I'm working on exercise 2.2 in The Art of Electronics (2nd ed. Horowitz & Hill, pg. 67)
The exercise is:
"Use a follower with base driven from a voltage divider to provide a stiff source of +5V from an available regulated +15V supply. Load current (max) = 25mA. Choose your resistor values so that the output voltage doesn't drop more than 5% under full load."
Based on the problem description I have the following circuit:
What I've got so far:
RL(min) = 200Ω (using the maximum load condition: 5V/25mA = 200Ω)
I need about 5.6V at the base (VB) so R1/R2 = 1.68
Output and input resistances:
Rout ≈ Rth/(hfe+1)
Rin ≈ (hfe+1)(RE//RL)
(note: Rth = R1//R2)
The book has been assuming a typical hfe of 100 in most of its examples, so I'm assuming that's the assumption used here as well.
I'm a bit confused about how to proceed from here. The variable load current and maximum voltage drop condition is really throwing me. I just need a nudge in the right direction.
Thanks
The exercise is:
"Use a follower with base driven from a voltage divider to provide a stiff source of +5V from an available regulated +15V supply. Load current (max) = 25mA. Choose your resistor values so that the output voltage doesn't drop more than 5% under full load."
Based on the problem description I have the following circuit:
What I've got so far:
RL(min) = 200Ω (using the maximum load condition: 5V/25mA = 200Ω)
I need about 5.6V at the base (VB) so R1/R2 = 1.68
Output and input resistances:
Rout ≈ Rth/(hfe+1)
Rin ≈ (hfe+1)(RE//RL)
(note: Rth = R1//R2)
The book has been assuming a typical hfe of 100 in most of its examples, so I'm assuming that's the assumption used here as well.
I'm a bit confused about how to proceed from here. The variable load current and maximum voltage drop condition is really throwing me. I just need a nudge in the right direction.
Thanks
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