Transistor Biasing: What is it? (Circuits And Types of Transistor Biasing)

What is Transistor Biasing?

Transistor Biasing is the process of setting a transistor’s DC operating voltage or current conditions to the correct level so that any AC input signal can be amplified correctly by the transistor.

Transistors are one of the most widely used semiconductor devices which are used for a wide variety of applications, including amplification and switching. However, to achieve these functions satisfactorily, a transistor must be supplied with a certain amount of current and/or voltage.

The process of setting these conditions for a transistor circuit is referred to as transistor biasing. Transistor biasing can be accomplished by various techniques that give rise to different kinds of biasing circuits.

However, 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 an input.

Moreover, it is crucial to select the collector resistor, R_C, appropriately. This ensures the collector-emitter voltage, V_CE, stays above 0.5V for germanium transistors and 1V for silicon transistors. Several biasing circuits are detailed below.

The types of transistor biasing include:

• Fixed Base Bias or Fixed Resistance Bias
• Collector Feedback Bias
• Dual Feedback Bias
• Fixed Bias with Emitter Resistor
• Emitter Bias
• Emitter Feedback Bias
• Voltage Divider Bias

Types of Transistor Biasing

Fixed Base Bias or Fixed Resistance Bias

In this circuit, the base-emitter junction is forward biased by the voltage drop across R_B, caused by the base current, I_B, flowing through it. The corresponding mathematical expression for I_B is shown in the figure.

Here the values of V_CC and V_BE are fixed, while the value for RB is constant once the circuit is designed.

This configuration ensures a constant I_B, establishing a fixed operating point—hence the name, fixed base bias. However, this type of biasing has a stability factor of (β+1), which contributes to lower 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 a 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, this kind of β dependent bias is prone to the changes in operating points 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 fewer components. Moreover, it offers the chance for the user to change the operating point anywhere in the active region by changing the value of R_B in the design.

Furthermore, it offers no load on the source as there is no resistor across the 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 because


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 design ensures the Q-point, or operating point, remains stable despite variations in the load current, keeping the transistor in its active region, independent of β value.

Furthermore, this circuit is also referred to as a 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 that is less than (β+1), which results in 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 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, the I_E also increases, increasing the voltage drop across R_E.

This results in the reduction of V_C, causing a decrease in I_B, bringing I_C back to its normal value. Thus, this kind of biasing network offers better stability compared to a fixed base bias network.

However, including R_E lowers the amplifier’s voltage gain due to unwanted AC feedback. The circuit includes mathematical equations for various voltages and currents, as follows:

Emitter Bias

This biasing network (Figure 5) uses two supply voltages, V_CC and V_EE, 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 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 and emitter feedback to result in higher stability. 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 degenerative feedback, which is nothing but 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 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 the required base current, I_B (i.e., I₂ = 10I_B).

This is done to avoid its effect on the voltage divider current or 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.

As I_C increases due to a temperature rise, IE increases, causing an increase in the emitter voltage V_{E, reducing} 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 most widely used despite 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.

Furthermore, 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).