I was reading the BJT section of The Art of Electronics on page 77 Figure 2.10.B. Shouldn't The divider voltage sit at 14.4v rather than 11.6v? the opposite what the author mentioned. Since if Q3 is in saturation, the base voltage of Q3 should be ~0.6v lower than the collector which is 14.4v.
Answer
The text clearly states that the base-emitter junction of Q3 dominates the voltage across R3. If Vcc supply is +15 volts, this 'locks' the voltage at the base of Q3 at +15-.6, or about 14.4 volts.
The 0.6 volt B-E drop is an average, as bjt transistors (NPN and PNP) have a B-E drop of about .55volts to .65 volts, depending on its beta and the manufactures needs.
Notice the comment that R3 could be increased a great deal, to 10K perhaps. The only minor penalty would be a slower response due to transistor capacitance. R1, R2 and R3 could also be lower by 50%, increasing the response speed of the circuit, but at the small penalty of a higher 'ON' current.
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