Posted on Jan 22, 2013

A MW AM radio like those you buy, or used to buy. Use just BC549 trannies, with the BC559 complement being allowed in the audio output stage. Buying RF antenna coils, oscillator coils and intermediate frequency transformers would be cheating - wind your own. 9V battery powered. The transistor Q4 is biased up as a class A amplifier with some gain. The collector circuit provides feedback to the base via the tuned circuit. The two diodes D1 and D2 make a limiter to stabilise the amplitude across the frequency range. This is a differential amplifier comprising Q2 and Q3 with the two collectors connected together instead of going to individual loads to +BAT. The normal difference voltage output that you'd expect is therefore cancelled and you are left with the much smaller signal which is the effect of more current flowing in one transistor reducing the current flowing in the other, thus reducing its partner's gain and vice-versa.

The turns ratios for these home wound Intermediate Frequency Transformers, T1, T2, and T3, were determined by looking up the turns ratios used in TOKO brand transformers for the 1st 2nd and 3rd IFTs. The number of turns for the primaries were first determined by calculating the desired inductance to resonate with the parallel capacitor at the IF of 455kHz. fo=1/2pi*sqrtLC where C is about 220pF typically. Then using the tables supplied with the coil formers and cores the approximate number of turns can be found using the "Al" values. This value gives you a nominal inductance per 1000 turns, and the inductance is proportional to the square of the number of turns. So if the quoted Al for a certain former and core is Al=10uH/1000Turns, and if you wanted 5uH you would wind (5^2/10^2)*1000=250 turns. The Al values work pretty well and I didn't need to rewind after testing, though I may have changed the parallel capacitor value for T1. An RF signal generator was used to check the resonant frequency and to set the cores in about the right positions prior to wiring up. An interesting fact reveals itself in the IF strip. The transistors do not need to be class A biased as you would normally do in a cascaded audio amplifier chain. Because the inductance of the IFTs stores energy, and we are dealing with an AM signal, you can bias the transistors pretty much in class C, only increasing the bias current when maximum gain is needed on...

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