Slide 1: Introduction to Bipolar Junction Transistors

  • A Bipolar Junction Transistor (BJT) is a three-layer, two pn-junction semiconductor device.
  • It is a current-controlled device, used for amplification and switching applications.
  • The two main types of BJTs are NPN (negative-positive-negative) and PNP (positive-negative-positive).
  • The basic structure of a BJT consists of three regions: emitter, base, and collector.
  • The emitter region is heavily doped, while the base region is lightly doped.
  • The collector region is moderately doped, wider than the base region, and separates the emitter and base regions.
  • BJTs are widely used in various electronic devices such as audio amplifiers, oscillators, and digital logic circuits.

Slide 2: BJT Symbols and Terminal Identification

  • The schematic symbol of an NPN BJT consists of three interconnected arrows, representing the emitter, base, and collector.
  • The arrowhead indicates the direction of conventional current flow.
  • The emitter is identified by the arrowhead pointing outward, while the collector and base are identified by arrowheads pointing inward.
  • The terminal identification for an NPN BJT is as follows:
    • Emitter (E)
    • Base (B)
    • Collector (C)

Slide 3: BJT Operating Modes

A BJT can operate in three different modes depending on the biasing conditions:

  1. Active Mode:
    • The base-emitter junction is forward-biased, and the base-collector junction is reverse-biased.
    • A small forward current flows from the emitter to the base.
    • The transistor amplifies the input signal.
  1. Cut-off Mode:
    • Both the base-emitter and base-collector junctions are reverse-biased.
    • No current flows through the transistor, and it remains switched off.
  1. Saturation Mode:
    • The base-emitter junction is forward-biased, and the base-collector junction is forward-biased as well.
    • A large forward current flows through the transistor, and it remains in saturation state.

Slide 4: BJT Characteristics - I-V Relationship

  • The relationship between the collector current (IC) and the base-emitter voltage (VBE) is given by the input characteristic curve.
  • In the active mode, the IC increases exponentially with VBE for a constant collector current.
  • The output characteristic curve shows the relationship between the collector-emitter voltage (VCE) and the collector current (IC) for a constant base current (IB).
  • The output characteristic curve exhibits three regions: cut-off, active, and saturation.
  • The cut-off region represents the transistor being fully off, while the saturation region represents the transistor being fully on.

Slide 5: BJT Operating Point

  • The operating point of a BJT is the point where the device operates when connected to a circuit.
  • It represents a specific combination of IC and VCE values.
  • The Q-point (quiescent point) is the operating point where no input signal is applied.
  • The Q-point is chosen considering the desired amplification and linearity of the transistor.

Slide 6: Transistor Regions of Operation

  • In the active region, the BJT operates as an amplifier for small input signals.
  • The input voltage is amplified at the output.
  • The transistor remains in the active region when VBE is forward-biased and VCE is reverse-biased.
  • In the cut-off region, the BJT is fully off, and no current flows through it.
  • In the saturation region, the BJT is fully on, and it operates as a switch allowing maximum current to flow.

Slide 7: Transistor Current Gain (β)

  • The current gain β (beta) of a BJT measures the amplification capability of the transistor.
  • It is defined as the ratio of collector current (IC) to base current (IB).
  • β = IC / IB
  • The value of β varies from transistor to transistor and is typically in the range of 20-100 for small signal transistors.

Slide 8: Transistor Configurations

BJTs can be used in different configurations depending on the application requirements:

  1. Common Emitter (CE) Configuration:
    • The input is applied to the base, and the output is taken from the collector.
    • It provides high voltage gain and moderate current gain.
  1. Common Base (CB) Configuration:
    • The input is applied to the emitter, and the output is taken from the collector.
    • It provides high current gain and low voltage gain.
  1. Common Collector (CC) Configuration:
    • The input is applied to the base, and the output is taken from the emitter.
    • It provides unity voltage gain and high current gain.

Slide 9: Common Emitter (CE) Configuration

  • The common emitter configuration is the most commonly used BJT configuration.
  • It provides both voltage and current amplification.
  • The input is applied to the base-emitter junction, while the output is taken from the collector-emitter junction.
  • The voltage gain (Av) of the CE configuration is given by the ratio of change in output voltage (ΔVCE) to change in input voltage (ΔVBE).
  • Av = ΔVCE / ΔVBE

Slide 10: Common Emitter (CE) Configuration - Example

Example: Consider a common emitter amplifier circuit with the following parameters:

  • IC = 2 mA
  • VCE = 6 V
  • β = 100 Find:
  1. The value of base current (IB).
  1. The current gain (Av) of the amplifier. Solution:
  1. IB = IC / β = 2 mA / 100 = 20 μA
  1. To calculate Av, we need additional information about ΔVCE and ΔVBE.

