what is a transistor and types of transistors?

Transistor a device which transfers a signal from low resistance (input forward biased) to high resistance(output reverse biased). This is also called as transfer of resistance. Basically there are two types of transistors.

  1. Bipolar junction transistor(current controlled)
  2. Field effect transistor(Voltage controlled)

What is bipolar junction transistor?

The device in which the conduction takes place due to both charge carriers (electrons and holes) is called a bipolar device. It consists of two PN junctions so that it is called bipolar junction device. It transfers from signal from input low resistance(forward biased junction) to high resistance(reverse biased) so that it is called bipolar junction transistor. There are types of BJT’s available, NPN transistor and PNP transistor.

What is power dissipation in transistor?

When a transistor is operating it dissipates a quite amount of the power in the form of heat across the collector emitter junction. The power dissipation is calculated as the product of collector current and voltage drop at collector emitter junction. This will be more in active region in the range of 500 mv(Vce= 5v and Ic=100ma) and it will be less in cutoff region of operation.

Why inverse active mode of transistor is not useful?

Bipolar junction transistor internal design is in such a way that it will have high gain in normal active mode. Also when you interchange the roles of emitter and collector with emitter base junction reverse biased then break down voltage decreases as break down voltage is inversely proportional to the amount of doping. As emitter is highly doped compared to collector it is advantageous to reverse bias collector base junction in order to have the advantage of high breakdown voltages.

What are the types of transistor configurations?

The BJT operates in three types of configurations based on its terminal connections

  1. Common emitter configuration
  2. Common collector configuration
  3. Common base configuration

The word common in these configurations means that the followed terminal is common to the other terminals. The input and output voltage at other terminals are taken with respect to this common terminal.

What are the differences or comparisons between the transistor configuration.


Characteristic  Items Common Emitter Common Base Common Collector
Common terminal Emitter Base Collector
Input resistance moderate (500-800 Ω) Low  (<100 Ω) Very high (>1000 Ω)
output resistance high  (40- 50 KΩ) Very High  (>100 kΩ) Low (<100 Ω)
Voltage gain high (>200) high(>100) almost equals to unity
current gain high (50) less than unity very high
power gain High low low
Amplifier Power amplifier Voltage amplifier current amplifier
Applications practical amplifier circuits (power gain is high) high frequency applications (switching over is high) impedance matching amplifier (input resistance is high

 What are α,β and γ in a transistor ?

ALPHA (α): It is a large signal current gain in common base configuration. It is the ratio of collector current (output current) to the emitter current (input current).

\alpha =\frac{Collector\, current}{Emitter\, current}

\alpha =\frac{I_{C}}{I_{E}}

It is a current gain in CB amplifier and it indicates that the amount of emitter current reaching to collector. Its value is unity ideally and practically less than unity.

Beta (β): It is a current gain factor in the common emitter configuration. It is the ration of collector current (output current) to base current (output current).

\\beta =\frac{I_{C}}{I_{B}}

normally Its value is greater than 100.

Gama (γ): It is a current gain in common collector configuration and it is the ration of emitter current (output current) to base current (input current).

\gamma =\frac{I_{E}}{I_{B}}

It is also called emitter efficiency that how much current is injected from the emitter to base after recombination of minority charge carriers in base. It’s value is high compared to α,β.

What is the relation between α,β and γ in a transistor?

α,β and γ are the current gain factors in three CB, CE and CC configurations respectively.

Relation between α, β and γ :

\alpha =\frac{\beta }{\beta +1}

\beta =\frac{\alpha }{1-\alpha }

\gamma =\beta +1

What are the conditions for cutoff and saturation in common emitter configuration?

Cutoff: When input current IB is zero then CE transistor is said to be in cutoff region.

Saturation :

\left | I_{B} \right |\geq \frac{I_{C}}{\beta }

i.e. the input current IB should be greater than the ration of output current to the current gain factor of a transistor( manufacturer defined). The second condition is about connection voltage i.e. VCB should be positive for PNP and negative for NPN transistor in CE.

What is ICB0 and ICE0 in a transistor and what is relation between ICB0 and ICE0?

ICB0 is the collector to base reverse saturation current.

ICE0 is the collector to emitter reverse saturation current.

Relation between ICB0 and ICE0:

I_{CE0}=(\beta +1)I_{CB0}


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