# Transistors: Emitter Follower

1. Oct 5, 2014

### irishhockey

1. The problem statement, all variables and given/known data
Design a +10V supply with the following specs:
Iout =0-100 mA
Vout =10V
Vin=20-25V (unregulated supply w/ ripple)

The circuit will have the diagram attached.

The book I am using is The Art of Electronics, which is very "practical" the authoppr says things like why worry about calculating something to 1 in 100, when you will then reach for a 10% resistor or 20% cap.

2. Relevant equations
Ibase * beta = Icollector
Iemitter = Ibase (1+beta) = Ibase + Icollector
Iemitter=Ibase +0.6V

3. The attempt at a solution
I start at the output, 10V and 0-100mA. The Voltage at the base and also zener must be equal to collector voltage +0.6V. The current in the base is 1/100 the collector current, and output current is 100mA(max) so I base = 1mA and Icollector=100mA. The zener also requires a constant 10mA, which is needed to keep the dynamic resistance under control. My issue is now the resistors.

So, I know that the resistoprs have 25V on the side facing the power supply, and for R, the voltage on the base/zener side is 10.6V. Does this mean I can say the voltage drop across the res is 14.4 V (using 25V and planning for worst case)? If so, can I just say IBASE + Izener=11mA, asnd the use V/I=R? if so Ic an figure everything else out I belive.

Thank you

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2. Oct 6, 2014

### rude man

The worst case is when power supply = 20V.

If you assume 25V and compute R your way, what would be the consequence of a "soft" zener voltage if the supply dropped to 20V?