Question E7D03:
What device is used as a stable voltage reference?  

Choices:  
A: A Zener diode  
B: A digital-to-analog converter  
C: An SCR  
D: An analog-to-digital converter    

Correct Answer is  A: A Zener diode  

A Zener diode

Why this is correct:
- A Zener diode operated in reverse breakdown provides a nearly constant voltage over a wide range of current. This makes it a classic, simple voltage reference.

Key idea:
- Reverse-bias the Zener above its breakdown voltage $V_Z$; then the voltage across it stays approximately $V_Z$ despite current and supply variations.

Why the others are incorrect:
- B: Digital-to-analog converter (DAC) converts numbers to voltages/currents; it uses a reference but is not itself the reference.
- C: SCR (silicon-controlled rectifier) is a latching switch for power control, not a precision reference.
- D: Analog-to-digital converter (ADC) converts analog signals to numbers; it requires a reference but does not inherently provide one.

Useful formulas and tips:
- Series resistor to bias a Zener from supply $V_S$ with load current $I_L$ and desired Zener current $I_Z$:
  $$R_S = \frac{V_S - V_Z}{I_Z + I_L}$$
- Ensure Zener current stays above the knee current $I_{ZK}$ and below the max rated current.
- Worst-case Zener power (e.g., if load is disconnected):
  $$P_{Z,\max} = V_Z \times I_{Z,\max}$$
- Small-signal regulation: if $r_Z$ is the Zener’s dynamic resistance,
  $$\Delta V_{\text{out}} \approx r_Z \,\Delta I_Z$$
- Temperature coefficient:
  - Around $V_Z \approx 5.1$–$5.6\ \text{V}$, the tempco is near zero (most stable vs temperature).
  - Below this, tempco tends to be negative; above, positive.

Practical rule of thumb:
- Choose a 5.1–5.6 V Zener for best temperature stability, bias it at a few mA (per datasheet), and size $R_S$ so $I_Z$ remains in range over supply and load variations.