A bipolar junction transistor (BJT) is commonly used as an amplifier or switch in electronic circuits. As a three-terminal device, it has an input stage and an output stage. At the input stage, the VI characteristics of a transistor are similar to the forward-bias characteristics of a signal diode. The transistor turns on (i.e., begins conducting) when the applied voltage at the input exceeds a certain threshold known as the cut-in voltage. The output characteristics of a transistor include three distinct regions: saturation, cutoff, and active.
For a transistor to function as a switch, it must transition between the cutoff and saturation regions. In the cutoff region, the transistor is completely OFF. In the saturation region, the transistor is fully ON. To operate as an amplifier, a transistor must function in the active region.
It’s commonly assumed that the transistor amplifies signals on its own. The word transistor is derived by combining “transfer” and “resistor.” The amplification function of the transistor involves the transfer of energy from a dc source to increase the amplitude of voltage, current, or both.
Whether used as a switch or an amplifier, a transistor must be properly biased to operate. Biasing involves applying suitable dc voltage and current to ensure the transistor is fully turned on, off, or acts as a signal amplifier. While there is no single way to bias a transistor to operate in a desired region, multiple methods exist to achieve this. The most common biasing techniques are fixed bias, voltage divider bias, collector feedback bias, and emitter bias.
In this article, we’ll discuss the various transistor biasing methods and explain their basic concepts. We’ll also explore their advantages and disadvantages, determining which method is ideal under certain circumstances.
The common-emitter configuration is the standard method for connecting a transistor in a circuit and the NPN transistor is the typical variant of a BJT. So, all biasing methods will be referred to in the context of an NPN transistor connected in a common-emitter configuration. The same principles apply to NPN and PNP transistors, except that the voltage polarities are reversed in a PNP transistor circuit.
The concepts discussed in this tutorial regarding the common-emitter configuration also apply to common-base and common-collector configurations.
Operating point
The operating point of a transistor is a fixed point in the output characteristics of a transistor that shapes and defines the transistor’s response to an input signal (analog signal). This operating point is a fixed point in the transistor’s output characteristics, also called a quiescent point or Q-point. The biasing of the transistor sets the operating point. The output characteristics of a common-emitter configuration are shown in the figure below.
When a transistor is unbiased, its operating point is situated near the origin of both axes (illustrated by point A in the above figure), placing it in the transistor’s cut-off region. In this state, the transistor is entirely off and non-conductive. If the operating point is positioned too close to either the vertical or horizontal axis (as shown by point B), the transistor functions in the active region, but a portion of the input signal — either positive or negative — may be clipped. This area of the characteristic curve is also excessively non-linear for reliable output.
Establishing the operating point at an optimal level (demonstrated by point C) enables the amplification of the entire input signal without distortion. This proper biasing ensures that the transistor operates in its most effective and linear region, allowing for signal reproduction and amplification without distortion.
Positioning the operating point too close to the transistor’s maximum limits can push it beyond its safe operating range. While a transistor may temporarily function beyond these limits — allowing collector current to exceed its maximum rating or applying collector-base voltage above the specified maximum — doing so risks permanent damage or significantly reduces the transistor’s lifespan. Examining the graph, the operating point represents a fixed position on the output characteristics.
This point corresponds to specific output current (collector current in common-emitter configuration) and output voltage (collector-emitter voltage in common-emitter configuration) for a given input current (base current). When the transistor is biased to an appropriate operating point, the input signal — typically, an analog waveform — oscillates around this point, resulting in an amplified signal at the output.
This balanced approach to setting the operating point ensures optimal performance and longevity of the transistor while allowing for effective signal amplification without distortion.
Transistor biasing
The operating point of a transistor can be set in the saturation, cut-off, or active region as required by the application by proper transistor biasing.
- To operate the transistor in the cut-off region, both the base-emitter junction and base-collector junction must be reverse-biased.
- To operate the transistor in the saturation region, the base-emitter and base-collector junction must be forward-biased.
- To operate the transistor in the active region, the base-emitter junction must be in forward-bias and the base-collector junction must be in reverse-bias.
Saturation
A transistor is in saturation when the collector current exceeds the maximum collector current-rating level. The collector current increases to a maximum level when the transistor is biased to operate in the saturation region.
For increases in current, the collector-base junction is reverse-biased, and the transistor enters the active region. When the reverse-bias voltage at the collector-base junction increases beyond the maximum collector-base voltage rating, the junction undergoes an avalanche effect. This means a large current flows through the emitter to the collector, causing a saturated transistor. A saturated transistor has zero resistance between the emitter and the collector. A short-circuit element replaces a saturated transistor in a circuit equivalent.
