How it Works
The circuit uses five transistors and is a broad band amplifier with a bandwidth of 5 MHz.
The signal is applied at points 1 and 2
(ground) and is taken through C1 to the first stage which is a preamplifier and is built
around Q1. In the output of Q1 are DC coupled Q2,3 which amplify the signal more, and as
they are DC coupled to the preamplifier there is virtually no distortion and the
amplification is quite high. Finally the signal from the out put Q3 is fed to the output
transistors which are Q4 & Q5. These two transistors are complementary and the signal
from their common emitters is taken to the signal distribution R-C network from where it
is sent to the various devices which are driven by the circuit. The circuit needs a 12 VDC
power supply and it is much better if it is a stabilised one like the circuit printed
elsewhere in the instructions.
Construction
First of all let us consider a few basics in building electronic circuits on a printed
circuit board. The board is made of a thin insulating
material clad with a thin layer of conductive copper that is shaped in such a way as to
form the necessary conductors between the
various components of the circuit. The use of a properly designed printed circuit board is
very desirable as it speeds construction up
considerably and reduces the possibility of making errors. Smart Kit boards also come
pre-drilled and with the outline of the components and their identification printed on the
component side to make construction easier. To protect the board during storage from
oxidation and assure it gets to you in perfect condition the copper is tinned during
manufacturing and covered with a special varnish that protects it from getting oxidised
and also makes soldering easier.
Soldering the components to the board is the only way to build your circuit and from the
way you do it depends greatly your success
or failure. This work is not very difficult and if you stick to a few rules you should
have no problems. The soldering iron that you use
must be light and its power should not exceed the 25 Watts. The tip should be fine and
must be kept clean at all times. For this
purpose come very handy specially made sponges that are kept wet and from time to time you
can wipe the hot tip on them to
remove all the residues that tend to accumulate on it.
DO NOT file or sandpaper a dirty or worn out tip. If the tip cannot be cleaned, replace
it. There are many different types of solder in
the market and you should choose a good quality one that contains the necessary flux in
its core, to assure a perfect joint every
time.
DO NOT use soldering flux apart from that which is already included in your solder. Too
much flux can cause many problems and is
one of the main causes of circuit malfunction. If nevertheless you have to use extra flux,
as it is the case when you have to tin
copper wires, clean it very thoroughly after you finish your work.
In order to solder a component correctly you should do the following:
- Clean the component leads with a small piece of emery paper.
- Bend them at the correct distance from the component’s body and insert the component
in its place on the board.
- You may find sometimes a component with heavier gauge leads than usual, that are too
thick to enter in the holes of the p.c.
board. In this case use a mini drill to enlarge the holes slightly. Do not make the holes
too large as this is going to make soldering
difficult afterwards.
- Take the hot iron and place its tip on the component lead while holding the end of the
solder wire at the point where the lead
emerges from the board. The iron tip must touch the lead slightly above the p.c. board.
- When the solder starts to melt and flow, wait till it covers evenly the area around the
hole and the flux boils and gets out from
underneath the solder. The whole operation should not take more than 5 seconds. Remove the
iron and leave the solder to cool
naturally without blowing on it or moving the component. If everything was done properly
the surface of the joint must have a bright
metallic finish and its edges should be smoothly ended on the component lead and the board
track. If the solder looks dull, cracked,
or has the shape of a blob then you have made a dry joint and you should remove the solder
(with a pump, or a solder wick) and redo
it.
- Take care not to overheat the tracks as it is very easy to lift them from the board and
break them.
- When you are soldering a sensitive component it is good practice to hold the lead from
the component side of the board with a pair
of long-nose pliers to divert any heat that could possibly damage the component.
- Make sure that you do not use more solder than it is necessary as you are running the
risk of short-circuiting adjacent tracks on
the board, especially if they are very close together.
- When you finish your work cut off the excess of the component leads and clean the board
thoroughly with a suitable solvent to
remove all flux residues that may still remain on it.
- The construction should not present any difficulties if you follow the circuit diagram
carefully and place the components in their
place as it is outlined on the component side of the p.c. board.
Solder first of all the pins, then the resistors, the capacitors making sure that the
electrolytic are connected correctly with respect to
their polarity and finally insert the transistors in their places and solder them very
carefully as overheating during soldering can
destroy them.
When all the components have been soldered make a careful inspection of the circuit and if
you are satisfied that there are no
mistakes make the following connections:
- Supply voltage at points 3 (+12 VDC) and 4 (-).
- Input signal at points 1 (signal) and 2 (common).
- Outputs 5,6,7 (signal) and 4 (common).
As it is only two of the outputs are connected initially. To enable the third you should
also connect the positive leads of C7 and C9
together.
