Bipolar Junction Transistors

Bipolar Junction Transistors

Contents

Recall the Concept of Bipolar Junction Transistors(BJT) 1

Recall the Structure and Symbol of NPN and PNP Transistors 1

Describe the Mode of Operations of Bipolar Junction Transistors 2

Recall the Structure and Symbol of NPN and PNP Common-Base Transistors 3

Describe the Construction and Working of Common-Base Transistor 4

Describe the Input and Output Characteristics of Common-Base Transistors 5

Recall the Current components in Common-Base Transistors 6

Recall the Structure and Symbol of NPN and PNP Common-Emitter Transistors 7

Describe the Construction and Working of Common-Emitter Transistor 8

Describe the Input and Output Characteristics of Common-Emitter Transistor 9

Recall the DC Load Line and Quiescent Point 10

Recall Thermal Run-away and Stability Factor 11

Recall the Structure and Symbol of NPN and PNP Common-Collector Transistors 11

Describe the Construction and Working of Common-Collector Transistor 12

Describe the Input and Output Characteristics of Common-Collector Transistor 13

Recall the need of Biasing in BJT 13

Recall the types of DC Biasing of BJT 14

Describe the Fixed-Bias Circuit 14

Describe the Emitter-Bias Circuit 15

Recall the Voltage Divider or Self-Bias Configuration 15

Recall the Collector-to-Base Bias Configuration 17

Recall the Collector-to-Base Bias Configuration with Emitter Resistance 17

Recall the following Bias Compensation Techniques used in the Transistor i. Bias Compensation using Diode ii. Bias Compensation using Thermistor iii. Bias Compensation using Sensistor 18

Describe AC Load Line in BJT 19

Recall Q-Point in AC Load Line 20

Compare AC and DC Load Lines 20

Describe the Graphical Analysis of BJT Amplifier 25

Recall the Ebers-Moll Model of BJT 25

Recall the h-parameter models for Low Frequency Signals 26

Compute the following parameters in CE Configuration of BJT: i. Input Impedance ii. Output Impedance iii. Voltage Gain and Current Gain 27

Describe π-Model of BJT 27

Calculate the following parameters in Common Emitter Amplifier using re-Model: 28

i. Input Impedance ii. Output Impedance iii. Voltage Gain 28

Describe the High Frequency Model of BJT 29

Find out the Low Frequency Response of Common-Emitter BJT Amplifier 29

Find out the High Frequency Response of Common-Emitter BJT Amplifier 30

Show the Frequency Response Curve of Common-Emitter BJT Amplifier 31

Recall the Concept of Bipolar Junction Transistors(BJT)

A Bipolar Junction Transistor (BJT) is a type of three-layer, solid-state device that can be used as an amplifier, switch, or oscillator. It consists of two p-n junctions connected back-to-back, forming three regions: the emitter, the base, and the collector. The emitter is heavily doped and provides a large amount of current to the base, which is lightly doped and controls the flow of current. The collector is moderately doped and collects the current amplified by the transistor.

The BJT operates by controlling the flow of current from the emitter to the collector by varying the amount of current that flows into the base. When a small current is applied to the base, it controls a larger current from the emitter to the collector, allowing the BJT to act as an amplifier. When the base-emitter junction is forward-biased, the BJT acts as a switch, allowing current to flow from the emitter to the collector.

There are two types of BJTs: NPN and PNP. In an NPN transistor, the emitter and collector are n-type regions, while the base is p-type. In a PNP transistor, the emitter and collector are p-type regions, while the base is n-type. The choice of the type of BJT depends on the application and the required polarity of the input and output signals.

Recall the Structure and Symbol of NPN and PNP Transistors

The NPN and PNP bipolar junction transistors (BJTs) have different structures and symbols.

Structure of NPN Transistor:

The NPN transistor consists of three layers of semiconductor material: an n-type emitter, a p-type base, and an n-type collector. The emitter and collector regions are heavily doped, while the base region is lightly doped. The emitter-base junction is forward-biassed and the base-collector junction is reverse-biassed.

Structure of PNP Transistor:

The PNP transistor consists of three layers of semiconductor material: a p-type emitter, an n-type base, and a p-type collector. The emitter and collector regions are heavily doped, while the base region is lightly doped. The emitter-base junction is reverse-biassed and the base-collector junction is forward-biassed.

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Describe the Mode of Operations of Bipolar Junction Transistors

Bipolar Junction Transistors (BJTs) have three modes of operation:

  1. Active mode: In active mode, the base-emitter junction is forward-biassed and the base-collector junction is reverse-biassed. This mode of operation allows for current amplification as the small current flowing into the base controls a larger current flowing from the emitter to the collector.
  2. Saturation mode: In saturation mode, both the base-emitter and base-collector junctions are forward-biassed. This mode of operation acts as a switch, allowing current to flow from the emitter to the collector with very little voltage drop across the collector-emitter terminal.
  3. Cut-off mode: In cut-off mode, both the base-emitter and base-collector junctions are reverse-biassed. This mode of operation acts as an open switch, preventing current from flowing from the emitter to the collector.

