Transistor Biasing

Transistors are one of the largely used semiconductor devices which are used for wide variety of applications including amplification and switching. However to achieve these functions satisfactorily, transistor has to be supplied with certain amount of current and/or voltage. The process of setting these conditions for a transistor circuit is referred to as Transistor Biasing. This goal can be accomplished by variety of techniques which give rise to different kinds of biasing circuits. However, all of these circuits are based on the principle of providing right-amount of base current, I_B and inturn the collector current, I_C from the supply voltage, V_CC when no signal is present at the input. Moreover the collector resistor R_C has to be chosen so that the collector-emitter voltage, V_CE remains greater than 0.5V for transistors made of germanium and greater than 1V for the transistors made of silicon. A few of the ample biasing circuits are explained below.

Fixed Base Bias or Fixed Resistance Bias

The biasing circuit shown by Figure 1 has a base resistor R_B connected between the base and the V_CC. Here the base-emitter junction of the transistor is forward biased by the voltage drop across R_B which is the result of I_B flowing through it. From the figure, the mathematical expression for I_B is obtained as
 

Here the values of V_CC and V_BE are fixed while the value for RB is constant once the circuit is designed. This leads to a constant value for I_B resulting in a fixed operating point due to which the circuit is named as fixed base bias. This kind of bias, results in a stability factor of (β+1) which leads to very poor thermal stability. The reason behind this is the fact the β-parameter of a transistor is unpredictable and varies up to a large extent even in the case of transistor with the same model and type. This variation in β results in large changes in I_C which cannot be compensated by any means in the proposed design. Hence it can be concluded that this kind of β dependent bias is prone to the changes in operating point brought about by the variations in transistor characteristics and temperature. However it is to be noted that fixed base bias is most simple and uses less number of components. Moreover it offers the chance for the user to change the operating point anywhere in the active region just by changing the value of R_B in the design. Further it offers no load on the source as there is no resistor across base-emitter junction. Due to these factors this kind of biasing is used in switching applications and to achieve automatic gain control in the transistors. Here, the expressions for other voltages and currents are given as

 

Collector Feedback Bias

In this circuit (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 these 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 indicates 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. Further 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 relatively simple bias has a stability factor which is less than (β+1), which results in a better stability when compared to fixed bias. However the action of reducing the collector current by base current leads to a reduced amplifier gain. Here, other voltages and currents are expressed as



Dual Feedback Bias

Figure 3 shows a dual feedback bias network which is an improvisation over the collector feedback biasing circuit as it has an additional resistor R₁ which increases the stability of the circuit. This is because an 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,

Fixed Bias with Emitter Resistor

As evident from Figure 4, this biasing circuit is nothing but a fixed bias network with an additional emitter resistor, R_E. Here, 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 brings I_C back to its normal value. Thus this kind of biasing network is seen to offer better stability when compared to fixed base bias network. However the presence of R_E reduces the voltage gain of the amplifier as it results in unwanted AC feedback. In this circuit, the mathematical equations for different voltages and current are given as

Emitter Bias

This biasing network (Figure 5) uses two supply voltages, V_CC and V_EE, which are equal but opposite in polarity. Here V_EE forward biases the base-emitter junction through R_E while V_CC reverse biases the collector-base junction. Moreover



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.

Emitter Feedback Bias

This kind of self-emitter bias (Figure 6) employs both collector-base feedback as well as emitter feedback to result in a higher stability. This is because, here, 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. An increase in the temperature 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 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

Voltage Divider Bias

This type of biasing network (Figure 7) employs a voltage divider formed by the resistors R₁ and R₂ to bias the transistor. This means that here the voltage developed across R₂ will be the base voltage of the transistor which forward biases its base-emitter junction. In general, 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. Further, from the circuit, one gets





In this kind of biasing, I_C is resistant to the changes in both β as well as V_BE which results in a stability factor of 1 (theoretically), the maximum possible thermal stability. This is because, as I_C increases due to a rise in temperature, IE also increases causing an increase in the emitter voltage V_E which in turn reduces the base-emitter voltage, V_BE. This results in the decrease of base current I_B which restores I_C to its original value. The higher stability offered by this biasing circuit makes it to be most widely used inspite of providing a decreased amplifier gain due to the presence of R_E. Apart from the analyzed basic types of biasing networks, Bipolar Junction Transistors (BJTs) can also be biased using active networks or by using either silicon or zener diodes. Further it is also to be noted that although the biasing circuits are explained for BJTs, similar bias networks also exist in the case of Field Effect Transistors (FETs).