Slide 11: Bipolar Junction Transistor Basics - I-V Characteristic

  • The I-V characteristic of a BJT describes the relationship between the collector current (IC) and the collector-emitter voltage (VCE).
  • The I-V curve is nonlinear and can be divided into three regions: active, cut-off, and saturation.
  • In the active region, the transistor operates as an amplifier and follows the exponential relationship given by the Ebers-Moll equation.
  • In the cut-off region, the transistor is fully off, and no current flows through it.
  • In the saturation region, the transistor is fully on, and the collector current is at its maximum value.

Slide 12: Ebers-Moll Equation

  • The Ebers-Moll equation is a mathematical model that describes the behavior of a BJT in the active region.
  • It relates the collector current (IC) to the base-emitter voltage (VBE) and the collector-emitter voltage (VCE).
  • The equation is given by: IC = IS * (e^(VBE / Vt) - 1) * (1 - e^(-VCE / VA)) where:
    • IC is the collector current
    • IS is the reverse saturation current of the base-emitter junction
    • VBE is the base-emitter voltage
    • Vt is the thermal voltage (approximately 26 mV at room temperature)
    • VA is the Early voltage, which represents the variation of IC with VCE

Slide 13: Early Voltage (VA)

  • The Early voltage (VA) is a parameter that characterizes the output resistance of a BJT in the active region.
  • It represents the variation of IC with VCE, i.e., how much the collector current changes for a given change in VCE.
  • A higher value of VA indicates a more gradual decrease in IC as VCE increases.
  • VA typically ranges from a few tens to a couple of hundred volts for most BJTs.

Slide 14: Common Emitter (CE) Circuit Configuration

  • The common emitter (CE) circuit configuration is widely used for amplification purposes.
  • It provides both voltage and current gain.
  • In this configuration, the input signal is applied to the base-emitter junction, and the output is taken from the collector-emitter junction.
  • The CE configuration offers high voltage gain but lower current gain compared to other configurations.

Slide 15: Common Emitter Amplifier - Small Signal Model

  • A common emitter amplifier can be represented using a small-signal model for analysis.
  • The small-signal model considers the variations of signals around the operating point (Q-point).
  • It includes the equivalent resistance (re) of the base-emitter junction, the current gain (β), and the output resistance (Ro) of the collector.
  • This model simplifies the analysis of the amplifier’s response to small input signal variations.

Slide 16: Common Emitter Amplifier - Small Signal AC Analysis

  • The AC analysis of a common emitter amplifier involves determining the voltage gain (Av) and input/output impedances.
  • The voltage gain (Av) is given by the ratio of change in output voltage (ΔVout) to change in input voltage (ΔVin).
  • The input impedance (Zin) determines how much the input signal is attenuated due to loading effects.
  • The output impedance (Zout) determines how well the amplifier can drive the load without significant signal degradation.

Slide 17: Common Emitter Amplifier - Frequency Response

  • The frequency response of a common emitter amplifier refers to how the gain of the amplifier varies with frequency.
  • The gain usually decreases at higher frequencies due to the internal capacitances of the transistor.
  • The lower cut-off frequency (fL) is the frequency at which the gain drops by 3 dB (half power).
  • The upper cut-off frequency (fH) is the frequency beyond which the gain drops significantly.
  • The bandwidth (BW) of the amplifier is the frequency range between fL and fH.

Slide 18: Common Emitter Amplifier - Biasing

  • The biasing of a common emitter amplifier ensures that the transistor operates in the active region.
  • A proper biasing is required to stabilize the operating point (Q-point) and prevent distortion of the amplified signal.
  • Biasing methods include fixed bias, emitter bias, and voltage-divider bias.
  • A stable Q-point provides good linearity and ensures that the amplifier can faithfully amplify the input signal.

Slide 19: Common Collector (CC) Configuration

  • The common collector (CC) configuration is also known as the emitter follower configuration.
  • In this configuration, the input is applied to the base, and the output is taken from the emitter.
  • The CC configuration provides unity voltage gain, high current gain, and low output impedance.
  • It is commonly used as a buffer stage between high impedance sources and low impedance loads.

Slide 20: Common Base (CB) Configuration

  • The common base (CB) configuration provides high current gain and low voltage gain.
  • It is useful for impedance matching and impedance transformation.
  • In this configuration, the input is applied to the emitter, and the output is taken from the collector.
  • The CB configuration is less commonly used compared to the CE and CC configurations.

