What Is Biasing & Need Of Biasing A transistor
In last blog we studied about output characteristics of transistors and from there we understood that transistor can be operated in 3 different regions.In this blog we will be studying about biasing of transistor.Biasing of transistor is mainly used in the application of amplification of weak signal.But many questions may arise in your mind that What is biasing ?,Why transistor needs to be biased ? and How to bias a transistor?
Let’s understand it in detail
1) What is biasing ?
Basically in electronics biasing means to apply fixed dc voltage to an electronic component (active component) like transistor in order to establish proper operating conditions for the component .In short by biasing a transistor we can operate it according to our will,mostly biasing is done to operate transistor in active region.Due to biasing of a transistor we can get the amplified and undistorted version of input signal.
Before understanding what is need of biasing first understand 2 parameters
1)Load-line and
2)Quiescent-point(q-point)
As you can see above,a loadline is a line drawn between 2 maximum points that is between Vce(max)=Vcc and Ic(max)=Vcc/Rc.There are 2 types of load-line Ac and Dc load-line.Since transistors are mainly used for amplification purpose so we are superimposing the weak Ac signal which may be of few mv with dc voltage and at the load resistor we are getting the amplified version of the input signal therefore this line is known as Ac load-line.When the Ac input signal is not applied and we are only dealing with dc analysis then the load-line is known as dc load-line.
Any point present on load-line is known as q-point.So basically you can operate the transistor according to your desire.Let’s take a small example so that the whole picture will be more clear consider that you have 100$ with you,you and your friend are going for party so how much money you spend in that party is like your q-point.You can spend 30$ or 80$ or 99$ but surely not less than 0$ and not more than 100$.Since 100$ is your maximum limit therefore you can consider it as load-line.
2) Need of biasing a transistor ?
Now just look at the above diagram
In the above diagram you can see that when q-point is present in middle of the load line figure(2) i.e. when [Ic=Ic(max)/2 & Vce=Vcc/2] then the amplification is maximum while if it shifts up or down then the amplified wave is clipped either positive half of the wave is clipped or negative half depending on the position of q-point.This q-point is not stationary at single point it keeps on changing because of following factors
@ Individual Variations
@Temperature dependence of Ic and Thermal runaway
@ Individual Variations:-When you replace a transistor it’s some parameters like Vbe and β changes as a result q-point also changes due to which a proper amplified version of input signal is not obtained at the output.
@ Temperature Dependence Of Ic and Thermal Runaway:-
As we had discussed earlier that P-region has more number of positive charges & it also contains very less number of electrons which we call as minority carriers.Similarly In N-region negative charges i.e. electrons are more in number and holes are less in number that is holes are minority carriers in N-region.
It is observed that when surrounding temperature increases electrons(minority carriers) present in p-type region[Base Terminal] moves towards n-type region[collector Terminal],similarly holes(minority carriers) present in n-type region[collector Terminal] moves towards p-type region[Base Terminal].When this minority carriers come close to depletion region an electric field is generated between electrons(minority carriers) present in p-region[Base Terminal] and positive charge present in depletion region due to this the electrons from p-region(Base Terminal) are transferred towards n-region(Collector Terminal).Therefore a current Icbo flows from collector terminal to base terminal due to minority carriers.
Icbo is known as collector to base current when emitter terminal is open
So basically the formula for Ic which we had studied from earlier blog is
Ic(real)=β .Ib
After increasing the surrounding temperature the formula for Ic changes and Ic becomes
Ic(total)=Ic(real)+Icbo*(1+β) [Derived]
So from here you can see that the current generated from minority carriers is also amplified by factor (1+β) due to which Ic(real) becomes Ic(total) and which in short leads to the shifting of q-point and if q-point gets shifted then the amplified wave get clipped as shown in above figure(1) or figure(3).Practically it is observed that Icbo doubles for every 10 degree rise in temperature.
Now if Ic increases that means electrons in collector region increases due to which collector region will burn out completely.Thus the self destruction of unstabilized transistor is known as thermal runaway.
CONCLUSION:-So we came to the conclusion that biasing is done for faithful or proper amplification.In next post we will see how to bias a transistor.
Originally published at https://www.practical-buddy.xyz on June 1, 2020.
