7/28/2023 0 Comments Two transistor fm receiverIt 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. 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. ![]() 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. Practically any small signal types are useable, even if they are not identical. The Q1 and Q2 transistors are old, high gain PNP, 2N3638 type made by the Fairchild company. It is important that the C2 capacitor be a very low loss polypropylene film or mica type. 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 may be air core or made with a powdered iron core or on a ferrite rod, or on a toroid. The L2 coil is a 240 uH type and it can be made in several different ways. The L1 antenna coil is a few turns wound near, or on top of the existing L2. So the product of 2,000 to 4,000 times 1,000 is a respectable two to four Megohms. 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. Usually it is two to four thousand ohms, depending on how are the two halves connected. The load resistor for this circuit is the magnetic headphone that is at least a thousand ohms. The input impedance of this transistor pair is the product of the load resistor and the gain. Otherwise just use whatever working transistor is available. If a transistor gain tester is available use it or use a VOM. Of the available transistors select the highest gain units. If the parts box overflow with unknown type transistors, now is the time to put them to good use. It is important to note that germanium transistors have a higher leakage current and lower operating temperature range. Since germanium transistors have half the voltage drop than silicon their use would allow reduction of the battery voltage to three volts. If germanium transistors available they would also be useable for this project. Using PNP type transistors are more advantageous because they usually and typically have a higher gain or amplification factor. In this circuit two PNP transistors are used. This kind of dual transistor Darlington pair connection can be considered as a single high gain device.ĭarlington pairs are usually made with similar type transistors. This radio here is unique that the two transistors are employed here are able to provide a high input impedance, low damping load design. 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.Īs transistors displaced the vacuum tubes there was only a few possible ways to obtain a high input impedance amplifier until the FETs appeared. In tuned, parallel, electronic radio L-C circuits, it is very important to operate resonant circuits with as light load as possible. 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 configuration to provide high input impedance and low damping for the tuned L-C circuit. This simple, no frequency conversion, straight AM transistor radio is made of only seven components. ![]() Nevertheless, they can and do work very well, and this article will make use of - Technology Note - TN1001 - Karl Nagyįigure 1. They were very popular with early radio experimenters since the advent of radio in the ’30s, but because they required a bit of skill and experimentation, they fell out of fashion. Regenerative receivers work differently from the typical FM receiver circuits we just discussed. This signal is then amplified by the AF amplifier. The IF is then sent to an FM detector, which converts frequency to voltage and extracts the audio modulation. One of these is the needed intermediate frequency (IF). This is the tuning part, and the result is the creation of two new frequencies-the incoming signal with and without the local oscillator (LO) frequency. Then follows the mixer, which mixes the incoming frequency with another frequency from the local oscillator. The RF amp is a VHF amplifier tuned to the FM band and provides rejection to other frequencies and the intermediate frequency image (more on this below). ![]() The FM radio signal is picked up by the antenna, which as we discussed in a previous article on FM transmitters, is best as a quarter wavelength.
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