Question E6B01:
What is the most useful characteristic of a Zener diode?  

Choices:  
A: A constant current drop under conditions of varying voltage  
B: A constant voltage drop under conditions of varying current  
C: A negative resistance region  
D: An internal capacitance that varies with the applied voltage    

Correct Answer is  B: A constant voltage drop under conditions of varying current  

The key property of a Zener diode is that, when reverse-biased into its breakdown region, it maintains nearly constant voltage across it even as the current changes over a wide range. This makes it ideal for voltage references and simple regulators.

Why the correct answer is correct:
- In reverse breakdown, the Zener’s voltage is approximately constant: $V \approx V_Z$ for a wide range of $I_Z$. This is the basis of Zener shunt regulation.

Why the other choices are incorrect:
- A: “Constant current drop under varying voltage” describes a current regulator, not a Zener. A Zener allows its current to vary; its voltage stays roughly constant.
- C: “Negative resistance region” is a characteristic of tunnel diodes (and certain devices), not Zener diodes.
- D: “Capacitance that varies with applied voltage” is the defining feature of a varactor (varicap) diode, not a Zener.

Useful formulas and rules of thumb:
- Shunt regulator with series resistor $R_S$ from supply $V_S$:
  - Output voltage: $$V_{out} \approx V_Z$$
  - Zener current: $$I_Z = \frac{V_S - V_Z}{R_S} - I_L$$
  - Keep $$I_{ZK} \lesssim I_Z \le I_{Z(max)} = \frac{P_{Z(max)}}{V_Z}$$ to stay in regulation and within power rating.
  - Series resistor selection (for a chosen design point): $$R_S = \frac{V_S - V_Z}{I_Z + I_L}$$
- Dynamic (small-signal) resistance in breakdown (small but nonzero): $$r_Z = \frac{\Delta V}{\Delta I}$$
- Power dissipation: $$P_Z = V_Z \, I_Z \le P_{Z(max)}$$
- Temperature coefficient: around $5.1$–$5.6$ V, the tempco is near zero; lower-$V_Z$ parts tend to have negative tempco, higher-$V_Z$ positive.