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Aa lecture1 (1)

Kidane Tesfay
Kidane Tesfay

This document discusses bipolar junction transistors (BJTs). It describes the construction and operation of BJTs, including that they consist of either two n-type and one p-type layers or vice versa. It also covers the three main BJT configurations: common-base, common-emitter, and common-collector. For each configuration, it explains the terminal names, biasing, and provides illustrations of their input and output characteristics curves.

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Analog Electronics - I Chapter 2 Transistors
outline Introduction Bipolar Junction Trnasistor (BJT) Construction of BJT Operation of Transistors BJT configurations Common- base configuration Common-emmiter configuration Common-collector configuration
2.1 Introduction The invention of transistors (in 1947) marked the beginning of revolution in electronics. The pace of the revolution was accelerated a decade later by the development of the integrated circuit (IC or silicon chip). Transistors are broadly classified into two types: Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET).
2.2 Bipolar Junction Transistor (BJT) Construction: The transistor is a three-layer semiconductor device consisting of either two n- and one p-type layers of material or two p- and one n-type layers of material. The former is called an npn transistor, while the latter is called a pnp transistor. The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material. If only one carrier is employed (electron or hole), it is considered a unipolar device.
2.2.1 Construction of BJT
Construction of BJT In the previous diagrams, the terminals have been indicated by the capital letters E for emitter, C for collector, and B for base.
2.2.2 Transistor Operation Consider the pnp trnasistor of previous figure (Fig. (a) ) to describe the basic operation of a BJT transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. We investigate the two junctions of pnp transistor (separately).
Forward-biased junction of a pnp transistor (without base-to- collector bias). The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p- to the n- type material. It is similar to forward biased diode.
Reverse-biased junction of a pnp transistor (without base-to-emitter bias). Flow of majority carriers is zero, resulting in only a minority-carrier flow. Similar to reverse biased diode. The number of uncovered negative ions in the depletion region of the P- type material will increase due to the large number of free holes drawn to the negative potential of the applied voltage. The effect is a widening of the depletion region.
Operation of BJT In general: One p-n junction of a transistor is reverse biased, while the other is forward biased. Both biasing potentials are applied (normal operation), which results in Majority and minority carrier flow of a pnp transistor, as follows.
Operation of BJT
Operation of BJT The widths of the depletion regions, indicating clearly which junction is forward-biased and which is reverse-biased. A large number of majority carriers will diffuse across the forward-biased p-n junction into the n-type material. Since the sandwiched n-type material is very thin and has a low conductivity, a very small number of these carriers will take this path of high resistance to the base terminal.
Operation of BJT The larger number of these majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal. Applying Kirchhoffs current law to the transistor of previous Fig. : IE = IC + IB The collector current, however, is comprised of two componentsthe majority and minority carriers. The minority current component is called the leakage current and is given the symbol ICO (IC current with emitter terminal Open). The collector current is then given by IC = ICmajority + ICOminority
2.2.3 BJT Configurations A) Common base configuration In common base configuration, the base is common to both the input and output sides of the configuration. The arrow in the graphic symbol defines the direction of emitter current (conventional flow) through the device.
Common base configuration
Common base configuration
Common base configuration To fully describe the behavior of a three-terminal device such as the common base amplifiers of above figure, it requires two sets of characteristicsone for the driving point or input parameters and the other for the output side. The input set for the common-base amplifier, as shown in Fig. below, relates an input current (IE) to an input voltage (VBE) for various levels of output voltage (VCB).
Common base configuration
Common base configuration The output set relates an output current (IC) to an output voltage (VCB) for various levels of input current (IE) as shown in following figure.
Common base configuration Click to edit Master text styles Second level Third level Fourth level Fifth level
Common base configuration The output or collector set of characteristics has three basic regions of interest, as indicated in the previous figure - the active, cutoff, and saturation regions. The active region is the region normally employed for linear (undistorted) amplifiers. In particular : In the active region the collector-base junction is reverse-biased, while the base- emitter junction is forward-biased.
Common base configuration The curves clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC IE . As inferred by its name, the cutoff region is defined as that region where the collector current is 0 A. In addition, in cutoff region the collector- base and base-emitter junctions of a transistor are both reverse-biased.
Common base configuration
Common base configuration The ac alpha is formally called the common-base, short-circuit, amplification factor
B) Common emitter configuration It is called the common-emitter configuration since the emitter is common or reference to both the input and output terminals (in this case common to both the base and collector terminals). Notation and symbols used with the common-emitter configuration (for both npn and pnp transistors) are shown in the following figure.
Common emitter configuration
Common emitter configuration Even though the transistor configuration has changed, the current relations developed earlier for the common-base configuration are still applicable . That is, IE = IC + IB and IC = 畩IE. For the common-emitter configuration the output characteristics are a plot of the output current (IC) versus output voltage (VCE) for a range of values of input current (IB). The input characteristics are a plot of the input current (IB) versus the input voltage (VBE) for a range of values of output voltage (VCE). Both plots are shown in the following figure.
Common emitter configuration
Common emitter configuration The curves of IB are not as horizontal as those obtained for IE in the common-base configuration, indicating that the collector-to-emitter voltage will influence the magnitude of the collector current. Note on the collector characteristics of the previous Fig. that IC is not equal to zero when IB is zero. For the common-base configuration, when the input current IE was equal to zero, the collector current was equal only to the reverse saturation current ICO, so that the curve IE = 0 and the voltage axis were, for all practical purposes, one. The reason for this difference in collector characteristics can be derived as follows:
Common emitter configuration If we consider the case IB = 0 A, and substitute a typical value of 畩 such as 0.996, the resulting collector current is the following:
Common emitter configuration
Common emitter configuration
Common emitter configuration
Common emitter configuration
Common emitter configuration
Common emitter configuration
C. Common collector configurations Reading assignment