The mode of operation of a BJT is determined by the voltage applied to its base-emitter and base-collector junctions. By controlling the base current, the BJT can be used as an amplifier or switch in a variety of electronic circuits.

Recall the Structure and Symbol of NPN and PNP Common-Base Transistors

The NPN and PNP bipolar junction transistors (BJTs) can be configured in different circuit arrangements, each with its own unique structure and symbol.

Structure of NPN Common-Base Transistor:

The NPN common-base transistor consists of three layers of semiconductor material: an n-type emitter, a p-type base, and an n-type collector. The emitter and collector regions are heavily doped, while the base region is lightly doped.

Symbol of NPN Common-Base Transistor:

The symbol of an NPN common-base transistor is similar to the general NPN transistor symbol, but with the emitter and collector reversed. The arrow in the symbol points towards the base and the emitter is on the right, while the collector is on the left.

Structure of PNP Common-Base Transistor:

The PNP common-base transistor consists of three layers of semiconductor material: a p-type emitter, an n-type base, and a p-type collector. The emitter and collector regions are heavily doped, while the base region is lightly doped.

Symbol of PNP Common-Base Transistor:

The symbol of a PNP common-base transistor is similar to the general PNP transistor symbol, but with the emitter and collector reversed. The arrow in the symbol points away from the base and the emitter is on the left, while the collector is on the right.

Describe the Construction and Working of Common-Base Transistor

Common-base transistors are one of the three basic configurations of bipolar junction transistors (BJTs) along with common-emitter and common-collector.

Construction:

The common-base configuration of a BJT consists of three layers of semiconductor material: an n-type emitter (for an NPN transistor) or a p-type emitter (for a PNP transistor), a base, and a collector. The emitter and collector regions are heavily doped, while the base region is lightly doped.

Working:

In a common-base configuration, the base terminal is common to both the input and output signals, while the emitter and collector terminals serve as the input and output terminals, respectively. When a small input current is applied to the emitter, it results in a larger output current flowing from the collector. The current gain in common-base configuration is usually less than unity, meaning the output current is less than the input current.

In an NPN common-base transistor, the emitter-base junction is forward-biassed, while the base-collector junction is reverse-biassed. In a PNP common-base transistor, the emitter-base junction is reverse-biassed, while the base-collector junction is forward-biassed.

The common-base configuration of a BJT is primarily used as an amplifier for low-frequency signals, where the input and output signals are both in phase. It is also used in switching applications, where it acts as a switch, connecting the input signal to the output signal when a small trigger voltage is applied to the base.

Describe the Input and Output Characteristics of Common-Base Transistors

The input and output characteristics of common-base (CB) transistors describe how the transistor behaves in terms of its input and output voltages and currents.

Input Characteristics:

The input characteristics of a CB transistor show the relationship between the base-emitter voltage (VBE) and the emitter current (IE) for a fixed collector current (IC). The input resistance of a CB transistor is the equivalent resistance between the emitter and base terminals, and it is typically low, on the order of a few hundred ohms.

Output Characteristics:

The output characteristics of a CB transistor show the relationship between the collector-emitter voltage (VCE) and the collector current (IC) for a fixed base current (IB). The output resistance of a CB transistor is the equivalent resistance between the collector and emitter terminals, and it is typically high, on the order of several kilo-ohms.

In general, the input and output characteristics of a CB transistor are relatively flat, meaning that small changes in the input or output voltages or currents result in relatively large changes in the other parameters. This behaviour is due to the low current gain of the CB configuration, which is typically less than unity.

The input and output characteristics of a CB transistor are useful for determining its operating point, which is the specific combination of input and output voltages and currents at which it operates in a particular circuit. The operating point is important because it determines the overall behaviour and performance of the transistor in a circuit, including its gain, stability, and linearity.

Recall the Current components in Common-Base Transistors

The common-base (CB) configuration of a bipolar junction transistor (BJT) involves three current components: emitter current (IE), collector current (IC), and base current (IB).

Emitter Current (IE):

The emitter current is the total current flowing into the emitter terminal of the CB transistor. It is composed of two components: the forward-biassed emitter current and the reverse-biassed collector current. The forward-biassed emitter current is due to the flow of majority carriers from the emitter to the base, while the reverse-biassed collector current is due to the flow of minority carriers from the base to the collector.

Collector Current (IC):

The collector current is the total current flowing out of the collector terminal of the CB transistor. It is equal to the sum of the forward-biassed emitter current and the reverse-biassed collector current.

Base Current (IB):

The base current is the current flowing into the base terminal of the CB transistor. It is much smaller than the emitter current, and it is responsible for controlling the emitter-base junction and the collector-base junction of the transistor. The base current is proportional to the emitter current, and it is used to determine the overall current gain of the CB configuration.

The three current components in a CB transistor are related by the current gain, which is the ratio of the collector current to the base current. The current gain of a CB configuration is usually less than unity, meaning that the output current is less than the input current. This low current gain is one of the main disadvantages of the CB configuration, compared to the common-emitter and common-collector configurations, which have higher current gains.

