How to Understand Transistor Biasing

The proper steady-state operation of the transistor needs the proper biasing. Transistors must be biased around the operating point for them to act like a proper linear amplifier. Properly biased resistors and load resistors are required for the transistor to produce appropriate current and voltage.

Transistor Biasing

When a transistor is biased in such a way the Q-point lies in between the operating line, in this situation, the transistor will act as a class A amplifier. There is no distortion of the output obtained by this class A amplifier, which means that the full cycle is obtained as output. Following are the transistor biasing combinations:

• Common Base biasing
• Fixed base biasing
• Collector feedback biasing
• Dual feedback biasing
• Emitter feedback biasing
• Voltage divider biasing

1: Common Base Biasing

The below given figure shows the common base basing, the best basing is obtained by connecting the proper biasing resistor and applying the proper value of DC voltage (V_cc). The Q- Q-point of the transistor is set in such a way that the DC biasing level must be set in the collector current to a constant value:

2: Fixed Base Biasing

The type of biasing in which the operating point of the transistor remains fixed because the V_cc provides the fixed value of current I_B, and this is the most frequently used biasing of the electronics circuits. The below figure shows the common emitter biasing. Two biasing resistors are used in this type of bias circuit:

The mathematical derivation of the above circuit is given as:

3: Collector Feedback Biasing

This type of biasing is also known as beta-dependent biasing, two biasing resistors are used to obtain the required DC biased voltage. The below given circuit shows the clear prototype of collector feedback biasing. This circuit acts in the active region by proper biasing of the collector to base, furthermore, this circuit provides the best stability:

The mathematical derivation of the above circuit is given as:

4: Dual Feedback Biasing

The type of biasing in which an extra bias resistor is added to the base of the common base bias configuration results in dual feedback biasing. The figure below shows the dual feedback biasing configuration in which the resistor R_B2 is an extra resistor added to improve the DC bias voltage:

The mathematical derivation of the above circuit is given as:

5: Emitter Feedback Biasing

The type of biasing in which both base and emitter are connected to the bias resistors to stabilize collector current. The drawback of this biasing is that it reduces the gain due to the base resistor. The below figure shows the configuration of emitter feedback biasing:

The mathematical derivation of the above circuit is given as:

6: Voltage Divider Biasing

In the voltage divider, a common emitter bias transistor is used. The main function of two resistances R_B1 and R_B2 is to divide the input voltage, this is the reason it is known as voltage divider bias configuration. The below given figure shows this configuration:

The mathematical derivative for the voltage divider is given by

Example of Biasing

The circuit is shown in the figure below, according to this solve example for the different parameters, V_cc=8 and V_BE=0.5, R_C=1.1 k-ohm

Firstly, find the value of I_B

Now calculate the I_C

Now calculate the V_CE

Now calculate the V_BC

Conclusion

Transistors must be biased around the operating point for them to act like a linear amplifier. Properly biased resistors and load resistors are required for the transistor to produce appropriate current and voltage. There are different types of biasing lies in the electronics circuits to perform different functions.