Chapter 3

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Diodes

Diode.jpg
Diode load line
  • Forward bias occurs when the P-type semiconductor material is connected to the positive terminal of a battery and the N-type semiconductor material is connected to the negative terminal, as shown below. This usually makes the p–n junction conduct. <ref> Wikipedia P-N junction</ref>
  • Because diodes are a nonlinear device, traditional circuit analysis will not work on them. One method of analyze the diodes is to do it graphically. This technique is called Load-line analysis.
  • The load-line equation is obtained by applying KVL or KCL. The equation plots as a straight line that can be drawn by locating two points.
  • In circuits with multiple diodes, it is not immediately apparent which diodes are on or off. The ideal-diode model assumes that the diode is a perfect conductor with zero voltage drop in the forward direction and an open circuit in the reverse direction.
  • Is there a better, less exhaustive method to determine which diodes are on or off? With the current system, for n diodes there are possibilities.
  1. Assume a set of states for the diodes
  2. Solve the circuit to find for diodes assumed to be on and for diodes assumed to be off
  3. Check to see if is positive for all diodes assumed to be on and if is negative for all diodes assumed to be off.
  • Explain peak inverse voltage, PIV.
  • Full-wave rectifier circuits: Center-tapped transformer & diode bridge. What are the pros and cons to each?
  • Diode clipper circuits and their zener diode equivalents

Full Wave Recitifier: Diode Bridge

Full Wave Recififier: Center-tapped Transformer

Clipper Circuit

  • The resistor is large enough that the forward diode current is within reasonable bounds and small enough so that the reverse bias current results in a negligible voltage drop.
    • Need help understanding the above sentence
  • A battery + diode can be used to clip the circuit. A Zener diode will often take the place of the battery in a clipper circuit.
  • Placing a resistor in series with the diodes can allow for more gradual transfer characteristics.

Clamp Circuit

  • The large capacitor acts as a DC offset and has a very small impedance for the AC signal. A large resistor is chosen to allow the capacitor to discharge slowly.
  • Diodes are then used to define the upper and lower limit by conducting if the voltage goes outside their specified range.
  • Why aren't the waveforms clipping in F3.20 (P149)

Linear Small-Signal Equivalent Circuits

  • Dc supply voltages are used to bias a nonlinear device at an operating point and a small ac signal is injected into the circuit. If we consider a small enough region of operation, we can find a linear small-signal equivalent circuit.<ref>Electronics p156</ref>
  • The DC supply voltage results in the circuit operating at the quiescent point. We can use this point to find the dynamic resistance of the diode.
  • Shockley Diode equation:
  • is the saturation current:
  • is the thermal voltage:
  • Dynamic small-signal resistance of the diode at the Q-point:

To Do

  • Start up Chapter 3 problems
  • Extend the bag of tricks post. Have subsections for op amps and diodes now. Then BJTs later.

References

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