Recall the Structure and Symbol of NPN and PNP Common-Emitter Transistors

The common-emitter (CE) configuration of a bipolar junction transistor (BJT) is a three-layer device composed of a P-type semiconductor (base), an N-type semiconductor (emitter), and another N-type or P-type semiconductor (collector).

For NPN transistors, the base is a P-type semiconductor, the emitter is an N-type semiconductor, and the collector is another N-type semiconductor. The NPN transistor symbol is a triangle pointing towards the emitter terminal, with the collector terminal on the right and the base terminal on the left.

For PNP transistors, the base is an N-type semiconductor, the emitter is a P-type semiconductor, and the collector is another P-type semiconductor. The PNP transistor symbol is a triangle pointing away from the emitter terminal, with the collector terminal on the left and the base terminal on the right.

In both NPN and PNP transistors, the emitter terminal is connected to the base terminal, forming a forward-biased junction, and the collector terminal is connected to the base terminal, forming a reverse-biased junction. The CE configuration is characterised by the common connection of the emitter terminal to both the input and output signals, and it is widely used for amplification and switching applications.

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Describe the Construction and Working of Common-Emitter Transistor

The common-emitter (CE) configuration of a bipolar junction transistor (BJT) is a three-layer device that functions as an amplifier and switch. The CE configuration is characterised by the common connection of the emitter terminal to both the input and output signals.

Construction:

The CE configuration consists of a P-type semiconductor (base), an N-type semiconductor (emitter), and another N-type or P-type semiconductor (collector). For NPN transistors, the base is a P-type semiconductor, the emitter is an N-type semiconductor, and the collector is another N-type semiconductor. For PNP transistors, the base is an N-type semiconductor, the emitter is a P-type semiconductor, and the collector is another P-type semiconductor.

Working:

In the CE configuration, the emitter-base junction is forward-biassed, and the collector-base junction is reverse-biassed. This creates a potential difference across the collector-base junction, causing a flow of minority carriers from the emitter to the collector. The flow of minority carriers is amplified by the forward-biassed emitter-base junction, resulting in an increased collector current.

The base current (IB) controls the flow of minority carriers from the emitter to the collector. When the base current increases, the collector current also increases, which results in an increased collector-emitter voltage (VCE). The collector current is proportional to the base current, and the relationship between the two currents is determined by the current gain of the CE configuration. The current gain is the ratio of the collector current to the base current and is represented by the symbol “β” (beta).

The CE configuration is widely used for amplification and switching applications because of its high current gain, low output impedance, and high input impedance. The CE configuration is commonly used in electronic circuits, including audio amplifiers, operational amplifiers, and switching power supplies.

Describe the Input and Output Characteristics of Common-Emitter Transistor

The input and output characteristics of a common-emitter (CE) configuration of a bipolar junction transistor (BJT) describe the relationship between the input and output signals of the transistor.

Input Characteristics:

The input characteristics of a CE configuration describe the relationship between the base-emitter voltage (VBE) and the base current (IB). The input characteristics are typically plotted on a graph with VBE on the x-axis and IB on the y-axis. The input characteristic curve is a nonlinear curve that is used to determine the input impedance of the CE configuration. The input impedance is the resistance seen by the input signal and is a measure of the transistor’s ability to accept signals.

Output Characteristics:

The output characteristics of a CE configuration describe the relationship between the collector-emitter voltage (VCE) and the collector current (IC). The output characteristics are typically plotted on a graph with VCE on the x-axis and IC on the y-axis. The output characteristic curve is a non-linear curve that is used to determine the output impedance of the CE configuration. The output impedance is the resistance seen by the output signal and is a measure of the transistor’s ability to deliver signals.

The input and output characteristics of the CE configuration are important for understanding the performance of the transistor and for selecting the appropriate transistor for a given application. For example, the input and output characteristics can be used to determine the maximum input and output voltages and currents for a given transistor, as well as the maximum frequency response and gain of the transistor. The input and output characteristics can also be used to determine the appropriate biassing conditions for a given transistor, which are necessary to ensure stable and reliable operation of the transistor.

Recall the DC Load Line and Quiescent Point

The DC load line and the quiescent point are related concepts in bipolar junction transistor (BJT) electronics.

DC Load Line:

A DC load line is a graphical representation of the relationship between the collector-emitter voltage (VCE) and the collector current (IC) of a bipolar junction transistor (BJT). It is a line that shows the range of possible operating points of the transistor under a given set of biassing conditions. The load line is plotted on a graph with VCE on the y-axis and IC on the x-axis.

Quiescent Point (Q-Point):

The quiescent point (Q-point) is the operating point of the transistor in a given circuit when it is not conducting or amplifying a signal. It is the intersection of the DC load line with the transistor’s characteristic curve, which represents the relationship between VCE and IC under a specific set of biassing conditions. The Q-point determines the bias conditions of the transistor, which can impact its performance.

Recall Thermal Run-away and Stability Factor

Thermal runaway refers to a phenomenon where an increase in temperature of a material or system leads to a further increase in temperature, resulting in a self-sustaining process. This can happen when there is a positive feedback loop between temperature and a property such as resistance or reaction rate.

