Transistors Configuration And Application


Computers have brought a massive breakthrough in the world in almost every field today. However, this would not be possible if it was not for transistors that came to replace the vacuum tubes that were used before.  Vacuum tubes were inefficient, and they consumed a lot of power when compared to the transistors. In this post, we shall dig deep into transistors configuration and application

Transistors were developed from diodes which were used to allow current to flow in one direction and not the other.  Combining two diodes produced a transistor.  There are several types of transistors today. Other than that, transistors are used depending on their characteristics. Transistors are configured in different ways.

One category include those that are activated by the current. the second category is made up of those  activated by voltage. It is because of transistors that people have managed to create registers, microchips, CPU, and controllers. transistors are attributed to miniaturization of devices such as the small form factor computers.

How it works
Transistors configuration and application
NPN type transistor

before we cover fully Transistors Configuration And Application we shall look at how they work.  Semi-conductor materials are used to make transistors. The materials do not conduct electricity on their own, but an electron or a proton has to be added so that it can conduct electricity. Electrons or protons are added through a process called doping. It was said earlier that transistors were created from a diode. Diodes, on the other hand, are created by joining two semiconductors that have been doped differently. One of the semiconductors has an extra electron added while the other part has a proton added.  Transistors are made by having either a proton doped semiconductor sandwiched between two electron-doped semiconductors or vise versa.

They are three terminal device. For a bipolar type of transistors, there are two categories; NPN and PNP type.  Diodes are of two categories, n-type and p-type. This depends on what is used to trigger the current flow. In the N-type diode, current is triggered by electron flow while in the p-type, the current flow is triggered by the movement of holes.

One of the terminals is called base labeled b, the second one emitter labeled e and the other one collector labeled c. among the many application of a transistor is to serve as a switch. In that case, one terminal is connected to the collector and the other to the emitter.  It is always in open mode until the base is connected to a source of current. The transmissivity of the transistor is directly proportional to the size of the current where the increase in current leads to a proportional increase in the transitivity. Other than a switch, transistors are used in amplification.

Operational Modes

Transistors are not linear devices such as the resistors as far as the relationship between the voltage and current is concerned. It, therefore, adopts four operational modes which are saturation, reverse active, cut-off and active mode. in  Saturation mode, the current just passes through without any resistance; it acts as a short circuit. in Reverse-active the current is proportional to the current passing through the base. However, the current flows in reverse. In the active mode, the current flows in the right direction, but it is proportional to a current flowing through the  Cut-off mode, it serves as an open switch where the current cannot pass through.

In order to determine the mode which, the transistor is operating on, one has to examine the relationship between the current in the three pins.  The voltage from base to collector and from the base to emitter usually determine the mode which the transistor is on.


There are only three ways to connect a transistor in a circuit.  One of the terminals is common to the input and the output.  Each method has a different response to the input signal because they behave different depending on the circuit arrangement.  The common configuration is common to the base, common emitter, and common collector configuration.

In the common base configuration, the base is common to the input and the output signal.  The input signal is connected to the base and the emitter terminals while the output is taken from between the base and the collector. The base is grounded or sometimes connected to reference voltage sources.

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The input signal is a large current since it is the sum of base and collector current. There is a current gain since the output current is less than the emitter current. It can be simply said that the input signal is attenuated.

For this type of amplification, the voltage in and voltage out at in phase. Due to the high voltage gain, this type of amplification is not very common.

 Common emitter (CE) configuration

The emitter is grounded where the input is applied between the emitter and the base. The output is collected between the emitter and the collector. This the “normal” connection method of bipolar transistors. It is capable of producing the highest current power gain of all the configuration method. The input impedance is very low due to the P-N junction forward biasing.

The current in the input signal and the output signal are equal to the emitter current.

Ie= Ic+ Ib.

The ratio of Ic/Ie is large and so is the current gain of the common emitter because the load resistance is in series.  This ratio is called alpha. For this type of configuration Ic/Ie is less than 1. The ratio Ic/Ib is called beta. This ratio is usually between 20 and 200.  It means that for every single electron from the base there are 20 electrons flowing between the collector and emitter if the ratio is 20.  Therefore, the following equations are true about the alpha and beta.

alpha=  Beta/(Beta+1)

Ic= alpha.Ie= βeta.Ib

Where Ie, Ib, and Ic are the current flowing into emitter, base and collector terminals respectively. High input impedance is a feature of this configuration, but there is low voltage gain.

Common collector configuration (cc)

The collector is grounded where the input current is connected to the base while the output is taken from the emitter. This configuration is called an emitter follower or voltage follower. There is high input impedance for this type of configuration in the input and very low impedance on the output. The voltage gain is usually less than 1.

Application of transistors
Computers and calculators

The digital signals are usually ones and zeros that are created using logic gates. The logic gates are created when several transistors are connected.  Different input signals are compared, and then an output signal is given depending on the configuration. Using the Boolean algebra, several logic gates have been created which are OR, NAND, NOT, NOR, and XOR gates. Through this, these computers are programmed through a set of instructions.

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 As amplifiers

They are used to amplify a small current into a large current. Base current is usually amplified when it is outputted in the collector terminal as a large current. This is how a microphone amplifies sound. The voice causes the diagram to vibrate creating a small current that is outputted after amplification. A small change in the base current has a corresponding large change in the collector current. The input sound and the output sound are of the same frequency but different amplitude.

As switches

Transistors are used as switches.  These switches  are used to light the florescent tubes in houses. Other than that, they are also used in the electronics a switch since they rarely fail like the normal on and off switches used today. Their advantage is that no may need to activate them manually, a current or a voltage is what they require to function.


Transistors are the reason why electronics work as they are today. They did well to render the vacuum tubes useless within years of their discovery.  It is a common knowledge that first computers were as big as a house, but today they fit in our bags and our pocket.

A smartphone, for example, has millions of transistors in their processor. It is this attribute that makes them highly  effective and  faster than the fastest computers in the year 2000.

Transistors have not only made electronics smaller, but also fasters and more energy efficient.  Engineers  believe that with time, transistors will be made even smaller. Smaller transistors will be such that billions of them will be fit in the CPU for higher speed and better performances in devices.

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