/diy/ - Do It Yourself

>>1419131
The chapter in the book is about the input and output impedance of an emitter follower circuit. The exercise is about minimizing the drive current under the given 5% drop conditions.

Basic approach in the book (assuming constant B=100 and constant Vbe=0.7V):
Rout = ∆Ve/∆Ie = 250mV/25mA = 10Ω -- this is a dynamic resistance, not a static resistor!
Rin = Rout*B -- this is the dynamic load resistance at the base
R1/R2 = 9.3/5.7 -- this is the divider ratio at no load for Ve = 5V
R1||R2 = 1000Ω -- this is the source resistance of the divider for Rout = 10Ω

Two equations, two unknowns, solve for R1,R2.

Different approach: R3max = inf, R3min = 4.75V/25mA = 190Ω
R1/R2 = 9.3/5.7 -- divider ratio at no load for Ve = 5V
∆Ib = 0,25mA -- no load to max load
∆Vb = 0.25V -- max Vb drop at max load
Ri = ∆Vb/∆Ib = R1||R2 = 1000Ω

Same result as above. The exercise is about understanding, not about replacing a regulator.