In the last blog, we studied about output characteristics of transistors and from there we understood that transistors can be operated in 3 different regions.
In this blog, we will be studying the biasing of transistors. The biasing of the transistor is mainly used in the application of amplification of weak signals. 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 a 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 transistors in the active region. Due to biasing of a transistor, we can get the amplified and undistorted version of input signal.
Before understanding what is needed for biasing first understand 2 parameters
1)Load-line and
2)Quiescent-point(q-point)
1) What is biasing?
Basically in electronics biasing means to apply fixed dc voltage to an electronic component (active component) like a 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 transistors in the active region. Due to biasing of a transistor, we can get the amplified and undistorted version of input signal.
Before understanding what is needed for biasing first understand 2 parameters
1)Load-line and
2)Quiescent-point(q-point)
1) Load-line :-
As you can see above,a load line 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 purposes so we are superimposing the weak Ac input signal which may be of a 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.
2) Quiescent Point or q-point:-
Any point present on the load-line is known as a 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, and you and your friend are going to a 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 a 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 the 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 the single point it keeps on changing because of the following factors
As you can see above,a load line 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 purposes so we are superimposing the weak Ac input signal which may be of a 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.
2) Quiescent Point or q-point:-
Any point present on the load-line is known as a 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, and you and your friend are going to a 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 a load line.
2) Need of biasing a transistor ?
In the above diagram, you can see that when q-point is present in the 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 the single point it keeps on changing because of the 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 the input signal is not obtained at the output.
@ Temperature Dependence Of Ic and Thermal Runaway:-
As we had discussed earlier the P-region has more positive charges & it also contains a 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 the surrounding temperature increases electrons(minority carriers) present in the p-type region[Base Terminal] moves towards the n-type region[collector Terminal], similarly holes(minority carriers) present in the n-type region[collector Terminal] moves towards p-type region[Base Terminal]. When these minority carriers come close to the depletion region an electric field is generated between electrons(minority carriers) present in the p-region[Base Terminal] and positive charge present in the depletion region due to this the electrons from the p-region(Base Terminal) are transferred towards n-region(Collector Terminal). Therefore a current ICBO flows from the collector terminal to the base terminal due to minority carriers.
ICBO is known as a collector-to-base current when the emitter terminal is open
So basically the formula for Ic which we had studied from an 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 a 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 the collector region increases due to which the collector region will burn out completely. Thus the self-destruction of unstabilized transistors is known as thermal runaway.
@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 the input signal is not obtained at the output.
@ Temperature Dependence Of Ic and Thermal Runaway:-
As we had discussed earlier the P-region has more positive charges & it also contains a 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.
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ICBO is known as a collector-to-base current when the emitter terminal is open
So basically the formula for Ic which we had studied from an 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 a 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 the collector region increases due to which the collector region will burn out completely. Thus the self-destruction of unstabilized transistors is known as thermal runaway.
VIDEO:-
CONCLUSION:-So we came to the conclusion that biasing is done for faithful or proper amplification.
Here is the video to make your concept more clear
VIDEO:-
CONCLUSION:-So we came to the conclusion that biasing is done for faithful or proper amplification.
Here is the video to make your concept more clear
VIDEO:-
In the next post, we will see how to bias a transistor.
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