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Explain the operation of PNP and NPN transistors.
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PNP Transistor Working 

 The circuit connection of PNP transistor with supply voltages is given below. Here the base terminal has negative bias with respect to emitter and the emitter terminal has positive bias voltage with respect to both base and collector because of PNP transistor.


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 The polarities and current directions are reversed here compared to NPN transistor. If the transistor is connected to all the voltage sources as shown above then the base current flows through the transistor but here the base voltage needs to be more negative with respect to the emitter to operate transistor.  Here the base- emitter junction acts as a diode. The small amount of current in the base controls the flowing of large current through emitter to collector region. The base voltage is generally 0.7V for Si and 0.3V for Germanium devices.  Here the base terminal acts as input and the emitter- collector region acts as output. The supply voltage VCC is connected to the emitter terminal and a load resistor (RL) is connected to the collector terminal.  This load resistor (RL) is used to limits the maximum current flow through the device. One more resistor (RB) is connected to the base terminal which is used to limit the maximum current flow through the base terminal and also a negative voltage is applied to the base terminal.  Here the collector current is always equal to the subtraction of base current from emitter current. Like NPN transistor, the PNP transistor also has the current gain value β. Now let us see the relation between the currents and current gain β.


The collector current (IC) is given by,

IC = IE – IB

The DC current gain (β) for the PNP transistor is same as the NPN transistor.

DC current gain = β = Output current/Input current

Here output current is collector current and input current is base current.

β = IC/IB

From this equation we get,

IB = IC/β

IC = β IB

And also we define the current gain as,

Current gain = Collector current/ Emitter current (In common base transistor)

α = IC/IE

The relation between α and β is given by,

image

The collector current in PNP transistor is given by,

IC = – α IE + ICBO where ICBO is the saturation current.

Since IE = -(IC + IB)

IC = – α (-(IC + IB)) + ICBO

IC – α IC = α IB + ICBO

IC (1- α) = α IB + ICBO

IC = (α/ (1- α)) IB + ICBO/ (1- α)

Since β = α / (1- α)

Now we get the equation for collector current

IC = β IB + (1+ β) ICBO


 The output characteristics of PNP transistor are same as NPN transistor characteristics. The small difference is that the PNP transistor characteristic curve rotates 1800 to calculate the reverse polarity voltages and current values. The dynamic load line also exists on the characteristic curve to calculate the Q-point value. The PNP transistors are also used in switching and amplifying circuits like NPN transistors.

NPN TRANSISTOR WORKING: 
 The NPN transistor is a bipolar junction transistor, In an NPN transistor, the first letter N indicates a negatively charged layer of material and a P indicates a positively charged layer. These transistors have a positive layer, which is located in-between two negative layers.  NPN transistors are generally used in circuits for switching, amplifying the electrical signals that pass through them. These transistors comprise three terminals namely, base, collector and emitter and these terminals connect the transistor to the circuit board.  When the current flows through the NPN transistor, the transistor base terminal receives the electrical signal, the collector makes a stronger electric current than the one passing through the base, and the emitter passes this stronger current on to the rest of the circuit. In this transistor, the current flows through the collector terminal to the emitter.
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 Generally, this transistor is used because it is so easy to produce. For an NPN transistor to work properly, it needs to be formed from a semiconductor material, which carries some electric current, but not the maximum amount as very conductive materials like metal. “Si” is one of the most commonly used semiconductor, and NPN transistors are the easiest transistors to make out of silicon.  The application of an NPN transistor is on a computer circuit board. Computers need all their information to be translated into binary code, and this process is accomplished through a plethora of small switches flipping on and off on the computers circuit boards. NPN transistors can be used for these switches. A powerful electric signal turns the switch on, whereas a lack of a signal turns the switch off.
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