Introduction to BJTs

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Questions and Answers

Who invented the transistor?

  • Dr. William Shockley and Dr. John Bardeen (correct)
  • Dr. Nikola Tesla and Dr. Albert Einstein
  • Dr. Thomas Edison and Dr. Guglielmo Marconi
  • Dr. Steve Wozniak and Dr. Bill Gates

What is the function of the emitter in a BJT?

  • To pass all charge carriers to the collector
  • To control input current to the base
  • To inject charge carriers into the base (correct)
  • To collect charge carriers from the base (correct)

Which type of transistor is predominantly used in electronic circuits?

  • PNP transistor
  • BJT transistor
  • NPN transistor (correct)
  • FET transistor

What characterizes the collector region of a BJT?

<p>Physically larger than the emitter (D)</p> Signup and view all the answers

Which rule is true regarding current conduction in NPN and PNP transistors?

<p>NPN conducts mainly by electrons (B)</p> Signup and view all the answers

What does the base region of a BJT do?

<p>Passes charge carriers to the collector (D)</p> Signup and view all the answers

What is the primary reason electrons are preferred over holes in transistors?

<p>Electrons are more mobile than holes (D)</p> Signup and view all the answers

In a BJT, which terminal is common to both input and output?

<p>Base (B)</p> Signup and view all the answers

What are the three transistor circuit configurations?

<p>Common base, Common emitter, Common collector (C)</p> Signup and view all the answers

What happens when a transistor is unbiased?

<p>The transistor has no external voltage connected (B)</p> Signup and view all the answers

In which mode of operation is the emitter-base junction forward-biased and the collector-base junction reverse-biased?

<p>Active (D)</p> Signup and view all the answers

Which of the following points is considered a suitable operating point for a transistor?

<p>Q3, in the active region (D)</p> Signup and view all the answers

What is a common issue when the operating point Q1 is chosen near the saturation region?

<p>The positive cycle of the signal gets clipped (C)</p> Signup and view all the answers

What effect does applying a DC load line to an amplifier circuit have?

<p>Analyzes the performance and biasing effects (D)</p> Signup and view all the answers

What condition is described as having no input AC signal in a common base amplifier?

<p>Quiescent condition (A)</p> Signup and view all the answers

What is the result of having both junctions of a transistor reverse-biased?

<p>Transistor is in cut-off mode (A)</p> Signup and view all the answers

What happens when the switch S is closed in a Common-emitter Configuration?

<p>Electrons are injected into the base region. (A)</p> Signup and view all the answers

What is the leakage current denoted by?

<p>$I_{CBO}$ (A)</p> Signup and view all the answers

What occurs when the switch S is opened?

<p>The collector current becomes a reverse saturation current. (D)</p> Signup and view all the answers

What is the relationship of $I_E$, $I_B$, and $I_C$ when the switch is closed?

<p>$I_E = I_C + I_B$ (B)</p> Signup and view all the answers

What is the definition of the stability factor (S)?

<p>The rate of change of collector current with respect to reverse saturation current. (A)</p> Signup and view all the answers

If the collector current $I_C$ is represented by the equation $I_C = eta I_B + I_{CBO}(1+eta)$, what does $I_{CBO}$ represent?

<p>The leakage current when the emitter is open (C)</p> Signup and view all the answers

In an NPN transistor, what contributes to the small base current $I_{CEO}$ when the switch is open?

<p>Thermally generated minority carriers and holes flowing across the emitter-base junction. (A)</p> Signup and view all the answers

When analyzing transistor stability, what does a higher stability factor indicate?

<p>Worse bias stability of the circuit. (D)</p> Signup and view all the answers

What is the expression for the stability factor S in terms of dI_C and dI_{CBO}?

<p>S = rac{dI_C}{dI_{CBO}} (A)</p> Signup and view all the answers

Which biasing method uses a voltage divider for stable operation?

<p>Self bias or voltage divider bias (D)</p> Signup and view all the answers

What h-parameter represents the input impedance in the hybrid equivalent circuit?

<p>h_{11} (B)</p> Signup and view all the answers

In the common-emitter configuration, what is the expression for the stability factor S?

<p>S = 1 + eta (B)</p> Signup and view all the answers

Which hybrid parameter represents the reverse voltage gain?

<p>h_{12} (D)</p> Signup and view all the answers

What are the advantages of emitter bias in transistor biasing?

<p>It provides a stable operating point. (B)</p> Signup and view all the answers

Which of the following is NOT a common method of transistor biasing?

<p>Gate bias (C)</p> Signup and view all the answers

What h-parameter defines the output conductance/admittance in the hybrid circuit?

<p>h_{22} (B)</p> Signup and view all the answers

What is the expression for current gain $A_i$ in a common-emitter amplifier?

<p>$A_i = - rac{h_{fe}}{1 + h_{oe}R_L}$ (C)</p> Signup and view all the answers

Which formula represents the voltage gain $A_v$ for a common-collector amplifier?

<p>$A_v = rac{A_iR_L}{R_i}$ (A)</p> Signup and view all the answers

How is the output resistance $R_o$ calculated for a common-emitter amplifier?

<p>$R_o = R_s + h_{ie}$ (D)</p> Signup and view all the answers

Which of the following describes the input resistance $R_i$ for a common-collector amplifier?

<p>$R_i = h_{ic} + A_i.h_{re}R_L$ (B)</p> Signup and view all the answers

What does the term $ rac{1}{ ext{h}}$ refer to in the context of output resistance calculation?

<p>It represents an adjustment factor based on the amplifier's parameters. (D)</p> Signup and view all the answers

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Study Notes

Introduction to BJTs

  • The Bipolar Junction Transistor (BJT) was invented in 1951 by Dr. William Shockley and Dr. John Bardeen, revolutionizing electronics by replacing bulky vacuum tubes.
  • BJTs are three-terminal devices, where input current controls output voltage, current, and power, hence called current-controlled devices.

Transistor Components

  • Emitter: Heavily doped, injects majority charge carriers (electrons for NPN, holes for PNP) into the base.
  • Base: Thin and lightly doped, responsible for passing charge carriers to the collector.
  • Collector: Doped in-between the emitter and base, collects charge carriers. Larger than the emitter.

Transistor Types

  • NPN: Emitter (N-type), Base (P-type), Collector (N-type)
  • PNP: Emitter (P-type), Base (N-type), Collector (P-type)
  • NPN transistors are more commonly used because electrons are more mobile, leading to higher conduction than holes in PNP.

Transistor Configurations

  • A transistor is a three-terminal device that can be configured in different ways to achieve various functionalities.
  • The common configurations are:
    • Common Base: The base is common to input and output.
    • Common Emitter: The emitter is common to input and output.
    • Common Collector: The collector is common to input and output.

DC Load Line Analysis

  • DC Load Line: A graphical tool used to analyze the performance of an amplifier circuit.
  • Quiescent Point (Q-point): The operating condition of the transistor without any input AC signal.
  • KVL: Kirchhoff's Voltage Law is applied to determine the relationship between voltage and current in the collector circuit.

Bias Stability & Operating Point

  • Operating Point Selection: The Q-point must be carefully chosen within the amplifier's active region for proper operation and minimal distortion.
  • Stability Factor (S): A measure of how stable the Q-point is with respect to changes in transistor parameters. A lower 'S' value indicates better bias stability.
  • Distortion: If the Q-point is too close to saturation or cut-off regions, the signal will be clipped, resulting in distortion.

Leakage Current in BJTs

  • Leakage Current (Icbo/Ico): A small current that flows between the collector and base when the emitter is open. This is due to thermally activated minority carriers.
  • Icbo: Leakage current with the emitter open.
  • Leakage current is a key factor in understanding transistor behavior at reverse bias.
  • Iceo: Leakage current between collector and emitter with the base open, present in common-emitter configuration.

ON & OFF Conditions

  • ON: When the emitter-base junction is forward biased, the transistor conducts, with the collector current approximately equal to the emitter current.
  • OFF: When the emitter-base junction is reverse biased, the transistor is off, with the collector current primarily determined by the leakage current (Icbo).

Transistor Biasing Methods

  • Fixed Bias: Uses two DC batteries ($V_{BB}$ and $V_{CC}$).
  • Collector-to-Base Bias: Offers stable operating condition.
  • Self Bias or Voltage Divider Bias: Provides stable operating point and good performance.
  • Emitter Bias: Provides stable operating point and good voltage and current gain.

Hybrid Parameters (h-parameters)

  • Hybrid parameters (h-parameters) are used for analyzing and modeling transistor circuits. These parameters are easily measured.
  • h11: Input impedance.
  • h12: Reverse voltage gain.
  • h21: Forward current gain.
  • h22: Output conductance/admittance.

h-parameter Representation of Different Configurations

  • The h-parameters are specific to each configuration:
    • CB: Common Base
    • CC: Common Collector
    • CE: Common Emitter

Hybrid Equivalent Circuit

  • Representing a transistor using h-parameters results in a simplified equivalent circuit for circuit analysis.

Gain Characteristics

  • Current Gain (Ai): The ratio of output current to input current.
  • Voltage Gain (Av): The ratio of output voltage to input voltage.
  • Output Resistance (Ro): Defined as the ratio of output voltage to the current drawn from the output source.
  • Input Resistance (Ri): The resistance seen at the input terminals of the transistor.

h-parameters for Common-Emitter Amplifier

  • The common-emitter is the most commonly used transistor configuration.
  • Current gain (Ai), **Voltage Gain (Av) **, Output Resistance (Ro), Input Resistance (Ri) can be calculated using h-parameters.

h-parameters for Common-Collector Amplifier

  • The common-collector is also known as the emitter-follower configuration.
  • Current gain (Ai), **Voltage Gain (Av) **, Output Resistance (Ro), Input Resistance (Ri) can be calculated using h-parameters.

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