerated under the high electric field, and it to may collide with the lattice. The hole,being positively charged moves in the opposite direction to the electron. If the field is

sufficiently strong sufficient numbers of collisions occur so that an effect known as

avalanche breakdown occurs. This happens only when a specific field is exceeded, i.e.when a certain reverse voltage is exceeded for that diode, making it conduct in the

reverse direction for a given voltage, just what is required for a voltage reference diode.

It is found that of the two effects the Zener effect predominates above about 5.5 volts whereas

impact ionisation is the major effect below this voltage.

The two effects are affected by temperature variations. This means that the Zener diode voltage

may vary as the temperature changes. It is found that the impact ionisation and Zener effects

have temperature coefficient in opposite directions. As a result Zener diodes with reverse

voltages of around 5.5 volts where the two effects occur almost equally have the most stableoverall temperature coefficient as they tend to balance each other out for the optimum

performance.

Zener diode circuits

The most basic Zener diode circuit consist of a single Zener diode and a resistor. The Zener

diode provides the reference voltage, but a series resistor must be in place to limit the current

into the diode otherwise a large amount of current would flow through it and it could be

destroyed.

The value of the resistor should be calculated to give the required value of current for the supply

voltage used. Typically most low power leaded Zener diodes have a maximum power dissipationof 400 mW. Ideally the circuit should be designed to dissipate less than about half this value, but

to operate correctly the current into the Zener diode should not fall below about 5 mA or they do

not regulate correctly.



Basic Zener diode circuit

2. With the help of circuit diagram and wave forms, Explain the

operation of bridge Rectifier.

When four diodes are connected as shown in figure 1, the circuit is called a BRIDGE

RECTIFIER. The input to the circuit is applied to the diagonally opposite corners of the network,

and the output is taken from the remaining two corners.

Figure 1- Bridge rectifier.



One complete cycle of operation will be discussed to help you understand how this circuit works.

We have discussed transformers in previous modules in the NEETS series and will not go intotheir characteristics at this time. Let us assume the transformer is working properly and there is a

positive potential at point A and a negative potential at point B. The positive potential at point A

will forward bias D3 and reverse bias D4. The negative potential at point B will forward bias D1

and reverse bias D2. At this time D3 and D1 are forward biased and will allow current flow topass through them; D4 and D2 are reverse biased and will block current flow. The path for

current flow is from point B through D1, up through R L, through D3, through the secondary of

the transformer back to point B. This path is indicated by the solid arrows. Waveforms (1) and(2) can be observed across D1 and D3.

One-half cycle later the polarity across the secondary of the transformer reverses, forward

biasing D2 and D4 and reverse biasing D1 and D3. Current flow will now be from point A

through D4, up through RL, through D2, through the secondary of T1, and back to point A. This

path is indicated by the broken arrows. Waveforms (3) and (4) can be observed across D2 andD4. You should have noted that the current flow through RL is always in the same direction. In

flowing through RL this current develops a voltage corresponding to that shown in waveform (5).Since current flows through the load (RL) during both half cycles of the applied voltage, thisbridge rectifier is a full-wave rectifier.

One advantage of a bridge rectifier over a conventional full-wave rectifier is that with a giventransformer the bridge rectifier produces a voltage output that is nearly twice that of the

conventional full-wave circuit. This may be shown by assigning values to some of the

components shown in views A and B of figure 4-9. Assume that the same transformer is used in

both circuits. The peak voltage developed between points X and Y is 1000 volts in both circuits.In the conventional full-wave circuit shown in view A, the peak voltage from the center tap to

either X or Y is 500 volts. Since only one diode can conduct at any instant, the maximum voltage

that can be rectified at any instant is 500 volts. Therefore, the maximum voltage that appearsacross the load resistor is nearly - but never exceeds - 500 volts, as a result of the small voltage

drop across the diode. In the bridge rectifier shown in view B, the maximum voltage that can be

rectified is the full secondary voltage, which is 1000 volts. Therefore, the peak output voltage

across the load resistor is nearly 1000 volts. With both circuits using the same transformer, thebridge rectifier circuit produces a higher output voltage than the conventional full-wave rectifier

circuit.

Figure 2. CONVENTIONAL FULL-WAVE RECTIFIER



Figure 3. FULL-WAVE BRIDGE RECTIFIER

3. Explain the Process of Conduction in Transistors

Energy can be added to electrons by applying heat. When enough energy is absorbed by the

valence electrons, it is possible for them to break some of their covalent bonds. Once the bondsare broken, the electrons move to the conduction band where they are capable of supporting

electric current. When a voltage is applied to a crystal containing these conduction band

electrons, the electrons move through the crystal toward the applied voltage. This movement of electrons in a semiconductor is referred to as electron current flow. There is still another type of

current in a pure semiconductor. This current occurs when a covalent bond is broken and a

vacancy is left in the atom by the missing valence electron. This vacancy is commonly referred

to as a "hole." The hole is considered to have a positive charge because its atom is deficient byone electron, which causes the protons to outnumber the electrons. As a result of this hole, a

chain reaction begins when a nearby electron breaks its own covalent bond to fill the hole,

leaving another hole. Then another electron breaks its bond to fill the previous hole, leaving stillanother hole. Each time an electron in this process fills a hole, it enters into a covalent bond.

