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, IB and inturn the collector current, IC from the supply voltage, VCC when no signal is present at the input. Moreover the collector resistor RC has to be chosen so that the collector-emitter voltage, VCE 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 RB connected between the base and the VCC. Here the base-emitter junction of the transistor is forward biased by the voltage drop across RB which is the result of IB flowing through it. From the figure, the mathematical expression for IB is obtained as
Here the values of VCC and VBE are fixed while the value for RB is constant once the circuit is designed. This leads to a constant value for IB 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 IC 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 RB 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 RB is connected across the collector and the base terminals of the transistor. This means that the base voltage, VB and the collector voltage, VC are inter-dependent due to the fact that
From these equations, it is seen that an increase in IC decreases VC which results in a reduced IB, automatically reducing IC. 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 RB. 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.
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, RE. Here, if IC rises due to an increase in temperature, then the IE also increases which further increases the voltage drop across RE. This results in the reduction of VC, causing a decrease in IB which in turn brings IC 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 RE 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
This biasing network (Figure 5) uses two supply voltages, VCC and VEE, which are equal but opposite in polarity. Here VEE forward biases the base-emitter junction through RE while VCC reverse biases the collector-base junction. Moreover
In this kind of biasing, IC can be made independent of both β and VBE by choosing RE >> RB/β and VEE >> VBE, 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, RE due to the flow of emitter current, IE. An increase in the temperature increases IC, causing an increase in the emitter current, IE. This also leads to an increase in the voltage drop across RE which decreases the collector voltage, VC and in turn IB, thereby bringing back IC 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, VC. This effect can be compensated by using a large bypass capacitor across the emitter resistor, RE. 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, IB (i.e. I2 = 10IB). 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, IC is resistant to the changes in both β as well as VBE which results in a stability factor of 1 (theoretically), the maximum possible thermal stability. This is because, as IC increases due to a rise in temperature, IE also increases causing an increase in the emitter voltage VE which in turn reduces the base-emitter voltage, VBE. This results in the decrease of base current IB which restores IC 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 RE.
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).