This information is now available in my "Beginners Guide to Electronics" book. One of the most commonly used circuits is that of the VOLTAGE REGULATOR. This method is OK for low currents but the resistor becomes too hot if larger currents are needed. To cope with this problem we can add the NPN transistor (Q1). Now the transistor passes the current required at the output. What is the output voltage? The first circuit can be improved upon to provide more current. All we need to do is to add a second transistor (which has a higher rating to handle the extra current) and change the zener diode to clamp at 6.2 volts in order to compensate for the b-e voltage of BOTH transistors. 6.2 - 0.6 - 0.6 = 5.0 volts 
Caravan
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Example Transistor Circuit
