In the last post, we studied what is biasing? and at the same time, we also understood why transistors should be biased? So in this post, we will see how the transistor is biased. Before going with biasing techniques you need to understand one term which is the stability factor.
# Stability Factor:-
The rate of change of collector current(Ic) with respect to collector leakage current(ICBO) at constant β and Vbe is known as a stability factor. So basically the change in Q-point due to a change in collector current(Ic) is known as q-point. Ideally, it should be one but practically it should be as low as possible.
Mathematically it is given as,
S = (β+1)/1-[β(dIb/dIc)]
Now let's move to the Biasing Techniques of Transistors:-
1) Fixed Bias
2) Modified Fixed Bias
3) Collector to Base Bias
4) Voltage Divider Bias
Let's understand each technique in depth
1) Fixed Bias:-
FIXED BIAS
As you can see above, this is a circuit diagram of fixed bias. The resistance Rb is applied from a positive terminal of the battery to the base of the transistor similarly Rc is applied between the positive terminal of the battery to the collector terminal. Now apply KVL on both the side that is on the input side of the transistor and then at the output side of the transistor. So you will get Ic and Vce which is q-point. So the advantage of using this circuit is that you can easily change the operating point just by changing the base resistor(Rb). Also, you require less space and components for this circuit. The disadvantage of this circuit is that its stabilization is very poor which in short means that its stability factor is very high.
Mathematically it is S =1+β
Due to this reason this circuit is very rarely used. So to decrease the stability factor a new technique was introduced and the name of this technique was modified fixed bias.
2) Modified Fixed Bias :-
MODIFIED FIXED BIAS
As you can see that the circuit is almost the same as that of fixed bias but resistance Re is introduced at the emitter side to decrease the stability factor and then the rest of the procedure is the same to calculate q-point of the circuit. Since we had introduced Re in the circuit, therefore, the stability factor changes and the formula becomes.
S = (1+β)/1+[ β*Re/(Rb+Re)]
3) Collector to Base Bias :-
COLLECTOR TO BASE BIAS
In collector to base bias the base resistor is connected to the collector terminal, as shown above, in short, it acts as a feedback resistor. Let's assume that collector current(Ic) increases due to an increase in the temperature as a result the voltage drop across Rc will also increase therefore the collector voltage(Vc) will reduce, as the voltage Vc has reduced therefore the base current(Ib) will also reduce due to which collector current(Ic) will also reduce since Ic = β.Ib
In this way, this circuit tries to stabilize any change in the collector current. To improve the stability Re is also introduced at the emitter terminal. Although the circuit provides better stabilization then the modified fixed bias but the gain of the amplifier is reduced because the circuit provides negative feedback.
The stability factor of this circuit is given as
S = 1+β/1+[β*Rc/Rb+Rc]
4) Voltage Divider Bias :-
VOLTAGE DIVIDER BIAS
This technique is used widely since it provides better stabilization to the transistor. In this method of biasing R1 and R2 divide supply voltage Vcc. The voltage divider bias is also known as a self-biasing circuit I will explain the reason for this in-depth when we will start designing a ce amplifier
The stability factor for voltage divider bias is given as
S = 1+β/1+[β*Re/Rth+Re].
So now you can understand why there is a 180 degree phase difference between the input and output signal of the transistor. For that let's take an equation of Vce from Fixed Bias.
Vce = Vcc-IcRc
So if Vin increases that means Ib increases as V is directly proportional to I and if Ib is increased that means Ic will also increase
since Ic=β*Ib
Now from the above equation, you can see that as Ic increases Vce decreases similarly if Ic decreases then Vce increases.
The concept would be more clear by watching the video
VIDEO :-
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