Concept:For small AC ripple variations, the Zener diode regulator acts as an AC voltage divider formed by the series resistor \( R_s \) and the small dynamic Zener impedance \( r_z \).
Formula:$$\frac{v_{\text{out,ripple}}}{v_{\text{in,ripple}}} = \frac{r_z \parallel R_L}{R_s + (r_z \parallel R_L)} \approx \frac{r_z}{R_s + r_z} \ll 1 \quad (\text{since } r_z \ll R_L, R_s)$$
Solution:- Because \( r_z \) is very small (typically \( 2\text{–}10\ \Omega \)) compared to \( R_s \) (typically hundreds of ohms), the voltage divider heavily attenuates input ripple.
- The ripple attenuation factor is approximately \( r_z / R_s \), providing effective output voltage stabilization.
Why other options are incorrect:- Option A: The DC voltage ratio sets operating points, not small-signal ripple attenuation.
- Option B: Current ratios determine power efficiency rather than the small-signal ripple division ratio.
- Option C: Transformer turns ratio sets secondary voltage levels, not Zener regulator attenuation.
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