Copyright of this circuit
belongs to smart kit electronics. In this page we will use this
circuit to discuss for improvements and we will introduce some
changes based on original schematic.
This is a small, broad band, signal amplifier which covers the frequencies from 40 to 900
MHz. These frequencies include TV in VHF
and UHF and also the radio broadcasting frequencies in the 88 – 108 MHz FM band.
It is connected between the antenna and the input of your receiver and boosts the signals
by up to 20 dB, thus making it possible to
receive even the weakest signals.
Dimensions (4,3cm x 5,4cm)
Technical Specifications -Characteristics
Frequency response: 40 – 900 MHz
Gain: . 20 dB
Maximum output level: 90 uV
Input – output impedance: 75 ohm
How it Works
The circuit is built around a single transistor a UHF low signal device, the BFW 92. This
transistor can operate in frequencies as high
as 1.6 GHz, and has a gain of 23 dB. The signal from the antenna is applied to the input
of the circuit and through C5 is fed to the
base of the transistor. It is amplified and from the collector of the BFW 92 through C2
and C1 is taken to the input of the radio or TV
The circuit operates off a small 9 V battery which, because of the very low power
consumption of the circuit, is going to last for a
very long time.
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
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
– 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
– Take care not to overheat the tracks as it is very easy to lift them from the board and
– 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 project is a very easy one, as the components which form the circuit are very few and
their outlines have been clearly stencilled
on the board for you. The only unusual thing is that the transistor must be soldered from
the copper side of the board.
This is, however, common with UHF devices and is usually done to avoid the introduction of
stray capacitances between the
transistor’s leads that could possibly alter the behaviour of the circuit.
Be careful to make good joints and try to keep the component leads as short as possible
because of the very high frequencies
involved. Solder first of all the pins and the resistors. The coils are supplied ready to
be soldered on the printed circuit and you
should take care not to deform them in the process. Place then the capacitors and solder
the diodes carefully trying to avoid
overheating them and making sure that they are correctly aligned.
Solder the transistor in its place, after you have finished soldering the other
components, to avoid overheating it, and be careful to
align it according to the diagram included in the instructions. (The lettering on the
transistor body should be facing away from the
The input of the circuit is at point 4 and ground and the output at point 1 and ground.
The battery is connected using the battery clip
supplied at points 2 (-) and 3 (+), and is a miniature 9 V one, alkaline if you prefer.
For best performance and to avoid unwanted interference during operation it is recommended
to place the circuit in a small metal
box, and use suitable connectors mounted on the box for the external connections. You can
use a box large enough to house the
amplifier and the battery or you can use an external power supply, but remember to use a
FEED THROUGH capacitor on the
positive supply line, where it passes through the metal box. If you plan to use the
amplifier for both VHF and UHF TV reception you
should use a common VHF/UHF mixer before the amplifier’s input.
R1 = 120 Ohm (brown, red, brown)
R2 = 1,5 KOhm (brown, green, red)
R3 = 270 Ohm (red, violet, brown)
R4 = 82 KOhm (gray, red, orange)
C1,C5 = 100pF (ceramic)
C2,C3 = 1nF (ceramic)
C4 = 2,2pF (ceramic)
D1,D2 = 1N4148 diode
Transistor = BFR90, BFR91, BFW92
Misc = PCB, 6pins, solder, 9V battery clip
diameter : 5mm
wire thickness : 0,5mm
turns : 8