One Transistor Radio

The one transistor AM radio receiver is something all whippersnappers try out when starting out on the royal road to radio building. This circuit is the one that I designed and made when I was a little whippersnapper. Although this circuit uses a transistor to amplify the signal from the crystal radio, it still requires a crystal earpiece, and you will be able to hear BBC Radio 4 very clearly if you are based in Croydon, UK.

The transistor used in this circuit is the common garden variety BC549C capable of providing maximum gain (hFE) of 800. However, BC547, and BC548 general purpose NPN transistors might also work as well. You could also try other transistors for experimentation, such as the 2SC3112 capable of providing gain (hFE) of 3600. If you have an MPSA18 high gain transistor, then that one is capable of providing maximum gain (hFE) of 1500. Back when I was a whippersnapper, I had an old Mullard Transistor Databook for reference and I was always experimenting with different high gain transistors to make my crystal radio louder.


Circuit Diagram

One Transistor Radio Circuit Diagram

As you can see in this circuit diagram, I have a centre tap in the ferrite coil from where I am taking the signal. For the coil wire, I am using Litz wire, which is the proper radio coil you should use to get a strong signal, and I have wound 45-turns on either side of the centre tap. All the turns must be in the same direction. The ferrite rod is a standard sized one, with 10 mm diameter.

The diode demodulates the RF (radio frequency) signal by removing the carrier, leaving the meaningful AF (audio frequency) part in AC form, and this is what we need to amplify using the transistor. The demodulation process also produces an average value, which is a DC component of the signal. We need to block this DC component and allow only AC through by using an electrolytic capacitor. One good reason for blocking the DC component of the signal is that it adversely affects the biasing of the transistor and this is undesirable. Since I simply wish to block DC I chose to use an electrolytic capacitor, which is usually something all hobbyists tend to have. A value in the µF range will have negligible effect on the signal, and one can use a wide range of values. I am using a 10 µF value simply because this is very common to have. The general-purpose transistor BC549C then amplifies the AC component of the signal, allowing one to hear it more loudly through the crystal earpiece. The power supply is a single 1.5 V cell. You can use any size single dry cell you wish for the power supply.

Comparison of Radio Circuits

Comparing these two circuits, we can see that they have identical components, and the one transistor radio circuit is just one small step further from the crystal radio circuit. Hence it is best to start by building the crystal radio first.

Project Outline

Stage 1 Stage 2

In electronic engineering it is good practice to build circuits in logical stages and testing each stage before moving further. The first step of this project is to build the Crystal Radio -- Budget Cuts 140 pF, which will involve building the ferrite rod coil, selecting the tuning capacitor, using a suitable germanium diode, and establishing proper earth and antenna. If you find that your crystal radio is operating successfully and you are receiving a number of strong stations, then this is a good point to continue the construction by adding a transistor stage to make the one transistor radio.

8.2 MΩ Biasing Resistor

The BC549C has maximum DC current gain of 800, with Ic rating of 2 mA. If you want to squeeze all of that amplifying capability then the biasing resistor has to be very large. The base junction of the BC549C transistor is biased by a positive charge from the very large 8.2 MΩ resistor. The reason why it is so large is that it requires a very tiny amount of positive charge to polarise the base junction. When you connect the battery, it will take around ten seconds for the base junction to charge up, and then you will start hearing the sound getting progressively louder. It is almost like a valve radio warming up. If you are having problems with this, then you could reduce the resistor value down to 1 MΩ, but it will not be as loud.

Germanium Diode D

The germanium diode in this circuit is for testing the reception of the tuned circuit by temporarily making a crystal radio. A germanium type, such as the OA90 that one can get from reputable stores usually works well. Please consult the Crystal Radio Diode article for more information.

Experimenting

If you are getting a strong audio signal through the crystal earpiece, and your circuit is working, then do some experimenting.

  1. Remove the germanium diode, and feed the signal directly to the 10 µF electrolytic capacitor, and see if it still works. You will find that the transistor still detects the signal without the diode! This is becasue the pn junction of the transistor behaves as a diode. However if you disconnect the battery, then the sound stops.
  2. What would happen if we replace the electrolytic capacitor with a direct link to the base junction of the transistor? You will see that the transistor no longer amplifies.
  3. What if you reverse the polarity of the electrolytic capacitor, does it make any difference?
  4. Replace the electrolytic capacitor with a ceramic or polyester (non-polarised capacitor) in the nano-farad (nF), and pico-farad (pF) ranges to see how much impact these values have on the signal. Notice any changes to the signal loudness, and frequency shifts causing you to retune.

Just experiment with the circuit and enjoy yourself, because that is what I did when I was a whippersnapper building transistor radios.


Related Pages

One Transistor Radio
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Crystal Radio - Budget Cuts 140-pF