1. Voltage Regulator Circuits

2. A Simple Zener-Diode Voltage Regulator

3. Example on source regulation
Calculate the source regulation for a Zener diode with characteristics shown below.
R = 1 KΩ
Slope of the load line is -1/R. Thus, change in supply voltage changes the position of the load line, but not the slope
In general:

4. Example: Analysis of a Zener-diode regulator with load
Found by considering open circuit in place of the diode in part (b)
Found by considering open circuit in place of the diode and assuming the source to be short circuit
From the Thevenin equivalent circuit, we can write the load equation as:

5. Solution continued
Thus, the solution is : Vd = -10 V, and id = -10 mA

6. Linear small signal equivalent circuit: Dynamic resistance
Note:
A dc supply voltage is used to bias a non-linear device to define an operating point
A small ac signal is injected into the circuit to model its transfer functions near the Q-point. The ac signal is small enough that the circuit operation can be considered to be linear.
From the diode I-V curve, we can see that for a small ac signal we can write:
Fig Diode characteristics showing the Q-point
……………(3.11)
The dynamic resistance of a diode can be found from the reciprocal of the slope of the diode I-V curve at the Q-point

7. The Shockley equation
(This is also our familiar diode equation)

8. The Shockley equation contd.
Thus, the resistance of the diode is inversely related to the dc diode current flowing through the diode.

9. Problems
A certain diode has Is = A and n = 1. Assume VT = 26 mV.
Determine the dynamic resistance of the diode if the forward current is 1 mA
Use the dynamic resistance to find the change in diode voltage if the current changes to 1.1 mA
Ans: (a) 26 Ω, (b) 2.6 mV