3.3 Simple Radio Receiver with TDA7050 IC
Readers that were able to browse through book 4 and, especially, book 5 of Practical Electronics edition, were able to convince themselves that there's a huge number of various audio amplifiers built with IC's, therefore amplifiers in discrete technique are practically no more being made. If we add on this the fact that integrated amplifiers outreach their discrete competition both by price and quality, it is then clear why we are going to use them in this book.
Electronic diagram of a simple radio receiver with LF stage built around the TDA7050 IC, where reproduction is being made through modern-type headphones, resistance being 32 or 64 Ù, is given on Pic.3.15. In book 5 of Practical Electronics you have acquainted yourself in more detail with this IC. Let us just repeat that it can be purchased both in 8-pin DIL package that we have been using, and in SO package, suitable for surface mount. Its label in the latter case is TDA7050T and it can also be used without any problems whatsoever. In that case changes on the PCB should be made considering that it is being mounted on the copper side of the board, and that drilling is now obsolete, since the pins are being soldered directly onto the copper foil. You can read the text that follows Pics.4.16 and 4.17 about soldering SMD components. Supply voltage for the IC is in range from 1.6 to 6 V. Idle current is 3 mA on 3 V supply voltage. Voltage amplification is 32 dB (40 x) on 6 V supply voltage and 32 Ù headphones resistance. Maximum output power is 150 mW, more than enough for the headphones amplifier device.
Regarding the Pic.3.15, both input and detection circuit described in two previous projects can be used, We have decided instead, to show you how to use the coil with multiple legs, since it offers more possibilities for experimenting in order to achieve optimal reception. The picture of such coil is given on Pic.3.16, the legs being made as previously described in this book. The first leg (numerated 6) is made after the 15-th quirk, the second (5) after 30-th, the third (4) after 45-th and the last one (2) after 55-th. Number of quirks between the legs isn't critical, you can have even more legs, being arranged more closely to each other. As seen on Pic.3.15, both the antenna and the detector are connected over these legs. Legs No. 4 and 5 have been used, but that is not a must. The closer the legwhere antenna is connected is to the ground (point Z), the less it damps the oscillatory circuit (therefore increasing the receiver's selectivity), and less it reduces the reception bandwidth. However, the signal that exits the antenna is then also smaller. The similar thing is with the leg where the diode (its anode end) is connected: The closer it is to the ground, the less the detector damps and tunes out the input circuit, but the signal being transferred to the detector is also smaller. It is clear now that a compromise must be made: Experimenting with various coil legs, those providing the optimal reception should be found.
Let us finally add that the antenna can also be connected to point 1, over the coupling capacitor (C1 on Pic.3.1), and the detector on one of the coil legs, or you can connect both the antenna and the diode's anode on the same leg, to connect the anode to point 1, etc. You should try various combinations out, observing their influence on the reception. You should certainly re-tune the resonance of the oscillating circuit after every change, with the aid of the variable capacitor C.
The resistor R2 and capacitor C2 create an LF filter, whose role is to pass through at the next stage the LF signal being detected, preventing simultaneously the HF voltage do the same (this voltage originates from the AM signal carrier). This filter circuit affects the LF signal tone colour. If you don't like it, you should alter the capacitance C2 in order to change it.
PCB for this device is shown on Pic.3.17. The receiver can be put in a box just as on Pic.3.14. The only significant difference is that a 4.5 V battery pack must be used instead of 1.5 V battery, but there's plenty of room for it.
