Transistor Operation

1.) Transistor construction
NPN transistor.

The transistor consists of two PN junctions to form three regions. And, these three regions of the transistors are called:

- Base, very thin and lightly doped, thus less mobile charge carriers.

- Emitter, highly doped, thus more mobile charge carriers.

- Collectors, largest region, and is less doped than the emitter.

PNP Transistor.

2.) Transistor Biasing

The setting up of dc voltage properly onto a transistor is called biasing.

For proper and correct biasing, please refer to the picture below,

- The emitter-base (or base-emitter) must be forward-biased.

- The collector-base must be reversed-biased.

Free electrons in the emitter region move to the base because the emitter-base is forward-biased. Because the base is deliberately made very thin and is very lightly doped. Most of these free electrons do not combine.

These free electrons diffused into the collector-base depletion where they are swept across the junction into the collector and are attracted by the higher positive potential (voltage) of collector supply.

Almost all these free electrons that enter the base, will pass the collector.

3.) Bipolar Transistor

For transistor, the charge carriers are:
- Electrons
- Holes

Since the conduction of a transistor is by electrons, as well as holes. a transistor is often called the bipolar transistor (BJT).

The two junctions of a bipolar transistor are known as:
- BE junction
- BC junction

Electrostatic Discharge (ESD)

1.) Electrostatic
It is the study of electrical charges at rest.

A charged object is created by the separation of charges:

- An atom is electrically neutral; it has the same number of protons (positive charges) as it does electrons (negative charges).

- Objects are charged by adding or removing electrons.

- A positive charge occurs when there are fewer electrons than protons; its classical definition is the charge accumulated by a glass rod rubbed with silk or wool.

- A negative charge occurs when there are more electrons than protons; its classical definition is the charge accumulated by a hard, rubber rod rubbed with fur.

2.) How is static charge generated?
Static charges are produced whenever there is friction between 2 materials or between human and material, for example touching or rubbing material, touching a plastic bag, clothing, foam coffee cup, or vinyl traveler. Demonstrate generation of static charges using a comb, small pieces of paper, a mylar  tape or balloon.

3.) What is Electrostatic Discharge?
ESD is a sudden discharge of the stored up of static charge. This sudden discharge can cause rupture on material inside devices. ESD sensitive devices therefore can be damaged when in contact with skin.

4.) What are ESD sensitive components?
Class 1 (0kV to 1kV)
- MOS IC
- VLS IC
- Linear IC, voltage regulators
- MOS capacitor
- FET, SCR
- Microwave, VHF transistors
- Thin film resistors

Class 2 (1kV to 4kV)
- MOS IC (CMOS)
- High speed bipolar logic IC
- Monolithic ceramic
- Schottky deiode

Class 3 (4kV to 15 kV)
- Small signal diodes
- Transistor
- Low speed bipolar TTL, DTL
- Quartz piezo-electric crystals

5.) ESD Safety Devices
The following safety precautions are used to prevent damages by ESD:

Handle static sensitive devices and PCB assemblies only at an anti-static workstation.

Discharge human body by wrist or heel straps.

Keep away static generators such as common plastic, tapes, and static generating tools away from work stations.

Use ionizers to reduce charge.

Store and transport sensitive devices in ESD protective packaging, (e.g. anti-static/conductive magazines, conductive bags/tote boxes) which are labeled clearly with a 'CAUTION" message.

Use treated plastics.

6.) Ionisers

An ioniser is a device that is used to clean the air of dust particles.

The ionisers uses a minute amount of electricity to generate billions and billions of negative ions (charged air molecules). These negative ions shoot down dirt particles that are circulating in the air by electrostatic action.

7.) How Static electricity is generated?

Examples:
- Pulling a tape from a reel.
- Clothing rubbing against a chair.
- Walking across a carpeted floow.
- Separating.rubbing 2 foils.
- Combing your hair.

Zener Diode Applications

1.) Zener Diode Application

It is commonly used as voltage regulator in DC power supplies.

Two types of regulation are:
- Line regulation
- Load regulation

2.) Line regulation

The zener diode regulates a varying input power supply as shown below.

Given: Izk = 0.25mA to Izm = 100mA.

Vin = Vr + Vz

For minimum current, voltage across 220 Ω:

Vr = IzkR = (0.25mA)(220Ω)

= 55mV

Therefore:

Vin = Vr + Vz

= 55mV + 10V

= 10.055V

For maximum current, voltage across 220Ω:

Vr = IzmR = (100mA)(200W)

= 22V

Therefore:

Vin = 22V + 10V

=32V

The current limiting resistor is included to prevent the zener from conducting too much and overheated.

Notice that the diode is reverse biased. The output voltage (across the zener doiode) is maintained constant in this manner.

3.) Load Regulation

A zener regulator with variable load resistor. A zener diode maintains constant voltage across RL as long as the zener current is greater than Izk and less than Izm.

At no load, IL = 0 & the total current, IT will flow through the zener diode.

When RL is connected, IT = Iz + IL.

When Iz reaches its minimum, Izk, load current is at its maximum.

Zener Diodes Principles

1.) Zener Diode

The zener diode is a special type of diode - for it is designed to operate at reversed biased condition.

2.) Characteristic of the Zener Diode

Zener diode is specially designed to operate at its reversed breakdown voltage region. This voltage is known as the Zener voltage, Vz.

At this region, a change in zener diode current will cause only a small change in the zener voltage. The zener voltage is reasonably stable.

3.) Zener Breakdown

Two types of reverse breakdown are avalanche and zener breakdown.

Avalanch breakdown occurs at a sufficiently high reverse voltage while Zener breakdown occurs at low reverse voltages.

Zener breakdown occurs at low reverse voltages.

4.) Breakdown Characteristic

As reverse voltage is increased, zener begins to breakdown at Izk.

A minimum value of reverse current, Izk, must be maintained to keep the diode in breakdown.

Maximum current, Izm, must not be exceeded otherwise the diode will be damaged.

Rectifier Calculations

1.) Calculations

-Full-wave Rectifier:

 

The average value of DC output voltage at the rectifier output

For full-wave rectifier: Vavg = 2Vp / π

For silicon diode: Vout = Vmax - 0.7V
= Vp

-Bridge Rectifier:

 

The average value of DC output voltage at the rectifier output

For bridge rectifier: Vavg = 2Vp/π

For silicon diodes (2 diodes) = Vout = Vmax - 0.7V - 0.7V

=V max - 1.4V 

=Vp

2.) Diode Rating
Diodes are generally reated by its voltage , current or power.

3.) Peak Inverse Voltage
It is defined as the maximum reverse bias voltage that can be applied to a doide without the diode breaking down.

The PIV across each reverse-biased diode = Vp + 0.7V

4.) Maximum forward Current Rating

It is referred to the maimum current that is allowed to flow through the diode.

5.) Ripple Voltage 

It is the AC variation of the DC output voltage after filtering.