Even though an electron has moved from one covalent bond to another, the most important thing

to remember is that the hole is also moving. Therefore, since this process of conductionresembles the movement of holes rather than electrons, it is termed hole flow (short for hole

current flow or conduction by holes). Hole flow is very similar to electron flow except that the

holes move toward a negative potential and in an opposite direction to that of the electron. Since

hole flow results from the breaking of covalent bonds, which are at the valence band level, theelectrons associated with this type of conduction contain only valence band energy and must

remain in the valence band. However, the electrons associated with electron flow have

conduction band energy and can, therefore, move throughout the crystal. A good analogy of hole

flow is the movement of a hole through a tube filled with balls (figure 1).



Figure 1 - Analogy of hole flow.

When ball number 1 is removed from the tube, a hole is left. This hole is then filled by ballnumber 2, which leaves still another hole. Ball number 3 then moves into the hole left by ball

number 2. This causes still another hole to appear where ball 3 was. Notice the holes are moving

to the right side of the tube. This action continues until all the balls have moved one space to theleft in which time the hole moved eight spaces to the right and came to rest at the right-hand end

of the tube. In the theory just described, two current carriers were created by the breaking of

covalent bonds: the negative electron and the positive hole. These carriers are referred to as

electron-hole pairs. Since the semiconductor we have been discussing contains no impurities, thenumber of holes in the electron-hole pairs is always equal to the number of conduction electrons.

Another way of describing this condition where no impurities exist is by saying the

semiconductor is INTRINSIC. The term intrinsic is also used to distinguish the puresemiconductor that we have been working with from one containing impurities.

4. Explain Input and output characteristics of CC Configuration.

It is called the common-collector configuration because (ignoring the power supply battery) both

the signal source and the load share the collector lead as a common connection point as in Figurebelow.

http://www.allaboutcircuits.com/vol_3/chpt_4/6.html#03101.png




Common collector: Input is applied to base and collector. Output is from emitter-collector

circuit.

It should be apparent that the load resistor in the common-collector amplifier circuit receives

both the base and collector currents, being placed in series with the emitter. Since the emitter

lead of a transistor is the one handling the most current (the sum of base and collector currents,since base and collector currents always mesh together to form the emitter current), it would be

reasonable to presume that this amplifier will have a very large current gain. This presumption isindeed correct: the current gain for a common-collector amplifier is quite large, larger than anyother transistor amplifier configuration. However, this is not necessarily what sets it apart from

other amplifier designs.

Let's proceed immediately to a SPICE analysis of this amplifier circuit, and you will be able toimmediately see what is unique about this amplifier. The circuit is in Figure below. The netlist is

in Figure below.

Common collector amplifier for SPICE.







common-collectoramplifier

vin 1 0q1 2 1 3 mod1

v1 2 0 dc 15rload 3 0 5k

.model mod1 npn

.dc vin 0 5 0.2

.plot dc v(3,0)

.end

Common collector: Output equals input less a 0.7 V V BE drop.

Unlike the common-emitter amplifier from the previous section, the common-collector produces

an output voltage in direct rather than inverse proportion to the rising input voltage. See Figure

above. As the input voltage increases, so does the output voltage. Moreover, a close examinationreveals that the output voltage is nearly identical to the input voltage, lagging behind by about

0.7 volts.

This is the unique quality of the common-collector amplifier: an output voltage that is nearly

equal to the input voltage. Examined from the perspective of output voltage change for a givenamount of input voltage change, this amplifier has a voltage gain of almost exactly unity (1), or 0

dB. This holds true for transistors of any β value, and for load resistors of any resistance value.

It is simple to understand why the output voltage of a common-collector amplifier is alwaysnearly equal to the input voltage. Referring to the diode current source transistor model in Figure

below, we see that the base current must go through the base-emitter PN junction, which is

equivalent to a normal rectifying diode. If this junction is forward-biased (the transistorconducting current in either its active or saturated modes), it will have a voltage drop of

approximately 0.7 volts, assuming silicon construction. This 0.7 volt drop is largely irrespective

of the actual magnitude of base current; thus, we can regard it as being constant:







Emitter follower: Emitter voltage follows base voltage (less a 0.7 V V BE drop.)

Given the voltage polarities across the base-emitter PN junction and the load resistor, we see that

these must add together to equal the input voltage, in accordance with Kirchhoff's Voltage Law.In other words, the load voltage will always be about 0.7 volts less than the input voltage for all

conditions where the transistor is conducting. Cutoff occurs at input voltages below 0.7 volts,and saturation at input voltages in excess of battery (supply) voltage plus 0.7 volts.

