Methods of Transistor Biasing

The biasing in transistor circuits is done by using two DC sources V_BB and V_CC. It is economical to minimize the DC source to one supply instead of two which also makes the circuit simple.

The commonly used methods of transistor biasing are

• Base Resistor method
• Collector to Base bias
• Biasing with Collector feedback resistor
• Voltage-divider bias

All of these methods have the same basic principle of obtaining the required value of I_B and I_C from V_CC in the zero signal conditions.

Base Resistor Method

In this method, a resistor R_B of high resistance is connected in base, as the name implies. The required zero signal base current is provided by V_CC which flows through R_B. The base emitter junction is forward biased, as base is positive with respect to emitter.

The required value of zero signal base current and hence the collector current (as I_C = βI_B) can be made to flow by selecting the proper value of base resistor RB. Hence the value of R_B is to be known. The figure below shows how a base resistor method of biasing circuit looks like.

Let I_C be the required zero signal collector current. Therefore,

This circuit helps in improving the stability considerably. If the value of I_Cincreases, the voltage across R_L increases and hence the V_CE also increases. This in turn reduces the base current I_B. This action somewhat compensates the original increase.

The required value of R_B needed to give the zero signal collector current I_Ccan be calculated as follows.

The required value of R_B needed to give the zero signal current I_C can be determined as follows.

Suppose that the current flowing through resistance R₁ is I₁. As base current I_B is very small, therefore, it can be assumed with reasonable accuracy that current flowing through R₂ is also I₁.

Now let us try to derive the expressions for collector current and collector voltage.

Collector Current, I_C

From the circuit, it is evident that,

From the above expression, it is evident that I_C doesn’t depend upon β. V_BE is very small that I_C doesn’t get affected by V_BE at all. Thus I_C in this circuit is almost independent of transistor parameters and hence good stabilization is achieved.

Collector-Emitter Voltage, V_CE

Applying Kirchhoff’s voltage law to the collector side,

Suppose there is a rise in temperature, then the collector current I_Cdecreases, which causes the voltage drop across R_E to increase. As the voltage drop across R₂ is V₂, which is independent of I_C, the value of V_BEdecreases. The reduced value of I_B tends to restore I_C to the original value.

Stability Factor

The equation for Stability factor of this circuit is obtained as

If the ratio R₀/R_E is very small, then R0/RE can be neglected as compared to 1 and the stability factor becomes

This is the smallest possible value of S and leads to the maximum possible thermal stability.