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Low Power Three Transistor Amplifier
This amplifier was originally designed to allow the Two Transistor Radio to drive a small loudspeaker.  
   
 
   
T1 BC109C T2 BD679 or BD683 T3 BD680  
   
As can be seen from the circuit diagram we are really using five transistors as the BD679 and BD680 are "Darlington" pairs of transistors.  
The led must be a green led for correct operation (the voltage across a green led is higher than the voltage across a red led).  
 When using a 12V supply the voltage at T1 collector should be (about) 5V and at T2 base (about) 7V.  
This amplifier will be unstable (it will oscillate) without the 4k7 variable resistor connected to the input.
   
For a method of making simple printed circuit boards, like this one, see here.  
   
Printed Circuit Board  
 
   
 
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