Three Transistor Radio - Whippersnapper 1

This three transistor radio circuit uses BC547B silicon transistors, and this is one of those age-old circuits that most whippersnappers will remember seeing it in books and magazines and making. I saw it back in 1979 and it was based on the BC107 transistor. I also remember seeing a similar circuit like this with OC71 germanium transistors! On the surface it appears to be a very simple circuit, however, it is much more complicated than it looks.

The quality of the construction is very important, and a working ferrite coil and tuning capacitor (C1) combination are vital to the success of this project. The BC547 documentation states that these silicon transistors may connect directly with each other, therefore, in this circuit, there is no decoupling, and the base junctions of successive stages connect directly to the collector junctions of the preceding stages. This is of course a good thing, as you will not need to buy any decoupling capacitors!

The 10 kΩ biasing resistors must have precisely identical values so that transistors T1 and T2 are conducting at the same precise level. This circuit also uses regeneration, and the 1 MΩ resistor brings some of the amplified signal back to the first stage. However, this feedback signal will contain RF and AF components. The electrolytic capacitor (C2) shorts the audio frequency signal to ground through the coil so that the base junction -- of the first transistor -- has only RF signal present. The value of the electrolytic capacitor can be almost anything from 1 µF to 10 µF.

The 22 nF capacitor (C3) shorts the RF signal to ground so that mainly audio frequency (AF) signals are amplified to the successive stages by transistors T2 and T3. As you can imagine, the values and placement of these capacitors are critical.

Tuned Circuit

The ferrite coil (L1) and tuning capacitor (C1) form a tuned circuit that is critical to the success of this build. If you can get it working as a crystal radio first, and are able to tune into stations, then it has a good chance of working with this transistor circuit! To test this circuit I used a tank combination from a previous crystal radio project.

If you are looking for coverage of the MW band then you will need around 100 turns of the coil. The coil has a single tap on the ‘earthy’ side, taken after 10 turns of the coil. This is a general rule-of-thumb that works well and provides good Q selectivity.

Load Resistance and RL

You need high impedance headphones for the load. Usually, 34 Ω works well. I used my vintage Sony MDR-005 and it was providing a clear sound. RL should be around 39 Ω. You could also use 47 Ω or 56 Ω to get the loudness you prefer.