Biasing and Working of PNP Transistor

For the normal operation of a transistor, it is necessary to provide voltage of right polarities to both of its junctions.

For the accurate and normal function of a transistor, its emitter-base junction should be forward biased and collector-base junction should be always reverse biased. As shown in the figure.

In the given method of biasing of PNP transistor, the emitter-base junction is forward biased, while collector-base junction is reverse biased.

Due to forward biasing of emitter-base junction, the majority carriers (holes) flow from emitter towards base. Thus, a big amount of holes crosses the junction and enters base. As the holes enter the base, they combine with electrons present there, (This process known as recombination).Since the base is lightly doped, so the recombination of holes and electrons in base is very low, (Almost 2% to 5%). Thus 95 to 98 percent of holes that came from emitter could not recombine with electrons.

Since the collector-base junction is reverse biased, so holes cannot flow from collector to base due to high resistance. But holes start to flow in the opposite direction (i.e. base to collector) because collector voltage attracts almost 95% holes present in base region. Thus the holes provided by emitter, travelling through base, go to collector-base junction and conventional current starts to flow in PNP transistor.

This should be kept in mind that collector current and emitter current is almost equal but base current is very low. 

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NPN Transistor

PNP Transistor

Biasing and Working of NPN Transistor 

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PNP Transistor

A transistor in which an N-type semiconductor is sandwiched between tow P-type semiconductors, is known as PNP transistor. (Shown in Figure)

In the p-region, majority carriers are holes or protons, and in the n-region, majority carriers are electrons.

Symbol of PNP Transistor

As conventional current flows in the transistors, so charge carriers start moving from emitter and they are collected at collector.

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Biasing and Working of PNP Transistor

Biasing and Working of NPN Transistor

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Biasing and Working of NPN Transistors

Due to forward biasing of emitter-base junction, majority carriers (Electrons) flow towards base. A big amount of electrons cross the junction and enter the base. As the base is lightly doped, so a small amount (2 to 5 percent) of electrons recombine with holes in the base and almost more than 95 percent electrons come under the effect of positively charged collector and enter the collector. This is possible due to attraction of collector voltage.

As collector-base junction is reverse biased, so electrons cannot flow from collector to base due to high resistance, infect, electrons always flow from base to collector.

 Thus due to diffusion and collection of electrons (provided by emitter) in the collector-base junction, the flow of electronic current starts. The direction shown in figure shows the flow of conventional current.

This should be kept in mind that the transistor will not conduct unless its emitter-base junction is forward biased.

Figures,

Biasing of NPN Transistor

Biasing of NPN Transistor (Symbol, Circuit Diagram)

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Biasing and Working of PNP Transistor

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Transistors

Definition:
"A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit." 
A voltage or current applied to one pair of the transistor's terminals changes the current flowing through another pair of terminals. Because the controlled (output) power can be much more than the controlling (input) power, a transistor can amplify a signal.
The transistor is the fundamental building block of modern electronic devices.

Usage:
The bipolar junction transistor (BJT) was the most commonly used transistor in the 1960s and 70s. Even after MOSFETs became widely available, the BJT remained the transistor of choice for many analog circuits such as simple amplifiers because of their greater linearity and ease of manufacture. Desirable properties of MOSFETs, such as their utility in low-power devices, usually in the CMOS configuration, allowed them to capture nearly all market share for digital circuits; more recently MOSFETs have captured most analog and power applications as well, including modern clocked analog circuits, voltage regulators, amplifiers, power transmitters and motor drivers.
Most common uses of transistors are:

1.Transistor as a switch

2. Transistor as an Amplifier

Types:

Transistors are categorized by

• Semiconductor material: graphene, germanium, silicon, gallium arsenide, silicon carbide, etc.
• Structure: BJT, JFET, IGFET (MOSFET), IGBT, "other types"
• Polarity: NPN, PNP (BJTs); N-channel, P-channel (FETs)
• Maximum power rating: low, medium, high
• Maximum operating frequency: low, medium, high, radio frequency (RF),microwave (The maximum effective frequency of a transistor is denoted by the term f_T, an abbreviation for "frequency of transition". The frequency of transition is the frequency at which the transistor yields unity gain).
• Application: switch, general purpose, audio, high voltage, super-beta, matched pair
• Physical packaging: through hole metal, through hole plastic, surface mount,ball grid array, power modules
• Amplification factor h_fe (transistor beta)

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NPN Transistor

Biasing and Working of PNP Transistor

Biasing and Working of NPN Transistor

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NPN Transistors

Definition:

A transistor in which a P-type semiconductor is sandwiched between tow N-type semiconductors, is known as NPN transistor.