Stability factor (SF) is a measure of how far a system is from thermal runaway. It is defined as the ratio of the change in heat produced by a reaction to the change in heat absorbed by the system. A SF greater than 1 indicates that the system is unstable and prone to thermal runaway, while a SF less than 1 indicates that the system is stable.

Recall the Structure and Symbol of NPN and PNP Common-Collector Transistors

NPN and PNP are two types of bipolar junction transistors (BJTs). NPN transistors have a structure with a layer of p-doped semiconductor material sandwiched between two n-doped layers, while PNP transistors have a structure with a layer of n-doped semiconductor material sandwiched between two p-doped layers.

The symbol for an NPN transistor in a common-collector configuration is a triangle pointing towards the emitter, with the collector at the bottom and the base in the centre. The symbol for a PNP transistor in a common-collector configuration is a triangle pointing away from the emitter, with the collector at the bottom and the base in the centre.

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Describe the Construction and Working of Common-Collector Transistor

A common-collector transistor, also known as an emitter-follower, is a configuration of a bipolar junction transistor (BJT) that is used as an amplifier or voltage buffer.

The construction of a common-collector transistor involves connecting the collector terminal to the output, the base terminal to the input, and the emitter terminal to a voltage source. The base-emitter junction of the transistor is forward-biassed, allowing current to flow from the emitter to the base. This, in turn, controls the current flow from the collector to the emitter, producing a voltage gain that is less than unity.

The working of a common-collector transistor can be explained by considering the input signal applied to the base and the resulting output signal at the collector. When a small input voltage is applied to the base, it generates a small current through the base-emitter junction. This current is amplified by the transistor’s current gain, and the resulting output current flows from the collector to the emitter. The collector voltage follows the input voltage with a slight lag, and the voltage gain of the circuit is less than unity but still greater than zero.

The common-collector configuration is often used for applications where a low voltage gain is acceptable, but a high input impedance and a low output impedance are desired. This is because the common-collector transistor has a high input impedance and a low output impedance, making it useful for interfacing between stages with different impedance levels.

Describe the Input and Output Characteristics of Common-Collector Transistor

Input characteristics are the relationship between the input current and input voltage keeping output voltage constant. Here input current is IB and input voltage VBE and output voltage is VCE.

The output voltage VCE initially kept at 3V and kept constant. The input voltage VBE is increased from zero gradually and the corresponding input current IB is noted.

Then the output voltage which is kept constant is increased, let’s say 5V and kept constant and output voltage is gradually increased and input current is noted. Graph is drawn with all the values noted.

Output characteristics are the relationship between the output current and output voltage keeping input current constant. Here output current is IE and output voltage is VCE and the input current is IB.

Initially the input current IB is kept at zero and kept constant. Slowly input current IB is increased like 10µA,20µA and kept constant and the output voltage VCE is increased gradually from zero and the corresponding output current IE is noted.

When the input current is zero no current flows in the transistor and it is called the cut off region. When the input current is very high the current through the transistor is also very high and the transistor will be in the saturation region.

The region where there is a change in the output current for the change in output voltage is the active region. Here the active region almost looks flat.

Recall the need of Biasing in BJT

The steady state operation of a bipolar transistor depends a great deal on its base current, collector voltage, and collector current values. Therefore, if the transistor is to operate correctly as a linear amplifier, it must be properly biassed around its operating point as improper transistor biassing will result in a distorted output.

Establishing the correct operating point requires the selection of bias resistors and load resistors to provide the appropriate input current and collector voltage conditions. The correct biassing point for a bipolar transistor, either NPN or PNP, generally lies somewhere between the two extremes of operation with respect to it being either “fully-ON” or “fully-OFF” along its DC load line. This central operating point is called the “Quiescent Operating Point”, or Q-point for short.

Recall the types of DC Biasing of BJT

There are three types of DC biassing of BJT (Bipolar Junction Transistor):

  1. Fixed bias: In this type of biassing, a voltage divider network is used to establish a fixed base voltage. It is the simplest type of biassing, but it is not very stable and sensitive to temperature variations.
  2. Collector to base bias: In this type of biassing, the collector-base junction is reverse biassed, and a voltage source is applied between the collector and the base. This biassing method provides good stability and high voltage gain.
  3. Emitter bias: In this type of biassing, a voltage divider network is connected between the emitter and ground, providing a stable bias voltage. This type of biassing is widely used in amplifier circuits as it provides good stability and temperature compensation.

Describe the Fixed-Bias Circuit

This type of transistor biassing arrangement is also beta dependent biassing as the steady-state condition of operation is a function of the transistors beta β value, so the biassing point will vary over a wide range for transistors of the same type as the characteristics of the transistors will not be exactly the same.

The emitter diode of the transistor is forward biassed by applying the required positive base bias voltage via the current limiting resistor RB. Assuming a standard bipolar transistor, the forward base-emitter voltage drop would be 0.7V. Then the value of RB is simply: (VCC – VBE)/IB where IB is defined as IC/β.

With this single resistor type of biassing arrangement the biassing voltages and currents do not remain stable during transistor operation and can vary enormously. Also the operating temperature of the transistor can adversely affect the operating point.