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Aa lecture1 (1)

  • 1. Analog Electronics - I Chapter 2 Transistors
  • 2. outline Introduction Bipolar Junction Trnasistor (BJT) Construction of BJT Operation of Transistors BJT configurations Common- base configuration Common-emmiter configuration Common-collector configuration
  • 3. 2.1 Introduction The invention of transistors (in 1947) marked the beginning of revolution in electronics. The pace of the revolution was accelerated a decade later by the development of the integrated circuit (IC or silicon chip). Transistors are broadly classified into two types: Bipolar Junction Transistor (BJT) and Field Effect Transistor (FET).
  • 4. 2.2 Bipolar Junction Transistor (BJT) Construction: The transistor is a three-layer semiconductor device consisting of either two n- and one p-type layers of material or two p- and one n-type layers of material. The former is called an npn transistor, while the latter is called a pnp transistor. The term bipolar reflects the fact that holes and electrons participate in the injection process into the oppositely polarized material. If only one carrier is employed (electron or hole), it is considered a unipolar device.
  • 6. Construction of BJT In the previous diagrams, the terminals have been indicated by the capital letters E for emitter, C for collector, and B for base.
  • 7. 2.2.2 Transistor Operation Consider the pnp trnasistor of previous figure (Fig. (a) ) to describe the basic operation of a BJT transistor. The operation of the npn transistor is exactly the same if the roles played by the electron and hole are interchanged. We investigate the two junctions of pnp transistor (separately).
  • 8. Forward-biased junction of a pnp transistor (without base-to- collector bias). The depletion region has been reduced in width due to the applied bias, resulting in a heavy flow of majority carriers from the p- to the n- type material. It is similar to forward biased diode.
  • 9. Reverse-biased junction of a pnp transistor (without base-to-emitter bias). Flow of majority carriers is zero, resulting in only a minority-carrier flow. Similar to reverse biased diode. The number of uncovered negative ions in the depletion region of the P- type material will increase due to the large number of free holes drawn to the negative potential of the applied voltage. The effect is a widening of the depletion region.
  • 10. Operation of BJT In general: One p-n junction of a transistor is reverse biased, while the other is forward biased. Both biasing potentials are applied (normal operation), which results in Majority and minority carrier flow of a pnp transistor, as follows.
  • 12. Operation of BJT The widths of the depletion regions, indicating clearly which junction is forward-biased and which is reverse-biased. A large number of majority carriers will diffuse across the forward-biased p-n junction into the n-type material. Since the sandwiched n-type material is very thin and has a low conductivity, a very small number of these carriers will take this path of high resistance to the base terminal.
  • 13. Operation of BJT The larger number of these majority carriers will diffuse across the reverse-biased junction into the p-type material connected to the collector terminal. Applying Kirchhoffs current law to the transistor of previous Fig. : IE = IC + IB The collector current, however, is comprised of two componentsthe majority and minority carriers. The minority current component is called the leakage current and is given the symbol ICO (IC current with emitter terminal Open). The collector current is then given by IC = ICmajority + ICOminority
