- 1 What is Transistor Biasing:
- 1.1 Methods of Transistor Biasing
What is Transistor Biasing:
In order to operate a Transistor for faithful amplification, a transistor biasing circuit is used to make the transistor voltages and transistor current to at correct levels so that a transistor must be able to produce faithful amplification.
The basic purpose of biasing is to keep the transistor input Base-Emitter junction forward bias, and Emitter-Collector Junction reversed bias. This can be achieved by bias battery of VBB and VCC or Biasing circuits.
The circuit which provides biasing of the transistor is known as Biasing circuits.
The transistors parameter like beta (β), VBE varies with every manufacturing scheme and due to inherent change of transistor parameter may change the operating point, resulting in unfaithful amplification so to operate a Transistor for faithful amplification a bias circuit is utilized which should make operating point stable independents of parameters variations.
Methods of Transistor Biasing
The biasing in the transistor circuit was done by the sources VBB and VCC, but it is more economical to reduce DC supply to one source instead of two DC sources and also it makes the circuit simpler.
Fixed Base Bias Method:
Fixed Base bias or Base resistor method is the basic type of transistor bias method. In which a high resistance RB resistor is connected between the base and +VCC for an NPN transistor as shown in the figure.
Here the Base-Emitter junction is forward bias because of the voltage drop across the RB or you can say the base is more positive with respect to the emitter, which is the result of IB following through it. By applying Kirchhoff’s law the mathematical equation for IB can be obtained.
Here the value of VCC and VBE are fixed and RB is constant once the circuit is designed. The constant RB will result in the constant value of IB resulting in fixed operating point due to which the circuit name is given Fixed Base Bias.
The fixed base bias method has the stability factor of (β + 1) which means very poor thermal stability. As we know Beta β parameter of a transistor is very sensitive to variation, so a small change in a β parameter can make a big change in transistor characteristics and temperature.
Advantages of Fixed Base Bias:
- The biasing circuit of fixed base bias is very simple because it only requires one resistor RB.
- The calculation of this bias method is very simple.
- There is no loading effect at input side because no resistor is present in the Base-Emitter junction.
Disadvantages of Fixed Base Bias:
- The transistor is very β sensitive and variation of beta can cause the temperature increase in transistor and hence this method is very unstable to temperature stability.
- This method has strong chances of thermal runaway.
Hence this fixed base bias method is rarely employed.
Collector Feedback Bias Method:
The collector feedback and Base bias circuits are almost same but the only difference is Base resistor RB is connected across the collector terminal. The current following from VCC first encounters by the RC then it goes to RB. Which means base voltage VB and collector voltages VC, are shared. As shown in figures below.
From these equations, it is obvious that by increasing the IC, VC will be decreased. Which will automatically reduce the IB and IC will be also reduced. Now, this all discussion indicates that in this type of collector feedback biasing method with the variation of transistor load current IC the Q-point will be stable.
This collector feedback bias method has the stability factor of less than (β + 1). Which results in better stability compared to fixed base bias. However, By controlling the collector current by base current leads to reduce amplifier gain.
Dual Feedback Transistor Bias Method:
By adding one more resistor to the base terminal we can improve the stability of circuit with respect to the variation of beta (β), By just increasing the current through the base bias resistor.
Connect one terminal of a resistor to base terminal and connect another terminal of a resistor to emitter terminal you have converted a circuit base bias to dual feedback bias circuit. As shown in the figure.
One of the main advantages of this Dual Feedback transistor bias method is that it provides automatic bias as well as resistor feedback bias.
Emitter Bias Method:
Emitter bias method provides the best stability with respect to the β variation or temperature, it used both +ve VCC and –ve VEE supply voltages. As shown in the figure below.
Here VEE forward biases the Base-Emitter junction using RE while VCC reverse biases the collector-Emitter junction.
Self-Emitter Bias Method:
Self-Emitter Bias method is the combination of Emitter bias and Collector Feedback bias method. And this Self-Emitter Bias method results in higher stability with respect to β variation. And the reason of high stability of this bias method is because, Emitter-Base Junction is forward biased by the voltage drop of emitter resistor RE, An increase in temperature increases collector current IC which causes an increase in IE, This lead to an increase in the voltage drop of RE.
This Self-Emitter Bias or Emitter feedback bias method has the disadvantage of reducing gain because of the base resistor. This type of bias is perfect for the operation of very low power supply voltages.
Voltage Divider Bias Method:
This is the most widely used method for providing biasing and stability to a transistor. In this method, two resistors say R1 and R2 are connected across the VCC which provide biasing. And the emitter resistance provides the stability. The name Voltage Divider Bias is given in the fact that the voltage is divided between the R1 and R2.
The voltage drop across the resistor R2 forward bias the Base-Emitter Junction, and the voltage divider bias circuit is designed so that the base current is much smaller than the I2 through R2.
To calculate the input applied voltage, to the base terminal or we can say to calculate the voltage drop across the R2 we simply use Voltage Divider formula.
Generally, the voltage drop across the R2 is much less than the R1 and the current following through the R2 is set 10 times greater than the input base current IB, because by doing that the effect on voltage divider current or in the change beta can be reduced.
This voltage divider bias is most widely used as I mentioned earlier because it has the maximum possible thermal stability its stability factor is 1. This is due to, If IC changes due to the rise in temperature, IE also changes with the Emitter voltages VE which cause the reduction in VBE. And that results in the decrease in IB which restores the IC to an original value.
Summary of Transistor Biasing Circuits
Remember the goal of Biasing is to generate or produce faithful amplification, and these biasing circuits provide us the Q-point parameters which are IC and VCE.
Here in this article, we have successfully covered all 6 types of different Transistor Biasing Methods. But remember that we can also bias the transistor by using either PN junction diodes or Zener diodes by connecting them to the base terminal of a transistor or by biasing the transistor with two supplies of VBE and VCC as I mentioned the side effect of that in the above description of this article. If you want to learn about how to bias transistor with PN junction diode or Zener diode comment below I will make sure to upload an article on it too.