Because of this behavior, the common-collector amplifier circuit is also known as the voltage-

follower or emitter-follower amplifier, because the emitter load voltages follow the input so

closely.Applying the common-collector circuit to the amplification of AC signals requires the same

input “biasing” used in the common-emitter circuit: a DC voltage must be added to the AC input

signal to keep the transistor in its active mode during the entire cycle. When this is done, the

result is the non-inverting amplifier in Figure below.

common-collector amplifier

vin 1 4 sin(0 1.5 2000 0 0)vbias 4 0 dc 2.3

q1 2 1 3 mod1

v1 2 0 dc 15

rload 3 0 5k .model mod1 npn

.tran .02m .78m

.plot tran v(1,0) v(3,0)

.end

Common collector (emitter-follower) amplifier.





The results of the SPICE simulation in Figure below show that the output follows the input. The

output is the same peak-to-peak amplitude as the input. Though, the DC level is shifteddownward by one VBE diode drop.

Common collector (emitter-follower): Output V3 follows input V1 less a 0.7 V VBE drop.

Here's another view of the circuit (Figure below) with oscilloscopes connected to several points

of interest.

Common collector non-inverting voltage gain is 1.

Since this amplifier configuration doesn't provide any voltage gain (in fact, in practice it actually

has a voltage gain of slightly less than 1), its only amplifying factor is current. The common-

emitter amplifier configuration examined in the previous section had a current gain equal to the βof the transistor, being that the input current went through the base and the output (load) current

went through the collector, and β by definition is the ratio between the collector and basecurrents. In the common-collector configuration, though, the load is situated in series with the

emitter, and thus its current is equal to the emitter current. With the emitter carrying collectorcurrent and base current, the load in this type of amplifier has all the current of the collector

running through it plus the input current of the base. This yields a current gain of β plus 1:







Once again, PNP transistors are just as valid to use in the common-collector configuration as

NPN transistors. The gain calculations are all the same, as is the non-inverting of the amplified

signal. The only difference is in voltage polarities and current directions shown in Figure below.

5. List the Differences between JFET and BJT?

S.No JFET BJT

1 It is an unipolar device It is a bipolar device

2 Voltage controlled device Current controlled device

3 Less noise More noise

4 High switching speed Low switching speed

5 Not possible to thermal runaway Possible to thermal runaway

6 Simple fabrication Difficult fabrication





7Source and drain terminlas areinterchangeable

Emitter and collector terminals are notinterchangeable

8 High input impedance Low input impedance

9 High power gain Low power gain

10Generally the input terminal is reverse

biased.Generally the input terminal is forward biased.

6. How FET is used as Amplifier?

The weak signal is applied between gate and source and the amplified output is obtained in

drain-source circuit. The input circuit is always reversed biased. A small change in the reversebias on the gate produces a large change in drain current. The large variation in drain current

produces large output across the load RL and so FET acts as an amplifier.

Output Characteristics Of Fet

The curve drawn between ID and VDS of a FET at constant VGS is known as output or staticcharacteristic of FET.

Let us first consider the characteristic for VGS = 0 (the gate being shorted with source).

When VDS = 0, there is no attracting potential at the drain and, therefore, ID = 0, although thechannel between the gates is fully open as VGS = 0. With the increase n VDS, ID increaseslinearly up to knee point i.e. the FET behaves as an ordinary resistor till knee point, at point A, is

reached.

With the increase in ID, gate junctions are reversing biased due to ohmic voltage drop in the

semiconductor material of channel and as a result the channel region begins to constrict. With thefurther increase in VDS, ID increases at reverse square law rate up to point B, called the pinch-

off point. The voltage corresponding to this point B is called the pinch-off voltage and is denoted



by VP. At this voltage the channel is more or less blocked. The worth noting point is that pinch-

off does not mean ID cut-off. Moreover the channel is not completely closed and so ID does notreduce to zero. With the further increase in VDS (beyond pinch-off voltage), the channel

resistance increases in such a way that ID practically remains constant up to point C. the region

BC is called saturation region or amplifier region. In this region the FET operates as a constant

current device. With constant increase of VDS corresponding to point C, called the avalanchebreakdown voltage VA, eventually breakdown across the gate junction takes place and current

ID shoots up to a high value.

(a)

(b)

Fig. 5.73

Fig. 5.73 (b) shows a family of ID versus VDS curves for different values of VGS. It is seen,

from fig. 5.73 (b), that the ID-VDS curves drawn for different values of VGS are similar to that

one for VGS = 0 except the following points.(i) ohmic region of portion reduces,

(ii) Maximum saturation drain current is smaller and



(iii) Avalanche breakdown occurs at progressively lower values of VDS because reverse bias

gate voltage adds to the drain voltage thereby increases the voltage effective across the gate junctions.

Transfer Characteristic

The curve drawn between ID and VGS for a constant value of VDS is called the transfer

characteristic .