Two Transistor Radio -- 6-V Option

This two transistor AM radio receiver uses the cheap and commonly available BC549B transistors to demodulate and amplify an AM radio signal. This circuit is a natural extension from the crystal radio and one-transistor radio circuits shown in the Radio section of this site. These examples are intended as a starting point and once you get a strong signal you will want to make modifications to see if you can make your own design that works better for you depending upon the components you have. The ferrite rod and coil design is exactly the same as that used in the previous crystal radio project and if you have been following the articles then the good news is that you can use it here. You will notice that in this circuit there is no germanium diode because the first transistor performs the demodulation and amplification of the radio frequency (RF) signal. At the collector output of the first stage, there is a 5 nF capacitor to ground. This capacitor behaves as a filter and shorts the high-frequency RF to earth. The resultant signal is then mainly audio frequency (AF), which is amplified by the second transistor to drive a crystal earpiece.

Even though there is a transistor dedicated to amplifying the audio frequency, the signal amplitude is very small and not large enough to drive a loudspeaker. However it is sufficiently large to drive a special high-impedance transducer known as a crystal earpiece. Hence a crystal earpiece is a vital component in this circuit.

Parts

The ferrite coil in this project is the same one used for the Spy Crystal Radio - Budget Cuts project. Once you have a good coil that works well and provides some stations, you can use it for other transistor radio projects. I obtained the specification of this one through trial and error, however I made dozens of coils, before getting one that worked very well for me here in the UK.

• Antenna: Copper wire (2-metres)
• Earth: Copper wire (2-metres)
• 4 × 1.5 V AA size dry cells, and battery holder
• Resistors: 10 kΩ, 100 Ω, 1.5 MΩ, 27 kΩ, 470 kΩ (all ¼-watt)
• Fixed Capacitors: 2 × 100 nF, 5 nF
• Variable Capacitor: 140 pF or more
• Ferrite Rod: 10 mm diameter
• Coil type: Litz wire
• Coil design: Centre tap, with 35-turns on each side, and winding in the same direction.

Compatible Transistors and Pinout Info

Back in my time I was using the BC109 transistors that came in a metal canister package, however, for this article, I am using BC549B, which works just as well. The BC549C is probably the better one to go with; however, I think this circuit will work fine with any general purpose NPN transistor such as the BC547, BC548, and BC549.

Test

• Location: Croydon, United Kingdom
• Weather: Clear
• Number of stations received: 4 with 2-metre high wire antenna

The radio worked well and I was able to receive many stations in Croydon. My neighbour, who had the Sinclair Micromatic Pocket Radio, preferred mine because it was louder and received more stations!

Other 2-Transistor AM Radio Circuits