Slide 21: Bipolar Junction Transistor Basics - I-V Characteristic

  • The I-V characteristic of a BJT describes the relationship between the collector current (IC) and the collector-emitter voltage (VCE).
  • The I-V curve is nonlinear and can be divided into three regions: active, cut-off, and saturation.
  • In the active region, the transistor operates as an amplifier and follows the exponential relationship given by the Ebers-Moll equation.
  • In the cut-off region, the transistor is fully off, and no current flows through it.
  • In the saturation region, the transistor is fully on, and the collector current is at its maximum value.

Slide 22: Ebers-Moll Equation

  • The Ebers-Moll equation is a mathematical model that describes the behavior of a BJT in the active region.
  • It relates the collector current (IC) to the base-emitter voltage (VBE) and the collector-emitter voltage (VCE).
  • The equation is given by: IC = IS * (e^(VBE / Vt) - 1) * (1 - e^(-VCE / VA))
  • Where:
    • IC is the collector current
    • IS is the reverse saturation current of the base-emitter junction
    • VBE is the base-emitter voltage
    • Vt is the thermal voltage (approximately 26 mV at room temperature)
    • VA is the Early voltage, which represents the variation of IC with VCE

Slide 23: Early Voltage (VA)

  • The Early voltage (VA) is a parameter that characterizes the output resistance of a BJT in the active region.
  • It represents the variation of IC with VCE, i.e., how much the collector current changes for a given change in VCE.
  • A higher value of VA indicates a more gradual decrease in IC as VCE increases.
  • VA typically ranges from a few tens to a couple of hundred volts for most BJTs.

Slide 24: Common Emitter (CE) Circuit Configuration

  • The common emitter (CE) circuit configuration is widely used for amplification purposes.
  • It provides both voltage and current gain.
  • In this configuration, the input is applied to the base-emitter junction, and the output is taken from the collector-emitter junction.
  • The CE configuration offers high voltage gain but lower current gain compared to other configurations.

Slide 25: Common Emitter Amplifier - Small Signal Model

  • A common emitter amplifier can be represented using a small-signal model for analysis.
  • The small-signal model considers the variations of signals around the operating point (Q-point).
  • It includes the equivalent resistance (re) of the base-emitter junction, the current gain (β), and the output resistance (Ro) of the collector.
  • This model simplifies the analysis of the amplifier’s response to small input signal variations.

Slide 26: Common Emitter Amplifier - Small Signal AC Analysis

  • The AC analysis of a common emitter amplifier involves determining the voltage gain (Av) and input/output impedances.
  • The voltage gain (Av) is given by the ratio of change in output voltage (ΔVout) to change in input voltage (ΔVin).
  • The input impedance (Zin) determines how much the input signal is attenuated due to loading effects.
  • The output impedance (Zout) determines how well the amplifier can drive the load without significant signal degradation.

Slide 27: Common Emitter Amplifier - Frequency Response

  • The frequency response of a common emitter amplifier refers to how the gain of the amplifier varies with frequency.
  • The gain usually decreases at higher frequencies due to the internal capacitances of the transistor.
  • The lower cut-off frequency (fL) is the frequency at which the gain drops by 3 dB (half power).
  • The upper cut-off frequency (fH) is the frequency beyond which the gain drops significantly.
  • The bandwidth (BW) of the amplifier is the frequency range between fL and fH.

Slide 28: Common Emitter Amplifier - Biasing

  • The biasing of a common emitter amplifier ensures that the transistor operates in the active region.
  • A proper biasing is required to stabilize the operating point (Q-point) and prevent distortion of the amplified signal.
  • Biasing methods include fixed bias, emitter bias, and voltage-divider bias.
  • A stable Q-point provides good linearity and ensures that the amplifier can faithfully amplify the input signal.

Slide 29: Common Collector (CC) Configuration

  • The common collector (CC) configuration is also known as the emitter follower configuration.
  • In this configuration, the input is applied to the base, and the output is taken from the emitter.
  • The CC configuration provides unity voltage gain, high current gain, and low output impedance.
  • It is commonly used as a buffer stage between high impedance sources and low impedance loads.

Slide 30: Common Base (CB) Configuration

  • The common base (CB) configuration provides high current gain and low voltage gain.
  • It is useful for impedance matching and impedance transformation.
  • In this configuration, the input is applied to the emitter, and the output is taken from the collector.
  • The CB configuration is less commonly used compared to the CE and CC configurations.