Transistor Radio with RF Amplifier
With this project you'll construct and test a simple radio which has an RF (radio frequency) amplifier ahead of the Diode detector.
The 2SA Transistor is fed the very low level RF voltage from the Antenna link. It then amplifies this voltage to a high enough level to be detected by the Diode and appear at the output to the Earphone with usable volume.
As with all simple radios, use a good Antenna and Ground system and experiment with connections as we've explained in other projects.
The RF amplifier is a simple RC coupled, class A stage using self-current bias. Because this Transistor has high gain at the RF frequencies, and because the circuit uses long leads and a large circuit layout, the collector circuit leads may have to be positioned away 10 pF and 100pF and any wires connected to these terminals (or the amplifier) will oscillate and cause a "beat" tone on all the stations.
Actually, when positioned near to any or all of these terminals, so that oscillations almost starts, you can use these wires as a regeneration control to obtain maximum sensitivity. This action occurs as the feedback to the Antenna end of the coil causes the amplifier to overcome the losses in the tank circuit.
Operation like this is used with the regenerative detector type radio. You will want to experiment with this circuit to determine if you can get-operation on any station at the optimum sensitivity arrangement of the lead wires. When this is done you will be able to receive all local stations with good selectivity and sensitivity.
Low frequency stations require the greatest amount of feedback coupling for optimum operation. RF amplifiers are usually built on a PC (printed circuit) board with proper layout to eliminate the possibility of unwanted oscillations. Amplifiers like the one in this project have been used as intermediate frequency (IF) amplifiers in communication receivers.