3.2. The Simplest Amplified Radio Receiver
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.
Please note that in the case of a very long antenna, when C1 of very small capacitance is being used, a hand made trimmer capacitor can be used. It is made by twisting two isolated wires, and its capacitance is changed simply by removing some wire from the ends.
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.
