Chapter 3

Diodes

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 $2^{n}$ possibilities.

1. Assume a set of states for the diodes
2. Solve the circuit to find $i_{D}$ for diodes assumed to be on and $v_{D}$ for diodes assumed to be off
3. Check to see if $i_{D}$ is positive for all diodes assumed to be on and if $v_{D}$ 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

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: $i_{D}=I_{S}\left(e^{\frac {v_{D}}{nV_{T}}}-1\right)$

• $I_{S}\,$ is the saturation current: $10^{-14}A\,$
• $V_{T}\,$ is the thermal voltage: $V_{T}={\frac {kT}{q}}=26mV$

• Dynamic small-signal resistance of the diode at the Q-point: $r_{d}={\frac {nV_{T}}{I_{DQ}}}$

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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