Describe the Emitter-Bias Circuit

One of the most frequently used biassing circuits for a transistor circuit is with the self-biassing of the emitter-bias circuit were one or more biassing resistors are used to set up the initial DC values for the three transistor currents, ( IB ), ( IC ) and ( IE ).

The two most common forms of bipolar transistor biassing are: Beta Dependent and Beta Independent. Transistor bias voltages are largely dependent on transistor beta, ( β ) so the biassing set up for one transistor may not necessarily be the same for another transistor as their beta values may be different. Transistor biassing can be achieved either by using a single feedback resistor or by using a simple voltage divider network to provide the required biassing voltage.

Recall the Voltage Divider or Self-Bias Configuration

Here the common emitter transistor configuration is biassed using a voltage divider network to increase stability. The name of this biassing configuration comes from the fact that the two resistors RB1 and RB2 form a voltage or potential divider network across the supply with their centre point junction connected to the transistors base terminal as shown.

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This voltage divider biassing configuration is the most widely used transistor biassing method. The emitter diode of the transistor is forward biassed by the voltage value developed across resistor RB2. Also, voltage divider network biassing makes the transistor circuit independent of changes in beta as the biassing voltages set at the transistors base, emitter, and collector terminals are not dependent on external circuit values.

To calculate the voltage developed across resistor RB2 and therefore the voltage applied to the base terminal we simply use the voltage divider formula for resistors in series.

Generally the voltage drop across resistor RB2 is much less than for resistor RB1. Clearly the transistors base voltage VB with respect to ground, will be equal to the voltage across RB2.

The amount of biassing current flowing through resistor RB2 is generally set to 10 times the value of the required base current IB so that it is sufficiently high enough to have no effect on the voltage divider current or changes in Beta.

Recall the Collector-to-Base Bias Configuration

The Collector-to-Base Bias Configuration is a type of DC biassing technique used in Bipolar Junction Transistors (BJTs). In this configuration, the collector-base junction is reverse biassed, and a voltage source is applied between the collector and the base.

The voltage divider network consisting of Rb and the base-emitter junction provides the base bias voltage, while the collector resistor Rc limits the collector current. The reverse-biassed collector-base junction ensures that the transistor remains in the active region of operation.

The Collector-to-Base Bias Configuration provides good stability and high voltage gain, making it a popular choice for amplifier circuits. However, it may suffer from thermal instability due to the dependence of the bias voltage on the transistor’s temperature coefficient.

Recall the Collector-to-Base Bias Configuration with Emitter Resistance

This type of transistor biassing configuration, often called self-emitter biassing, uses both emitter and base-collector feedback to stabilise the collector current even further. This is because resistors RB1 and RE as well as the base-emitter junction of the transistor are all effectively connected in series with the supply voltage, VCC.

The downside of this emitter feedback configuration is that it reduces the output gain due to the base resistor connection. The collector voltage determines the current flowing through the feedback resistor, RB1 producing what is called “degenerative feedback”.

The current flowing from the emitter, IE (which is a combination of IC + IB) causes a voltage drop to appear across RE in such a direction, that it reverse biases the base-emitter junction.

So if the emitter current increases, due to an increase in collector current, voltage drop I*RE also increases. Since the polarity of this voltage reverse biases the base-emitter junction, IB automatically decreases. Therefore the emitter current increases less than it would have done had there been no self biassing resistor.

Generally, resistor values are set so that the voltage dropped across the emitter resistor RE is approximately 10% of VCC and the current flowing through resistor RB1 is 10% of the collector current IC.

Thus this type of transistor biassing configuration works best at relatively low power supply voltage.

Recall the following Bias Compensation Techniques used in the Transistor i. Bias Compensation using Diode ii. Bias Compensation using Thermistor iii. Bias Compensation using Sensistor

These are the circuits that implement compensation techniques using diodes to deal with biassing instability. The stabilisation techniques refer to the use of resistive biassing circuits which permit IB to vary so as to keep IC relatively constant.

There are two types of diode compensation methods. They are −

  • Diode compensation for instability due to VBE variation
  • Diode compensation for instability due to ICO variation

Thermistor is a temperature sensitive device. It has a negative temperature coefficient. The resistance of a thermistor increases when the temperature decreases and it decreases when the temperature increases.In an amplifier circuit, the changes that occur in ICO, VBE and β with temperature, increases the collector current. Thermistor is employed to minimise the increase in collector current. As the temperature increases, the resistance RT of the thermistor decreases, which increases the current through it and the resistor RE. Now, the voltage developed across RE increases, which reverse biases the emitter junction. This reverse bias is so high that the effect of resistors R1 and R2 providing forward bias also gets reduced. This action reduces the rise in collector current.

Thus the temperature sensitivity of the thermistor compensates for the increase in collector current, occurring due to temperature.

A Sensistor is a heavily doped semiconductor that has a positive temperature coefficient. The resistance of a Sensistor increases with the increase in temperature and decreases with the decrease in temperature. The Sensistor may be placed in parallel with R1 or in parallel with RE. As the temperature increases, the resistance of the parallel combination, thermistor and R1 increases and their voltage drop also increases. This decreases the voltage drop across R2. Due to the decrease of this voltage, the net forward emitter bias decreases. As a result of this, IC decreases.