Load line
The operating conditions of a transistor circuit are determined by superimposing the transistor characteristics with the network equation involving the same network parameters. For a common-emitter circuit, the output current (i.e., the collector current) and the output voltage (i.e., the collector-emitter voltage) are the key network parameters.
When plotting the network equation relating the collector current, load resistance connected at the collector terminal, and the collector-emitter voltage, a straight line is formed. This line intersects with the transistor’s output characteristics and both axes, and is called the load line.
The load line is useful in deriving the actual operating points of the transistor with a given load resistance connected at the output terminal. It also helps in analyzing changes in the operating conditions of the transistor if the load resistance at the output terminal or the supply voltage is altered.
The load line intersects the horizontal axis, which represents the collector-emitter voltage at the VCC level. On the vertical axis, which represents the collector current, it intersects at VCC/RL. The points where this line intersects the transistor’s characteristic curves in the active, saturation, and cut-off regions represent the potential operating points of the transistor with a specific load resistance connected at the collector terminal.
If the load resistance is altered, the slope of the load line changes. For smaller values of load resistance, the slope becomes steeper. When the supply voltage VCC is decreased, the slope of the load line remains constant, but its points of intersection with the characteristic curves shift downward. This graphical representation allows for a quick visual analysis of how changes in load resistance or supply voltage affect the transistor’s operating conditions across its different regions of operation.
Temperature effects
In addition to load resistance and supply voltage changes, temperature variations significantly impact a transistor’s operating conditions. As temperature increases, the current gain (β) and leakage current (ICEO) rise. These changes can shift the load line and the transistor’s operating points.
For a circuit to function as intended, it must be designed to accommodate these shifts in operating conditions within tolerable limits. The stability factor S quantifies the temperature stability of a transistor, indicating the degree of shift in the transistor’s operating points due to temperature changes.
Fixed bias
In fixed biasing, a single resistor is connected to the transistor’s input terminal, and another is connected to its output terminal. The diagram below illustrates a fixed bias circuit for an NPN transistor in a common-emitter configuration.
If Kirchhoff’s voltage law is applied at the input stage, we get the following equation:
VCC – IBRB – VBE = 0
Or, IB = (VCC-VBE)/RB
The input current (i.e., the base current) is directly proportional to the supply voltage and inversely proportional to the fixed resistance RB connected at the input terminal. As the supply voltage VCC and the voltage drop across the base-emitter junction VBE are constant, the value of the base current only depends on the resistor RB. By changing the value of the resistor, the magnitude of the base current in the circuit can be controlled.
If Kirchhoff’s voltage law is applied at the output stage, we get the following equation:
VCC – VCE – ICRC = 0
Or, VCE = VCC-ICRC
Or, VCE = VCC-βIBRC
The current through the load RC, connected at the output, only depends on the input current IB and is related to the input current by the following equation (where β is a transistor constant):
IC = βIB
There’s no load-resistance effect on the output current IC. However, the voltage drop across the output terminal (i.e., VCE) decreases on when the RC load resistance increases Both the output and input currents depend upon the value of the resistor connected at the input terminal, and the output voltage depends on the value of the load resistance.
It’s possible to get the exact operating points of the circuit by finding the intersection points of the load line of both axes.
For IC = 0, VCE = VCC
For VCE = 0, IC = VCC/RC
Fixed bias is the simplest method for biasing a transistor, requiring only two resistors and a dc power supply. Due to the circuit’s simplicity, it’s relatively easy to predict the actual operating points of a fixed-bias transistor circuit. However, despite its simplicity, fixed bias has some significant drawbacks:
- Temperature sensitivity: The operating points of a fixed-bias transistor circuit may shift considerably with increasing temperatures.
- Limited current gain: The current gain of a fixed-biased transistor is capped at β and cannot be increased beyond this value.
Emitter bias
In an emitter bias configuration, a resistor is connected to the transistor’s emitter terminal and resistors at the collector and base terminals. This arrangement significantly improves the circuit’s thermal stability.
When applying Kirchhoff’s voltage law at the input stage, we get the following equation:
VCC – IBRB – VBE – IERE = 0
Or, VCC – IBRB – VBE – (β + 1)IBRE = 0
Or, IE = (VCC – VBE)/(RB + (β + 1)RE)
The input current (i.e., the base current) is a function of both the resistor connected at the base terminal and the resistor connected at the emitter terminal. As the value of β is always high (50~200), the resistor connected to the emitter adds stability to the value of the base current.
When applying the Kirchhoff’s voltage law at the output stage, we get the following equation:
VCC-ICRC-VCE-IERE = 0
As IE ≈ IC, VCC-ICRC-VCE-ICRE = 0
Or, VCE = VCC – IC (RC + RE)
As the collector current is a function of base current, it’s not affected by resistance connected at the collector terminal but now depends on both the resistance connected at the base terminal and the emitter terminal. The output voltage also now depends on resistance connected to the collector terminal and to the emitter terminal.