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor (the "base") between two N-doped layers. (As shown in the Diagram)

NPN Transistor

In this type of transistor, a small amount of current flows because P-region(Base) has a small amount of charge carriers.

The holes move from base towards emitter, they cannot go towards collector because collector has smaller amount of charge carriers as compared to emitter.

Symbol of NPN (Circuit Diagram)

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Biasing and Working of PNP Transistor

Biasing and Working of NPN Transistor

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Half wave rectifier

A device that converts one polarity (positive or negative) of input AC into output DC is known as Half-wave rectifier. OR

A device that converts half-cycle of input AC into output DC is known as Half-wave Rectifier.

As shown in the figure below.

Half Wave Rectifier (Circuit Diagram)

When the positive half cycle of input AC wave comes, than the diode is forward biased and switched into ON condition, So it conducts and positive half-cycle of input AC is dropped across RL.

When the negative half cycle of input AC comes, the diode is reverse biased and switched to OFF condition. Thus the diode doesn’t conduct and no voltage is dropped across RL. So negative half cycle is skipped.

So, the output is not a steady DC but a pulsating DC, having a specific frequency equal to that of the input voltage frequency.

Since only half cycle of input wave is used, it is called half wave rectifier.

If it is required to step up or step down the input voltage, we will have to use a power transformer as shown in figure, below.

Power Transformer (Step-up action)

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Full wave rectifier (Definition and Working)

A device that converts both polarities (positive and negative) of input AC into output DC is know as Full Wave Rectifier. OR

A rectifier that converts complete cycle of input AC into output DC is known as Full-wave Rectifier.

Full Wave Rectifier (Circuit Diagram and Output)

 When the positive half cycle comes, then upper end ‘A’ of secondary winding becomes positive and lower end ‘B’ becomes negative while central point ‘G’ remains at zero potential. In this condition the diode D1 is forward biased and conducts, while the diode D2 is reversed biased and switched to OFF condition and doesn't conduct.

Conventional current travels from point ‘A’ through diode D1 and reaches center tap point ‘G’ of transformer, Hence during positive half cycle D1 is ON and D2 remains OFF. (As shown in figure1)

Figure 1 (Output of Diode D1)

 When the negative half cycle comes, then upper point ‘A’ of secondary winding becomes negative and lower point ‘B’ becomes positive while central point ‘G’ remains at zero potentioal. In this condition, the diode D2 conducts because it is forward biased and the diode D1 is reverse biased and doesn’t conduct.

Conventional current travels from point ‘B’ towards load and reaches the center taps point ‘G’ of secondary winding. Hence, during negative half cycle D1 is Off and D2 is ON. (As shown in figure 2).

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Bridge Rectification

It is use the most frequently used circuit in electronic DC power supplies. It converts the full cycle of AC in to DC.

Four diodes and a transformer are used in it. Since, the whole secondary winding of transformer is used in this circuit, so it produces double volts than full wave centre tapped rectifier.

Working Principle

The input of transformer is provided an AC supply.

When the input gets a positive half cycle, the upper end ‘A’ of secondary winding becomes positive and lower end ‘B’ becomes negative. In this condition the diodes D1 and D3 become forward biased and switch to ON condition. And the diodes D2 and D4 are reversed biased and switched to OFF condition. The conventional current starts from point ‘A’ and travels through D1 load and D3 to complete its circuit.( As shown in figure 1).

Figure 1

When the input gets negative half cycle then point ‘A’ becomes negative and point ‘B’ becomes positive. In this condition, the diodes D2 & D4 are forward biased and switched to ON condition. While the diodes D1 & D3 become reverse biased and switched to OFF condition. The current starts from point ‘B’ travels through D2 , load and D4 to complete its circuit. (Shown in figure 2).

Figure 2

                                     

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