Hence by employing the Sensistor, the rise in the collector current which is caused by the increase of ICO, VBE and β due to temperature, gets controlled.

Describe AC Load Line in BJT

In a Bipolar Junction Transistor (BJT), the AC Load Line represents the locus of all possible instantaneous collector current and voltage combinations for a given load resistance under small-signal conditions. It is a graphical representation of the small-signal amplifier circuit behaviour.

The AC Load Line is obtained by superimposing the small-signal AC signal on top of the DC operating point, which is given by the DC Load Line. The DC Load Line is the locus of all possible collector current and voltage combinations for a given fixed biassing condition.

The slope of the AC Load Line is determined by the load resistance, while its intercept with the vertical axis represents the maximum possible collector current for the given load resistance. The intersection of the AC Load Line with the DC Load Line represents the small-signal operating point of the amplifier circuit.

The AC Load Line is useful in determining the small-signal performance of an amplifier circuit. It helps in identifying the maximum output voltage swing, the voltage gain, the output impedance, and the power dissipation of the amplifier.

A properly designed AC Load Line ensures that the amplifier operates in the linear region of the BJT’s transfer characteristics, which results in faithful reproduction of the input signal with minimum distortion.

Recall Q-Point in AC Load Line

The Q-point in the AC Load Line is the operating point of a transistor amplifier under small-signal conditions. It is the intersection point of the AC Load Line with the DC Load Line and represents the DC biassing point of the transistor.

The Q-point is important because it determines the amplifier’s operating point in terms of collector current and voltage. It also affects the amplifier’s voltage gain, power dissipation, and distortion.

If the Q-point is too close to the saturation region, the amplifier may clip the output signal, resulting in distortion. On the other hand, if the Q-point is too close to the cutoff region, the amplifier may not provide sufficient gain or may be unable to deliver the required output power.

Therefore, it is essential to choose the correct biassing conditions to ensure that the Q-point is located in the linear region of the transistor’s transfer characteristics. This ensures that the amplifier operates in a stable and predictable manner and provides faithful reproduction of the input signal with minimum distortion.

The Q-point can be adjusted by changing the biassing resistors or by using a biassing circuit such as a voltage divider or a biassing network. The AC Load Line provides a graphical representation of the amplifier’s small-signal performance and helps in selecting the appropriate biassing conditions for the desired operating point.

Compare AC and DC Load Lines

The AC Load Line and the DC Load Line are two graphical representations used in Bipolar Junction Transistor (BJT) amplifier circuits, but they serve different purposes and have some key differences.

The DC Load Line represents the locus of all possible combinations of collector current (Ic) and collector-emitter voltage (Vce) for a given fixed biassing condition under DC (direct current) operating conditions. The DC Load Line is used to determine the DC operating point, also known as the Q-point, which is the DC biassing point of the transistor. The DC Load Line is a straight line that is determined by the collector resistance (Rc) and the supply voltage (Vcc).

On the other hand, the AC Load Line represents the locus of all possible combinations of Ic and Vce for a given load resistance under small-signal AC (alternating current) conditions. The AC Load Line is obtained by superimposing the small-signal AC signal on top of the DC operating point. The AC Load Line helps in determining the maximum output voltage swing, voltage gain, output impedance, and power dissipation of the amplifier circuit.

The key differences between the AC Load Line and the DC Load Line are:

  1. Representation: The DC Load Line represents the DC operating point, while the AC Load Line represents the small-signal performance of the amplifier circuit.
  2. Shape: The DC Load Line is a straight line, while the AC Load Line is a curve that depends on the load resistance.
  3. Slope: The slope of the DC Load Line is determined by the collector resistance, while the slope of the AC Load Line is determined by the load resistance.
  4. Intercept: The intercept of the DC Load Line with the vertical axis represents the bias voltage, while the intercept of the AC Load Line with the vertical axis represents the maximum possible collector current for the given load resistance.

A tabular comparison highlighting the differences between the AC load line and the DC load line:

AC Load Line DC Load Line
Definition Represents the locus of all possible instantaneous AC voltages and currents in a circuit for a given load. Represents the locus of all possible DC voltages and currents in a circuit for a given load.
Analysis Purpose Used to determine the operating point and the maximum power transfer in AC circuits. Used to determine the operating point and the biasing conditions in DC circuits.
Type of Circuit AC circuits, which involve time-varying signals. DC circuits, which involve constant (or time-invariant) signals.
Representation Plots the relationship between voltage and current for each instant in an AC cycle. Shows the relationship between voltage and current at a steady state or constant voltage level.
Load Resistance Takes into account both the resistance and the reactance (inductive or capacitive) of the load. Considers only the resistance of the load.
Shape The AC load line is typically nonlinear due to reactance. The DC load line is a straight line with a slope determined by the load resistance.
Maximum Power Transfer The maximum power transfer occurs when the load line intersects the AC load line at its peak value. The maximum power transfer occurs when the load line intersects the DC load line at its midpoint.
Impedance Matching AC load lines can be adjusted to match the load impedance for optimal power transfer. DC load lines are not adjusted for impedance matching as they represent fixed operating points.
Application Used in analyzing amplifiers, oscillators, and other AC-based circuits. Used in analyzing transistor biasing, amplifier quiescent points, and other DC-based circuits.

