Crystal Radio RF Amplifier

One of the best places to add a transistor to a simple crystal radio is at the front end in the form of an RF amplifier. The circuit below is a simple but effective amplifier which will give surprising performance improvement. This amplifier can exhibit negative resistance for low settings of the 500 ohm pot which results in extra gain or even oscillation. So, the circuit can actually be considered to be a regenerative receiver with an external detector. The sensitivity is so high that no cold water pipe ground is needed and the antenna is short.



The behavior of the amplifier depends on how it is connected to the tuned circuit. When connected to a lower impedance tap as shown in the schematic, the gain will be lower with less tendency to oscillate. Higher taps or even connection directly to the antenna will give higher gain and even oscillation. The 500 ohm pot is adjusted to give adequate gain without squealing as stations are tuned. High regeneration settings will actually narrow the bandwidth of the tank enough to give the sound a "mellow" quality which sounds pretty good in a "tinny" crystal earphone! Lower settings are best when using an audio amplifier and the fidelity is quite good thanks to the linear detector (typical regens use changes in the operating point of the transistor to demodulate the RF). As with any regen, the gain may be increased after the station is tuned in and the circuit will oscillate, locked to the station's frequency.
bullet Current consumption is about 1 mA which may be reduced by increasing the 1.8k but the RF envelope begins to distort below about 500 uA.
bullet A 4.5 volt battery may be used if the 220 k resistor is reduced to 68 k.
bullet The transistor may be just about any NPN small-signal transistor.
bullet No ground is shown but performance is better with a good ground connected to the bottom of the tuner.
bullet Longer antennas should be connected to taps instead of across the whole coil. A ferrite loopstick will pick up stronger stations with no antenna at all but use more audio gain after the diode detector and reduce the regeneration to get adequate bandwidth or the sound will be muffled.