OddMix.com - Technology Note - TN1001 - Karl Nagy
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This simple, no frequency conversion, straight AM transistor radio is made of only seven components. It uses the "grid" detector concept for audio signal recovery and an unusual two transistor Darlington pair that are employed in a direct-coupled CE [Common Emitter] configuration to provide high input impedance and low damping for the tuned L-C circuit.
In tuned, parallel, electronic radio L-C circuits, it is very important to operate resonant circuits with as light load as possible. It was easy for vacuum tubes that are voltage operated devices to work with tuned circuits in resonance while still maintained sharp peeks and a high Q.
As transistors displaced the vacuum tubes there was only a few possible ways to obtain a high input impedance amplifier until the FETs [Field Effect Transistor] appeared. This radi