Note: The AC load line and DC load line are analysis tools used in different contexts to understand the behavior and operating points of AC and DC circuits, respectively. They serve different purposes and considerations based on the characteristics of the circuits and the signals involved.

Describe the Graphical Analysis of BJT Amplifier

The graphical analysis of a bipolar junction transistor (BJT) amplifier involves using two plots to determine the transistor’s operating point and the transfer characteristic of the amplifier circuit.

  1. Load Line Plot: It is a graph of the output current and voltage of the BJT amplifier. The load line is drawn based on the voltage and current limits of the transistor, and it helps to determine the operating point of the transistor.
  2. Transfer Characteristic Plot: It is a graph of the output current and input voltage of the BJT amplifier. The transfer characteristic is plotted by sweeping the input voltage and measuring the corresponding output current. The transfer characteristic shows the relationship between the input and output signals, and it helps to determine the gain of the amplifier.

By combining the load line and transfer characteristic plots, the operating point of the transistor can be determined and the gain of the amplifier can be calculated. This graphical analysis method is useful for understanding the behaviour of BJT amplifiers and for optimising the performance of the amplifier circuit.

Recall the Ebers-Moll Model of BJT

The Ebers-Moll model of a bipolar junction transistor (BJT) is a mathematical model that describes the behaviour of a BJT in terms of its current and voltage characteristics. The model is based on the physical structure of a BJT and the principles of charge transport through a semiconductor.

The Ebers-Moll model uses three equations to describe the current flow through the BJT:

  1. The Collector Current Equation: describes the current flow from the collector to the emitter as a function of the base-emitter voltage, the collector-emitter voltage, and the temperature.
  2. The Base Current Equation: describes the current flow from the base to the emitter as a function of the base-emitter voltage, the collector-emitter voltage, and the temperature.
  3. The Emitter Current Equation: describes the current flow from the emitter to the collector as a function of the base-emitter voltage, the collector-emitter voltage, and the temperature.

The Ebers-Moll model is a useful tool for analysing the behaviour of BJT circuits and for designing and optimising BJT-based amplifiers. The model provides a quantitative description of the relationships between the input and output signals and the internal voltages and currents in the BJT.

Recall the h-parameter models for Low Frequency Signals

The h-parameter models for low frequency signals are mathematical models used to describe the behaviour of bipolar junction transistors (BJTs) and field-effect transistors (FETs) in common-emitter and common-source configurations. These models provide a simplified and equivalent representation of the device’s transfer characteristics.

The h-parameters are defined as:

  1. hfe (forward current transfer ratio): represents the ratio of the collector current to the base current.
  2. hoe (output conductance): represents the ratio of the change in collector current to the change in emitter-to-collector voltage.
  3. hie (input impedance): represents the impedance looking into the base terminal.
  4. hre (reverse transfer coefficient): represents the ratio of the change in collector voltage to the change in base-to-emitter voltage.

The h-parameter models are useful for analysing and designing low frequency amplifier circuits, as they provide a simple and intuitive representation of the device’s behaviour. The h-parameters can be used to calculate the input and output impedances, the voltage and current gains, and the frequency response of the amplifier circuit.

Compute the following parameters in CE Configuration of BJT: i. Input Impedance ii. Output Impedance iii. Voltage Gain and Current Gain

In a common-emitter (CE) configuration of a bipolar junction transistor (BJT), the following parameters can be calculated:

  1. Input Impedance (hie): The input impedance of a BJT in CE configuration is given by the formula hie = (beta + 1) * re, where beta is the current gain of the BJT and re is the emitter resistance.
  2. Output Impedance (hoe): The output impedance of a BJT in CE configuration is given by the formula hoe = beta / (beta + 1) * re, where beta is the current gain of the BJT and re is the emitter resistance.
  3. Voltage Gain (Av): The voltage gain of a BJT in CE configuration is given by the formula Av = – beta * (Rc / (Rc + re)), where beta is the current gain of the BJT, Rc is the collector resistance, and re is the emitter resistance.
  4. Current Gain (hfe): The current gain of a BJT in CE configuration is given by the formula hfe = beta, where beta is the current gain of the BJT.

It is important to note that the values of beta, Rc, and re depend on the operating conditions of the BJT and may vary with temperature and other factors.

Describe π-Model of BJT

The model and the π-model are two equivalent circuit models used to describe the behaviour of bipolar junction transistors (BJTs) in common-emitter (CE) configuration.

This model represents the BJT as a simple resistor, connected in series with the base-emitter junction. The value of re is equivalent to the dynamic resistance of the base-emitter junction and represents the resistance seen looking into the base terminal of the BJT. The e model is useful for simple calculations and for understanding the basic behaviour of BJTs.

This model represents the BJT as a combination of two resistors and a current source. The two resistors, re and rπ, are connected in series between the base and the collector, and the current source is connected between the collector and the emitter. The value of re represents the dynamic resistance of the base-emitter junction and π represents the internal resistance of the BJT. The π-model provides a more accurate representation of the BJT’s behaviour and is useful for more complex calculations and analysis.

