Armstrong Amplifier and Oscillators

When assembling the circuit in Fig. 16.2, a pair of 4.7 mfd capacitors are put in the collector and emitter positions, C and E, but none is in the optional base position labeled B. The antenna (radio aerial), indicated as a triangle, is not 

The 12 volt coil (yellow wires) of the transformer and its 4.7 mfd capacitor are a parallel resonator (page 126). A sine wave of the right frequency will continue oscillating inside that resonator for a while, until resistance damps it and causes it to die out, but other frequencies would die out much sooner. 

The transformer inductance plus the parallel capacitance is called a tank circuit. It can be attached to any of the three transistor wires, but in this case (position C), it is being used on the collector, which is the most commonly used position. (If the inductor on the base has no capacitor, it is not a tank circuit.) 

The IK emitter resistor helps to adjust the bias, and it also prevents excessive current from flowing. The 4.7 mfd at position E is sometimes called a speedup 

A piezoelectric transducer can be put across any of the capacitors, which might make a high-pitched audible sound, because it responds to higher frequencies. More will be said about piezo devices at the end of this chapter. Using the center-tap would also raise the frequency. This can be tried as another optional experiment. (The collector capacitor can be moved to position B in the diagram, and oscillations will be visible on the scope, but at a lower frequency, with the higher inductance of the 120 volt coil in the tank circuit.) 

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