This three-transistor AM radio circuit is a clean and minimalistic design that faithfully amplifies radio signals so that you can hear them through a loudspeaker. I am using the MPSA13 transistor for this design because internally it is a Darlington and therefore has very high gain. At each stage, I am using very high value self-biasing resistors to get the maximum out of each Darlington. The base junctions receive an extremely small amount of positive current so they self-centre automatically.
Ideally you should use the screwcup system for this build because breadboard and stripboard builds may introduce crosstalk issues. If you decide to install it inside a case, then use an ABS plastic or wood case, instead of a metal one, because the metal will prevent the ferrite rod from operating properly. This circuit utilises the LT700 audio transformer to drive the loudspeaker, however if you are unable to find this component, then I have an alternative design in the Whippersnapper 3 article that utilises an IC power amplifier instead.
The diode junction of the transistor in the first stage does the job of detection. Since the base has high impedance it keeps the Q of the tuned circuit high helping to improve the selectivity. I am using C1 and C2 to nail the bandwidth in the medium wave (MW) band, where C2 behaves more like an RF bypass and shorts any unwanted frequencies to ground. I am also controlling the gain to use the linear part of the transistor characteristic, which is where the Darlington transistor amplifies radio frequency (RF) signals most efficiently. The taps in the coil are there for experimentation purposes, but you can eliminate them. The selector switch is a two pole locking type for extending the coil by an additional 18 turns.
In the second stage, I am ramping up the gain and here it amplifies audio frequency (AF) and some RF. This stage is concerned mainly with small-signal voltage amplification to increase the amplitude of the signal. The 1-µF capacitors are simply there to decouple the middle stage.
In the third stage, the transistor is concerned with producing current gain to drive a load such as a loudspeaker, however the signal is still very weak, and therefore we must use the LT700 audio transformer to drive the speaker. The primary side has three terminals and the centre terminal is not used. The secondary side has two terminals and this is where one connects the speaker.
Matching the loudspeaker impedance with the secondary side of the transformer produces maximum power transfer, hence one should use a minature speaker with 3-Ω to 4-Ω impedance for the loudest sound. Although one could use 6-Ω and 8-Ω loudspeakers, they will not be as loud.
The capacitor C3 earths any high frequency and sharp RF noises. Although I am using a 10-kΩ potentiometer, ideally a 100 kΩ may work better.
The objective of this project was to design a radio that was as simple as possible to make, using the least number of readily available components. Had I introduced bootstrapping and regeneration, the circuit complexity would have increased by a factor of 10, and the skill requirement by a factor of 20 and that would have brought the project scope well outside the reach of students. As it is, all that is required is some experience of building crystal radios. If you build this radio and it works for you, then feel free to publish and show a video on YouTube. It is always great to see variations and improvements.
I have shown the main circuit and the hardest part of the radio; however, things like a reverse polarity protection diode or a power level LED, or power switch, are optional. If you have the extra time and money then you could add these.
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Three Transistor Radio – Whippersnapper 2 Circuit Diagram Ferrite Coil Antenna Loudspeaker and Transformer Variable Capacitor Baseboard Layout Parts List Completed Build