By equating VCE= 0 and IC= 0, we receive the intersection points of the load line with the horizontal and vertical axes. Connecting the two points provides the other operating points of the transistor in the emitter bias configuration.
The emitter bias configuration, while simple, offers more predictable output characteristics than the fixed bias. It provides improved and reliable thermal stability compared to the fixed-bias configuration. However, it does have the drawback of providing lower gain for low-frequency signals.
Voltage divider bias
In voltage divider biasing, a voltage divider circuit controls the input current (i.e., the base current). The transistor’s base is connected to the midpoint voltage of this divider circuit. This arrangement effectively introduces a Thevenin equivalent voltage source with an equivalent resistance at the transistor’s base.
In fixed or emitter biasing, the current flowing through the base resistor is identical to the current flowing through the base itself, making the input current directly dependent on transistor gain. In contrast, with voltage divider biasing, the input current becomes dependent on the equivalent resistance of the voltage divider network.
This configuration offers several advantages:
- Improved stability against variations in transistor parameters
- Better control over the operating point
- Reduced sensitivity to temperature changes
The circuit diagram of voltage divider biasing for a common-emitter NPN transistor is shown below.
When solving the network equations for the voltage divider biased circuit, we receive the following equations:
IB = (VR2 – VBE)/((R1||R2) + (β+1)RE)
IC = βIB
VCE = VCC – IC(RC + RE)
The base current or the input current is now a function of the voltage drop across R2 and the equivalent resistance of the voltage divider circuit. The output current (i.e., the collector current) is still dependent on the input current and has no effect from RC or RE. The output voltage is still a function of RC and RE.
By equating VCE = 0 and IC = 0, the same intersection points of the load line are received with the horizontal and vertical axes that we received for the emitter bias configuration. When connecting the two points, we get the transistor’s other operating points in the voltage divider bias configuration.
Voltage divider biasing offers better stability because of the independence of the base and collector currents from the transistor’s beta. This configuration facilitates a more straightforward setting of the circuit’s operating point. However, it comes with trade-offs, including:
- Increased overall power consumption of the circuit
- Slightly higher complexity in assembly compared to simple series resistor biasing
The most significant advantage of voltage divider biasing is its transistor interchangeability. Transistors can be replaced in the circuit without concern for variations in beta, as the input and output currents are mostly independent of transistor gain. This feature enhances the circuit reliability and simplifies maintenance.
Collector feedback bias
An alternative approach involves connecting a feedback resistor between the transistor’s collector and base. This method produces effects similar to voltage divider biasing, offering:
- Improved stability
- Better control over the operating point
- Reduced sensitivity to transistor parameter variations
The collector feedback configuration is a simplified version of voltage divider biasing, potentially offering a balance between performance and circuit complexity.
A collector-feedback bias circuit for the common-emitter NPN transistor is shown in the below diagram.
When solving the network equations for the input and the output stage, we get the following results:
IB = (VCC – VBE)/(RF + β(RC + RE))
IC = βIB
VCE = VCC – IC(RC + RE)
The equation for the input current (i.e., the base current) is similar to the equation for the voltage divider biasing. The input current is a function of the feedback resistor RF, as the load resistance RC and the emitter resistor RE. Notably, the input current in this configuration is not dependent on the transistor’s beta. Consequently, the output current, directly related to the base current, also becomes independent of the transistor’s gain. The output voltage in collector feedback biasing, similar to other configurations, depends on RC and RE.
The collector feedback biasing has the same intersection points of the load line with the horizontal and vertical axes as the emitter bias and voltage-divider bias. It improves the stability of the transistor circuit but suffers from a similar lower gain as emitter bias. Assembling a collector feedback bias is easier than a voltage divider, but collector feedback is less common.
Other biasing techniques
The biasing techniques discussed in this tutorial are just the tip of the iceberg. There are several ways to bias transistors, all stemming from these basic techniques or their combinations.
Regardless of the biasing configuration, the fundamental purpose of a biasing network remains the same: controlling the input current and the output voltage. Understanding these techniques and their practical applications is essential if working with transistor circuits.
Conclusion
There are many different biasing techniques for transistors. These techniques are derived from the basic biasing methods: fixed bias, emitter bias, collector feedback bias, and voltage divider bias. The fixed bias is simple but has stability issues. The emitter bias is relatively stable but still has low gain output. The collector feedback and voltage divider biasing provide improved stability and slightly better gain. Even among them, voltage dividers are the most common, while the collector feedback is rarely used.
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