Voltage regulator circuit calculations (zener diodes, transistors)

Question

The circuit given in the following Figure is a voltage regulator which is used to supply a constant voltage. It has the following properties: Vin = 20V, Rs = 1 kΩ, R1 = 200 Ω, R2 = 800 Ω, R3 = 2 KΩ, RL = 100 Ω, Vz = 5.6V, βdc = 100.

Vin = 20V Rs = 1kΩ R1 = 200Ω R2 = 800Ω R3 = 2kΩ Rl = 200Ω Vz = 5.6V βdc = 100

Vf = 0.7V (?) Vout = 5.6V (?) Ic2 = ? Ib2 = ? Ic1 = ? Iz = ? PQ2 = ? Pz = ? RL = ?

I am having immense trouble with this one. So far, Vz = Vout when using a Zener diode? So 5.6v? Vf = 0.7v? (Threshold voltage of a diode?)

Any help would be appreciated!

Answer 1

Any help would be appreciated!

Maybe you should think along these lines; ask yourself what voltage has to appear on the base of Q1 (with respect to ground) such that base current begins to flow. I mention this because, when Q1's base current starts to flow, you reach the onset of the circuit turning into a voltage regulator.

Then, once you have established this value, you can calculate what the output voltage must be. This requires recognizing that the R1 and R2 form a potential divider from the output to Q1's base.