## ^Work

^Work

1. A force is said to be doing work on a system if it displaces the system from its initial position on which it acts.
2. Work is not defined at a position or at an instant. It is defined for some displacement only.
3. Work can be +ve, – ve or zero.

## ^Motion of a body on a fixed rough horizontal surface

^Motion of a body on a fixed rough horizontal surface Suppose a block of mass ‘m’ placed on a rough horizontal plane is pulled by a horizontal force F.

Following are the various possibilities:

1. If F = 0, then the frictional force acting on the block will be zero & the block will remain stationary.
2. If F < fL, then the frictional force acting on the block will be equal to F & the block again will remain stationary.
3. If F = fL, then the frictional force acting on the block will be equal to fL & the block will be at the verge of motion i.e. about to move.
4. If F > fL, then the frictional force acting on the block will be equal to fK & the block will move with acceleration   a= f- fk /m   in the direction of force applied F.

## ^Logic Gates

### ^Logic Gates

Basic building blocks of digital electronics made of semiconducting material & used to control the flow of information from input to output in a logical manner & are used in calculators, digital watches, computers, robots, industrial control systems, and in telecommunications. ## ^SCs Versus VT devices

### ^SCs Versus VT devices

Semiconducting devices are the basic building blocks of all the modern electronic circuits & have following advantages over conventional vacuum tubes:

1. don’ require a heating battery & thus set in to operation as soon as the circuit is switched on.
2. require comparatively low voltage for their operation.
3. don’t produce any humming noise during their operation.
4. shock proof, small & compact in size, cheaper than vacuum tubes, have a very long life & are free from vacuum deterioration trouble.

### Limitations

1. higher noise level.
2. can’t handle as much as power as ordinary tubes can.
3. poor response in high frequency range.
4. temperature sensitive (maxi. tolerance 50 0C).

## ^Oscillator

^Oscillator

An oscillator is self sustained transistor amplifier with a positive feedback which produce electric oscillation of constant frequency  & amplitude without requiring any external input signal. It converts dc energy obtained from a battery into ac energy in same oscillatory circuit. ## ^CEA

#### Output is 1800 out of phase with input. Transconductance is also called mutual conductance.

As β > > α, thus the ac power gain of a CEA is much larger than that of a CBA. Remember the transistor is not generating any power. The energy for the higher ac power at the output is supplied by the battery.

Saturation state:

Both the junction are forward biased & here IC is maximum & does not depend on the input current IB. Cut off state:  Both the junctions are reverse biased as a result IC = 0. Between cut off & saturation state a transistor works as switch as here it turns over rapidly from OFF state (i.e. IC = 0 or cut off) to the ON state (i.e. IC is maximum or saturation state).

Active state: Emitter base junction is forward biased and the collector base junction is reverse biased. A transistor works as an audio amplifier in this regions.

Relation between α and β: As the value of IB is about 1 – 5 % of IE or IC is 95 – 99 % of IE, α is about 0.95 and 0.99 and β is about 20 to 100.  It is found that α and β are independent of current if the emitter base junction is forward biased and the collector base junction is reverse biased. Also the above definitions of α and β do not hold when both the junctions of a transistor are forward biased or reverse biased.

The CE configuration is frequently used as it gives high current gain as well as voltage gain.

## ^CBA

##### Output is in phase input ac current gain (α ac): ## ^Transistor

^Transistor

1. Emitter is the section on one side of transistor that supplies charge carriers. It is of moderate size & heavily doped. Collector is the section on the other side of transistor that collects the charge carriers. It is moderately doped but larger in size. Base is the middle section of transistor that forms two pn junctions with emitter and collector. It is very thin and lightly doped so as to pass most of the emitter injected charge carriers to the collector.
2. A transistor is also called Bipolar Junction Transistor (BJT), as in it current is due to both majority & minority charge carriers.
3. Input of a transistor is always forward biased, output is reverse biased & the common terminal is grounded.
4. Current in both npn & pnp transistor is IE = IB + IC. As the base current IB is very small thus IE » IC.. A transistor transfer almost the same electric current from low resistance path to high resistance path and hence is named as transistor. This transistor action makes it useful in transistor amplifier.
5. A transistor can be used as a switch, amplifier, oscillator & NOT gate.

## ^Zener diode

^Zener diode

It is a specially designed junction diode, which can operate in the reverse breakdown voltage region continuously without being damaged.  In forward bias behaves like ordinary diode.  Zener diode does not gets damaged at breakdown voltage, but it does so at some higher reverse voltage, known as its ‘burn out value’. The magnitude of zener voltage VZ can be decreased by increasing doping level in p and n type materials of zener diode.

I – V characteristics of a Zener diode are as shown. For a voltage more than zener breakdown voltage current through diode increases without increase in voltage across it. This feature is exploited to provide voltage stabilization across a circuit. Suppose a load RL connected between a & b is connected to a fluctuating dc voltage supply Vs between c & d. Let the load can’t tolerate a voltage VL but the supply Vs can be more than VL. To protect the load from any increase voltage a zener diode of breaking voltage VZ = VL is connected to the input supply with a safety resistor RS in the input loop.

If at any time VZ increases than VL the zener diode works in the breakdown region increasing the current through it without increasing the voltage across it & thus the load always remains protected. ## ^Photodiodes

^Photodiodes

Light incident near the junction of reverse biased (because the fractional change due to the photo-effects on the minority carrier dominated reverse bias current is more easily measurable than the fractional change in the forward bias current) diode produce electron hole pairs which are moved by the junction electric field (electrons towards n end & holes near the p end).  Thus p-side becomes positive and n-side becomes negative giving rise to emf. When an external load is connected, a current flows which can be controlled by changing the intensity of light falling on it. It is used in, light operated switches, optical counters, reproduction of sound from film in move projectors & in photo detectors to detect radiations.

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