  • 14. 2.2.3 BJT Configurations A) Common base configuration In common base configuration, the base is common to both the input and output sides of the configuration. The arrow in the graphic symbol defines the direction of emitter current (conventional flow) through the device.
  • 17. Common base configuration To fully describe the behavior of a three-terminal device such as the common base amplifiers of above figure, it requires two sets of characteristicsone for the driving point or input parameters and the other for the output side. The input set for the common-base amplifier, as shown in Fig. below, relates an input current (IE) to an input voltage (VBE) for various levels of output voltage (VCB).
  • 19. Common base configuration The output set relates an output current (IC) to an output voltage (VCB) for various levels of input current (IE) as shown in following figure.
  • 20. Common base configuration Click to edit Master text styles Second level Third level Fourth level Fifth level
  • 21. Common base configuration The output or collector set of characteristics has three basic regions of interest, as indicated in the previous figure - the active, cutoff, and saturation regions. The active region is the region normally employed for linear (undistorted) amplifiers. In particular : In the active region the collector-base junction is reverse-biased, while the base- emitter junction is forward-biased.
  • 22. Common base configuration The curves clearly indicate that a first approximation to the relationship between IE and IC in the active region is given by IC IE . As inferred by its name, the cutoff region is defined as that region where the collector current is 0 A. In addition, in cutoff region the collector- base and base-emitter junctions of a transistor are both reverse-biased.
  • 24. Common base configuration The ac alpha is formally called the common-base, short-circuit, amplification factor
  • 25. B) Common emitter configuration It is called the common-emitter configuration since the emitter is common or reference to both the input and output terminals (in this case common to both the base and collector terminals). Notation and symbols used with the common-emitter configuration (for both npn and pnp transistors) are shown in the following figure.
  • 27. Common emitter configuration Even though the transistor configuration has changed, the current relations developed earlier for the common-base configuration are still applicable . That is, IE = IC + IB and IC = 畩IE. For the common-emitter configuration the output characteristics are a plot of the output current (IC) versus output voltage (VCE) for a range of values of input current (IB). The input characteristics are a plot of the input current (IB) versus the input voltage (VBE) for a range of values of output voltage (VCE). Both plots are shown in the following figure.
  • 29. Common emitter configuration The curves of IB are not as horizontal as those obtained for IE in the common-base configuration, indicating that the collector-to-emitter voltage will influence the magnitude of the collector current. Note on the collector characteristics of the previous Fig. that IC is not equal to zero when IB is zero. For the common-base configuration, when the input current IE was equal to zero, the collector current was equal only to the reverse saturation current ICO, so that the curve IE = 0 and the voltage axis were, for all practical purposes, one. The reason for this difference in collector characteristics can be derived as follows:
  • 30. Common emitter configuration If we consider the case IB = 0 A, and substitute a typical value of 畩 such as 0.996, the resulting collector current is the following:
  • 37. C. Common collector configurations Reading assignment
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