The most obvious shortcoming of the receiver described in the previous chapter is that it can perform the sound reproduction loud enough only in case when the programme from some local or very powerful radio transmitter is being received, which can create very strong signal in the reception antenna. The reception of signals from other transmitters is too weak. The only thing that can be done is either to increase the length of the antenna, which, of course, does have its limits, or to insert an amplifying stage into the receiver. The simplest way to perform the latter is to add a LF amplifying stage behind the detector in the detection receiver, Pic.3.1. The electrical diagram of one such receiver is given on Pic.3.12. The electrical load in the detection stage are no longer headphones, but an ordinary resistor R1. An NF signal is obtained on its ends, which is then being lead into the LF amplifier with the transistor T, over the coupling capacitor C3. The electrical load in the collector circuit of the transistor are the headphones, which transform the amplified LF signal into sound.
The voltage negative feedback is being obtained with the capacitor C4, and the current negative feedback with the resistor R3. They enhance the characteristics of the amplifier (increase its stability, reduce distortion, widen the reception band), but they also reduce the amplification. The capacitor C5 prevents the AM signal carrier which, although very weak, also appears on the detector output, from entering the headphones. These 3 components however, can be omitted in most cases, for the sake of simplifying the device. C4 and C5 can be simply removed, and a piece of wire should be soldered instead of R3. Transistor operation point where the optimal reproduction (the biggest amplification, the smallest distortion) is being set by adjusting the resistence value of the resistor R2. The simplest way to do it is to connect the trimmer of couple of MÙ instead of the resistor, set the receiver on some station, then change the resistence until the optimal reception is being reached. The trimmer is then put out, its resistence measured, and a resistor of similar resistivity is then soldered on its place. The transistor T is any universal NPN - type.
The component data is given on the electrical diagram and in table on the rightmost side of Pic.3.12. If you don't have a 100 pF capacitor (C2) you can put some of bigger capacitance, but you should then use smaller R1.
The PCB layout is given on Pic.3.13. On 3.13-a is a picture that should be copied with the thin alcohol marker onto well cleaned copper side of the pertinax plate. Etching is to be performed then, as well as drilling the 0.8 mm holes, in the way that has been described in detail in PE2 issue (Practical realization of electronic devices). The component layout is given on Pic.3.13-b. The printed circuit is also visible on this picture, and that can be achieved by using vitroplast plate instead of pertinax. The look of board upon completion is on 3.13-c. Before soldering the wires that connect the variable capacitor, battery, switch etc. with the board, put some tin on the wire ends, using the calofonium or the tinol wire. You should by all means do this, especially if you 're not using the wire that is not pre tinned, in order to avoid cold solders, very unpleasant surprises that are hard to detect and locate.
The layout of the entire receiver is given on Pic.3.14, in a scale 1:1. As in the previous receiver example, the coil is being glued to the top side of the box, over two small pieces of wood. The board is being tightened with a screw, that is screwed into a piece of wood being glued to the front end box plate. The battery is attached with a rubber band for the pieces of wood glued at the back plate of the box. We did this, however, only for the sake of having a clear and understandable drawing. Battery can be put inside the box in the same way as before, or some other way.
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.
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.
