Transistor Biasing

Transistor biasing

To achieve transistor functions satisfactorily, a transistor has to be supplied with a certain amount of current and/or voltage. The process of setting these conditions and parameters for a transistor circuit is term as a Transistor Biasing. This can be achieved by a variety of techniques which give rise to different kinds of biasing circuits. All of these circuits are based on the principle of providing the right amount of base current, I_B and in turn the collector current, I_C from the supply voltage, V_CC when no signal is present at the input.

Different methods of transistor biasing are:

1.      Fixed Base Bias or Fixed Resistance Bias

The biasing circuit shown as in figure has a base resistor R_B connected between the base and the V_CC supply. In this circuit base-emitter junction of the transistor is forward biased because of a voltage drop across R_B which is the result of I_B flowing through it. The mathematical expression for I_B is obtained as

Here the values of V_CC and V_BE are fixed while the value for R_B resistance is constant once the circuit is designed. This causes a constant value for I_B resulting in a fixed operating point because of which the circuit is named as fixed base bias.

The expressions for other voltages and currents are given as

2.      Collector Feedback Bias

In this circuit, as shown in Figure 2, the base resistor R_B is connected across the collector and the base terminals of the transistor. This means that the base voltage, V_B and the collector voltage, V_C are inter-dependent due to the fact that                                                    Where,

From above equations, it is seen that an increase in I_C decreases V_C which results in a reduced I_B, automatically reducing I_C. This shows that, for this type of biasing network, the Q-point (operating point) remains fixed irrespective of the variations in the load current causing the transistor to always be in its active region regardless of β value. This circuit is also referred to as self-biasing negative feedback circuit as the feedback is from output to input via R_B. This kind of has a stability factor which is less than (β+1), which causes a better stability when compared to fixed bias. Here,

other voltages and currents are expressed as

3.      Dual Feedback Bias

Figure 3 shows a dual feedback bias network. It has an additional resistor R₁ which increases the stability of the circuit results in an improvisation over the collector feedback biasing circuit this is because of increase in the current flow through the base resistors results in a network which is resistant to the variations in the values of β. Here,

4.      Fixed Bias with Emitter Resistor

As shown in Figure 4, this biasing circuit is simply a fixed bias network with an additional emitter resistor, R_E. In this circuit, if I_C rises due to an increase in temperature, then the I_E also increases which further increases the voltage drop across R_E. This results in the reduction of V_C, causing a decrease in I_B which in turn causes I_C back to its normal value. Thus this type of biasing network is observed to have better stability when compared to fixed base bias network. But because of the presence of R_E reduces the voltage gain of the amplifier as it results in unwanted AC feedback. Mathematical equations for different voltages and current are given as

5.      Emitter Bias

Biasing network shown in Figure 5 uses two supply voltages, V_CC and V_EE, which are equal having opposite polarity. Here V_EE forward biases the base-emitter junction through R_E while V_CC reverse biases the collector-base junction. In this kind of biasing, I_C can be made independent of both β and V_BE by choosing R_E >> R_B/β and V_EE >> V_BE, respectively; which results in a stable operating point.

6.      Emitter Feedback Bias

This kind of self-emitter bias as shown in Figure 6uses both collector-base feedback as well as emitter feedback to result in a higher stability. In this circuit the emitter-base junction is forward biased by the voltage drop occurring across the emitter resistor, R_E due to the flow of emitter current, I_E. As an increase in the temperature leads to increases I_C, causing an increase in the emitter current, I_E. This also leads to an increase in the voltage drop across R_E which further decreases the collector voltage, V_C and in turn I_B, thereby bringing back I_C to its original value.

However this results in a reduced output gain due to the presence of a degenerative feedback which is nothing but an unwanted AC feedback, wherein the amount of current flowing through the feedback resistor is determined by the value of the collector voltage, V_C. This effect can be compensated by using a large bypass capacitor across the emitter resistor, R_E. The expressions corresponding to various voltages and currents in this low-power-supply-voltage suitable biasing network are given as

7.      Voltage Divider Bias

This type of biasing network as shown in Figure 7 employs a voltage divider network created by the resistors R₁ and R₂ to bias the transistor. It means that here the voltage developed across R₂ will be the base voltage of the transistor which forward biases its base-emitter junction, the current through R₂ will be fixed to be 10 times required base current, I_B (i.e. I₂ = 10I_B). This is done to avoid its effect on the voltage divider current or on the changes in β. The mathematical equation for current and voltage are: