Avionics Electronics Circuits Study Notes

Avionics Electronics Circuits Study Notes
Introduction

  • This document covers key topics related to Avionics Electronics, focusing on Field-Effect Transistors (FETs) and Metal-Oxide Semiconductor FETs (MOSFETs).

  • Key areas of study include Junction FET (JFET) and MOSFET, with emphasis on their introduction, biasing methods, and their applications in amplifiers.

JFET (Junction Field-Effect Transistor)

1. Introduction to JFETs

  • A JFET is a 3-terminal semiconductor device where current conduction takes place by a single type of carrier (either electrons or holes).

  • Current conduction is controlled by an electric field between the gate and the conduction channel, resulting in high input impedance and low noise levels.

2. Types of JFETs

  • Two basic types of JFETs:

    • N-Channel JFET: Requires positive supply voltages.

    • P-Channel JFET: Requires negative supply voltages.

3. Operation Overview

  • The drain current in a JFET (

    $ID$) is regulated by varying channel width using gate-source voltage ( $V{GS}$) and drain-source voltage (

    $V ext{DS}$).

  • $V ext{GS}$ modifies the depletion layer, thus changing channel width.

  • When $V ext{DS}$ is applied, it also affects the depletion layer.

4. Key Parameters

  • Pinch-Off Voltage ( $VP$): The value of

    $V{DS}$ where further increases only lead to augmented channel resistance.

  • Gate-Source Cutoff Voltage ( $V{GS(off)}$): The value of $V{GS}$ that causes $I_D$ to drop near zero.

    • Example: If

      $V{GS(off)} = -8 V$, then $VP = 8 V$.

  • JFET Biasing: The gate-source junction must not be forward biased to avoid damage.

    • Reverse-biasing allows for very high gate impedance, typically in the megohm (MΩ) range.

5. Control Mechanism

  • The drain current ( $ID$) can be calculated using the equation: I</em>D=I<em>DSS(1−V</em>GSVextGS(off))2I</em>D = I<em>{DSS} \bigg(1 - \frac{V</em>{GS}}{V ext{GS(off)}}\bigg)^2

    • Where

      $I{DSS}$ is the shorted-gate drain current rating, $V{GS}$ is the gate-source voltage, and

      $V ext{GS(off)}$ is the gate-source cutoff voltage.

6. Biasing Circuits for JFETs

  • Common JFET Biasing Circuits:

    • Self-bias Circuit: Simple but provides poor Q-point stability.

    • Voltage-Divider Bias Circuit: Stable Q-point but more complex circuitry.

Common JFET Amplifier Configurations

1. Common-Source Amplifier

  • Configuration: Input at the Gate, output from the Drain.

  • Advantages: High input impedance and good voltage amplification.

  • Widely used due to the aforementioned features.

2. Common-Gate Amplifier

  • Configuration: Input at the Source, output from the Drain, Gate connected to Ground.

  • Drawback: Loses high input impedance but gains high output impedance.

  • Suitable for high frequency circuits or impedance matching where low input impedance is necessary.

3. Common-Drain Amplifier

  • Configuration: No signal at the Drain; it's used for biasing. The output is in-phase with the input.

MOSFET (Metal-Oxide-Semiconductor FET)

1. Introduction to MOSFETs

  • Compared to the JFET, a MOSFET can operate without necessitating reverse bias.

  • A MOSFET is a four-terminal device: Source (S), Gate (G), Drain (D), and Body (B), but often works as a three-terminal device with the body linked to the source.

2. Types of MOSFETs

  • Depletion-mode MOSFETs (D-MOSFETs): Can be operated in both depletion and enhancement modes.

  • Enhancement-mode MOSFETs (E-MOSFETs): Operate only in enhancement mode.

3. Operation Mechanics

  • The operation of a MOSFET hinges on the MOS capacitor.

  • A positive gate voltage can invert the semiconductor surface from p-type to n-type, forming a conducting channel by repelling holes and attracting electrons from the source and drain regions.

  • When the gate voltage is negative, a p-type channel is formed.

4. D-MOSFET Application and Biasing

  • Similar biasing circuits as those used for JFETs are applicable.

  • The characteristics tie closely to the JFET, allowing ID (current) to surpass IDSS under certain conditions.

5. E-MOSFET Applications

  • E-MOSFETs only turn on in enhancement mode.

  • The Threshold Voltage ( $V ext{GS(th)}$) is the minimum positive voltage to activate the device.

  • Its current can be calculated by:

    I<em>D=k(V</em>GS−VextGS(th))2I<em>D = k \bigg(V</em>{GS} - V ext{GS(th)}\bigg)^2

  • Where

    $k$ is a MOSFET-specific constant.

Amplifier Frequency Response

1. Key Terms

  • Bandwidth: The range within which a circuit operates between its upper and lower cut-off frequencies where the power decreases to half its maximum value (resulting in a gain drop of 3dB).

  • Lower Cut-off Frequency: The frequency boundary below which energy is diminished in the output signal due to filtering effects.

  • Output Coupling Capacitor: Allows desired AC signals through but blocks unwanted DC components, which can induce noise in systems.

  • Upper Cut-off Frequency: Duty in amplifier gain fall-off due to internal capacitive effects.

Reference

  • Textbook: Electronics Technology Fundamental, Paynter and Boydell, 2009