Chapter 3 Study Notes

Electronic Devices

Chapter 3 Outline

• 3-1 The Zener Diode
• 3-2 Zener Diode Applications
• 3-3 The Varactor Diode

3-1 The Zener Diode

• Definition:
  • A zener diode is a silicon pn junction device designed to operate in the reverse breakdown region.
• Functionality:
  • Operates ideally at a constant breakdown voltage, which is critical for its use as a voltage reference.
• Symbol Representation:
  • Zener diode symbol showing Anode (A) and Cathode (K).

Breakdown Characteristics

• Voltage-Current Characteristics:
  • As the reverse voltage (VR) increases, the reverse current (IR) remains small until the knee of the characteristic curve.
  • The reverse current is also termed zener current (IZ).
  • At the knee, the breakdown effect begins, leading to:
  • Decrease in internal zener resistance (zener impedance, ZZ).
  • Zener breakdown voltage (VZ) stays nearly constant, though it increases slightly with rising zener current (IZ).
• Key Feature:
  • The ability to maintain constant reverse voltage across terminals is essential for zener diodes, requiring a minimum reverse current (IZK) to sustain breakdown for voltage regulation.

Zener Equivalent Circuits

• First Approximation Model:
  • Represents a zener diode in reverse breakdown with ideal characteristics.
• Second Approximation Model:
  • Incorporates zener impedance (ZZ) into calculations.
• Ohm’s Law Application:
  • The impedance can be calculated using the equation:
    $ext{ZZ} = \frac{∆VZ}{∆IZ}$
  • Where:
  • $∆VZ$ = Change in zener voltage
  • $∆IZ$ = Change in zener current

Example 3-1

• Problem Statement:
  • Given a change in $VZ$ of 50 mV over a current change from 10 mA to 15 mA, find the zener impedance.
• Calculation:
  • $ext{ZZ} = \frac{50 mV}{5 mA} = 10 ext{Ω}$
• Related Problem:
  • Find the zener impedance for a change in zener voltage of 100 mV with a current change of 20 mA.

Temperature Coefficient of the Zener Diode

• Definition:
  • Percent change in zener voltage per degree Celsius change in temperature, defined as:
    $ext{TC} = \frac{∆VZ}{∆T}$
  • Units:
  • $ext{TC}$ in %/°C or mV/°C depending on context.

Maximum Current Calculation

• Power Specification:
  • The maximum current ($ext{IZM}$) is derived from power specifications, where maximum power dissipation is given.

3-2 Zener Diode Applications

• Basic Voltage Regulation:
  • A zener diode used in low-current applications is an 8.2 V regulator via a practical circuit example.
• Calculation of Load Resistor:
  • Using Ohm's law in configurations:
  • With circuit conditions, smallest load resistor computation is done using:
    $R = \frac{(V_{IN} - V_{Z})}{I_{max}}$
  • For example, using a zener diode like the 1N756 with a zener voltage of 8.2 V, assume a maximum load current of 9.8 mA leads to:
    $R = \frac{18 V - 8.2 V}{9.8 mA} ext{ resulting in } R ext{ value}.$

Example 3-7

• Technical Specifications of IN4744A Zener Diode:
  • Zener voltage at certain currents defined.
  • Computed values of output voltage (VOUT) applied with maximum and minimum currents.
  • Voltage development under different current conditions, specifically noting resistor value to maintain current without overcoming maximum threshold.

Applications of Zener Diodes:

• Zener diodes can serve in three-terminal regulators:
  • Establish a reference voltage capable of larger load currents compared to basic zener regulators.

3-3 The Varactor Diode

• Definition:
  • A varactor diode is specialized to function in reverse-bias as a voltage-controlled capacitor.
• Depletion Region Behavior:
  • The depletion region widens as the reverse bias increases.
• The relationship between capacitance and bias voltage is defined as:
  $C = \frac{A ext{e}}{d}$
  Where effective plate separation (d) increases with reverse-bias, decreasing capacitance.

Capacitance Ratios and Applications

• Capacitance Tolerance and Ratios:
  • The capacitance tolerance range represents minimum and maximum values from data sheets.
  • The capacitance ratio depends on variation of capacitance with reverse voltage.
• Example Calculation:
  • Zener capacitance ratios indicate behavior across biases, informing frequency tuning applications.
• Tuning Applications:
  • Varactor diodes are widely used in oscillators to adjust tuning due to changes in applied voltage.

Example 3-8

• Varactor Circuit Calculations:
  • Detailed calculations under conditions of different applied reverse biases leading to resonant frequency outcomes for varying inductor values (L).
  • Utilization across specific applications yielding results across frequency spectra.

Summary of Diode Types

• Summary of Common Diode Symbols:
  • Zener, Light-emitting, Photo, Varactor, Laser, Schottky, PIN, Tunnel, and Current-regulator diodes.
• Each diode type serves distinct functionalities within the greater scope of electronic applications, emphasizing their operational principles and application domains.

Conclusion

• The comprehensive understanding of Zener and Varactor diodes allows for their effective implementation in voltage regulation, tuning applications, and robust electronic circuit design. Proper calculations and device specifications are pivotal to engineering sound electronic systems.