The π-model provides useful representations of the behaviour of BJTs in common-emitter configuration, and they are commonly used in the design and analysis of BJT-based amplifier circuits.

Calculate the following parameters in Common Emitter Amplifier using re-Model:

i. Input Impedance ii. Output Impedance iii. Voltage Gain

i. Input Impedance: In a common emitter amplifier using the hybrid-π model, the input impedance can be calculated as the parallel combination of the base-emitter junction resistance (rπ) and the emitter resistance (re).

Input impedance = (rπ || re)

ii. Output Impedance: The output impedance in a common emitter amplifier can be calculated as the collector resistance (RC).

Output impedance = RC

iii. Voltage Gain: The voltage gain of a common emitter amplifier using the hybrid-π model can be calculated as the ratio of the change in collector voltage to the change in base voltage. It can be given by the following equation:

Voltage gain = -α * (RC || re) / rπ

where α is the common-emitter current gain (typically around 0.98).

Describe the High Frequency Model of BJT

The high frequency model of a bipolar junction transistor (BJT) is a model used to describe the behaviour of the transistor at high frequencies, typically above a few megahertz. This model is an extension of the hybrid-π model and is also known as the “pi-equivalent circuit”.

The high frequency model of a BJT includes the addition of two capacitors: a capacitance between the base and the collector (Cbc) and a capacitance between the base and the emitter (Cbe). These capacitors account for the transit time of electrons and holes as they travel through the base region of the transistor.

In the high frequency model, the input impedance is given by the sum of the base-emitter junction resistance (rπ) in parallel with the base-collector capacitance (Cbc), and the output impedance is given by the collector resistance (RC) in parallel with the base-emitter capacitance (Cbe).

The high frequency model of a BJT is an important tool for the design of high frequency circuits and is used to determine the stability, bandwidth, and frequency response of transistor amplifiers.

Find out the Low Frequency Response of Common-Emitter BJT Amplifier

The low frequency response of a common-emitter bipolar junction transistor (BJT) amplifier is the behaviour of the amplifier with respect to changes in frequency at low frequencies, typically below a few megahertz.

At low frequencies, the common-emitter BJT amplifier provides a high voltage gain that is approximately equal to the current gain (beta or hFE) of the transistor. The voltage gain can be calculated as follows:

Av = – β * (RE / (RE + REE))

where Av is the voltage gain, beta (β) is the current gain of the transistor, RE is the emitter resistance, and REE is the effective emitter resistance.

In the low frequency response of a common-emitter BJT amplifier, the input impedance is determined by the base-emitter resistance (rBE) of the transistor and the emitter resistance (RE), and the output impedance is determined by the collector resistance (RC).

It’s important to note that at higher frequencies, the voltage gain of the common-emitter BJT amplifier will decrease, as the frequency response becomes limited by the internal capacitances of the transistor and the parasitic capacitances of the circuit.

Find out the High Frequency Response of Common-Emitter BJT Amplifier

The high frequency response of a common-emitter bipolar junction transistor (BJT) amplifier is the behaviour of the amplifier with respect to changes in frequency. It is an important consideration in the design of high frequency circuits and is typically characterised by the 3 dB cutoff frequency, also known as the half-power frequency.

The 3 dB cutoff frequency of a common-emitter BJT amplifier can be calculated using the high frequency model of the BJT and the RC circuit of the emitter circuit. The 3 dB cutoff frequency is given by the following equation:

f3 dB = 1 / (2 * π * (RC || re) * (Cbe + Cbc))

where RC is the collector resistance, re is the emitter resistance, Cbe is the base-emitter capacitance, and Cbc is the base-collector capacitance. The 3 dB cutoff frequency represents the frequency at which the voltage gain of the amplifier has decreased by a factor of 1/√2 or -3 dB relative to its low frequency gain.

It’s important to note that at frequencies higher than the 3 dB cutoff frequency, the gain of the common-emitter BJT amplifier will continue to decrease and eventually become negligible, limiting the useful frequency range of the amplifier.

Show the Frequency Response Curve of Common-Emitter BJT Amplifier

The frequency response curve of a common-emitter bipolar junction transistor (BJT) amplifier is a graph that shows the relationship between the frequency and the voltage gain of the amplifier. It is used to visualise the behaviour of the amplifier with respect to changes in frequency.

Typically, the frequency response curve is plotted on a logarithmic scale, with frequency on the x-axis and voltage gain (in dB) on the y-axis. The curve starts at a high gain at low frequencies and decreases as the frequency increases, reaching a minimum at the 3 dB cutoff frequency (also known as the half-power frequency). Beyond the 3 dB cutoff frequency, the gain continues to decrease, eventually becoming negligible at very high frequencies.

The frequency response curve is a useful tool for the design of high frequency circuits, as it provides a clear visual representation of the amplifier’s behaviour with respect to frequency. It allows designers to determine the frequency range over which the amplifier will provide an acceptable level of gain, and to make design decisions to improve the performance of the amplifier, such as adding frequency-compensating components.