Two Transistor Straight No Frequency Conversion Unique AM Radio Receiver

Figure 1. Two Transistor Darlington Radio

This simple, no frequency conversion, straight AM transistor radio is made of only seven components. It uses the "grid" detector concept for audio signal recovery and an unusual two transistor Darlington pair that are employed in a direct-coupled CE [Common Emitter] configuration to provide high input impedance and low damping for the tuned L-C circuit.

In tuned, parallel, electronic radio L-C circuits, it is very important to operate resonant circuits with as light load as possible. It was easy for vacuum tubes that are voltage operated devices to work with tuned circuits in resonance while still maintained sharp peeks and a high Q.

As transistors displaced the vacuum tubes there was only a few possible ways to obtain a high input impedance amplifier until the FETs [Field Effect Transistor] appeared. This radio here is unique that the two transistors are employed here are able to provide a high input impedance, low damping load design. This kind of dual transistor Darlington pair connection can be considered as a single high gain device.

Darlington pairs are usually made with similar type transistors. In this circuit two PNP transistors are used. Using PNP type transistors are more advantageous because they usually and typically have a higher gain or amplification factor. If germanium transistors available they would also be useable for this project. Since germanium transistors have half the voltage drop than silicon their use would allow reduction of the battery voltage to three volts. It is important to note that germanium transistors have a higher leakage current and lower operating temperature range.

If the parts box overflow with unknown type transistors, now is the time to put them to good use. Of the available transistors select the highest gain units. If a transistor gain tester is available use it or use a VOM. Otherwise just use whatever working transistor is available.

The input impedance of this transistor pair is the product of the load resistor and the gain. The load resistor for this circuit is the magnetic headphone that is at least a thousand ohms. Usually it is two to four thousand ohms, depending on how are the two halves connected. And the combined gain of the two transistor is the product of the two unit gains and that is easily comes to a thousand or more. So the product of 2,000 to 4,000 times 1,000 is a respectable two to four Megohms.

The L1 antenna coil is a few turns wound near, or on top of the existing L2. The L2 coil is a 240 uH [micro Henry] type and it can be made in several different ways. It may be air core or made with a powdered iron core or on a ferrite rod, or on a toroid. The rotary capacitor has to be low loss, good grade, a larger air insulation is preferred but a smaller one obtained from an old transistor radio would be just as useable. It is important that the C2 capacitor be a very low loss polypropylene film or mica type. The Q1 and Q2 transistors are old, high gain PNP, 2N3638 type made by the Fairchild company. Practically any small signal types are useable, even if they are not identical. If only NPN transistors are at hand, reverse the battery such that the positive terminal would connect to the collector instead of the emitters as presently.

With a good long wire antenna, or strong local stations, it is possible with this radio to use a loudspeaker in place of the headphones. It is important to remember, that because the gain of the Darlington amplifier is a function of the load resistor, when using a speaker, an output transformer - hopefully an old tube type - is required.

                           Parts list for the two transistor Darlington radio:

                            B1 - Battery 9 Volt transistor type - Carbon Zinc is fine
                            C1 - Capacitor 500 pF, Rotary
                            C2 - Capacitor 0.01 uF, low loss polypropylene film or mica
                            L1 - Coil Antenna - Part of L2, taps at 1/3 and 2/3 turns
                            L2 - Coil 240 uH - taps at 1/3 and 2/3 turns
                            Q1, Q2 - Transistor, PNP, Germanium or silicon
                            2000 Ohms magnetic headphones or small speaker