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.

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.


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