Transistor

What is transistor?

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Simply you have to just remember 3 or 4 lines only

1^st transistor is a semiconductor device, yes, it is a device not a component.

2^nd transistor is capable of amplifying the electronic signals.

3^rd transistor acts like a switch.

These three lines are capable of little introduction of transistor

Or simply we can say that it is the combination of two diodes.

Ok,

Now,

Come on its types & construction which is very important to understand.

According to construction there are basically two types of transistor.

NPN and PNP

It is very simple to understand.

In PNP transistor

It used 2 p type semiconductor and 1 n type semiconductor

Definition

Those transistors which have two p type and one n type semiconductors, the n type semiconductor is sandwiches between two layers of p type semiconductor is called pnp type transistor.

&

In NPN transistor

It used 2 n type semiconductor and 1 p type semiconductor

Definition

Those transistors which have two n type and one p type semiconductors, the p type semiconductor is sandwiches between two layers of n type semiconductor is called NPN type transistor.

In its construction we can see that it has 3 terminals named as emitter (E), base (B) & collector (C).

From emitter base junction we give input signal and from collector base junction we take output of amplified signal.

In circuit symbol one thing you always take care, the arrow mark on emitter line, this arrow mark always inward the base side in PNP, and outward from the base in NPN transistors. This arrow shows the direction of flow of current in transistor.

After that, we always have problem of polarity in input and output.

So you always remember that      P side positive supply

                    &            N side negative supply

In circuit symbol you can see that which side have positive and negative.

These transistors also known as BJT (Bipolar Junction Transistor)

According to name it is clear that it forms more than one junction.

I think you should clear that what is junction. Junction is a place or point of meeting.

So can see in the circuit symbol it has three junctions EB (emitter base junction), CB (collector base junction) & EC (emitter collector junction), it has three junctions but always between the two poles so it is called BIPOLAR JUNCTION TRANSISTOR.

I think it’s sufficient for basic concept.

Now,

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible ways to connect it within an electronic circuit with one terminal being common to both the input and output.

· Common Base Configuration – has Voltage Gain but no Current Gain.

· Common Emitter Configuration – has both Current and Voltage Gain.

· Common Collector Configuration – has Current Gain but no Voltage                                      Gain.

Hear, amplification = gain

1st 

Common base configuration

As its name suggests, in the Common Base or grounded base configuration, the BASE connection is common to both the input signal AND the output signal. The input signal is applied between the transistors base and the emitter terminals, while the corresponding output signal is taken from between the base and the collector terminals .

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signal voltages Vin and Vout are “in-phase”. This type of transistor arrangement is not very common due to its unusually high voltage gain characteristics.

Also this type of bipolar transistor configuration has a high ratio of output to input resistance or more importantly “load” resistance ( RL ) to “input” resistance ( Rin ) giving it a value of “Resistance Gain”. Then the voltage gain ( Av ) for a common base configuration is therefore given as:

Common Base Voltage Gain

Where: Ic/Ie is the current gain, alpha ( α ) and RL/Rin is the resistance gain.

The common base circuit is generally only used in single stage amplifier circuits such as microphone pre-amplifier or radio frequency ( Rf ) amplifiers due to its very good high frequency response.

2^nd

The Common Emitter (CE) Configuration 

In the Common Emitter or grounded emitter configuration, the input signal is applied between the base and the emitter, while the output is taken from between the collector and the emitter as shown. This type of configuration is the most commonly used circuit for transistor based amplifiers and which represents the “normal” method of bipolar transistor connection. 

The common emitter amplifier configuration produces the highest current and power gain of all the three bipolar transistor configurations. This is mainly because the input impedance is LOW as it is connected to a forward biased PN-junction, while the output impedance is HIGH as it is taken from a reverse biased PN-junction.

In this type of configuration, the current flowing out of the transistor must be equal to the currents flowing into the transistor as the emitter current is given as

Ie = Ic + Ib.

(Emitter current = collector current + base current)

therefore the current gain of the transistor can be given as:

The common emitter configuration is an inverting amplifier circuit. This means that the resulting output signal is 180^o “out-of-phase” with the input voltage signal.

3^rd

The Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the β value (β = Ic/Ib) of the transistor itself. In the common collector configuration the load resistance is situated in series with the emitter so its current is equal to that of the emitter current.

As the emitter current is the combination of the collector AND the base current combined, the load resistance in this type of transistor configuration also has both the collector current and the input current of the base flowing through it. Then the current gain of the circuit is given as:

The Common Collector Current Gain

This type of bipolar transistor configuration is a non-inverting circuit in that the signal voltages of Vin and Vout are “in-phase”. It has a voltage gain that is always less than “1” (unity). The load resistance of the common collector transistor receives both the base and collector currents giving a large current gain (as with the common emitter configuration) therefore, providing good current amplification with very little voltage gain.

But you never confuse with the above given configuration because you noticed that, all described for the NPN transistors.

Ok,

For more clarification

You should know this

Thank you