Schematics > RF circuit > SW RF Pre-Amplifier
SW RF Pre-AmplifierDescription:
A radio frequency amplifier to boost SW reception. Frequency range approximately
5 to 20 MHz.
SW RF Pre-Amplifier circuit
Notes:
The problem with amplifying weak radio signals is that you also amplify the
noise. What you can receive depends on how much background noise is present,
whether it be man made interference or static. In this design the RF signal is
first met by a resistive attenuator, this is necessary as strong signals could
otherwise overload your receiver. The transformer T1 is would on a 1 inch
diameter ferrite loop. The primary (antenna side) is 2 turns of 22 swg wire. The
secondary is 4 turns of 22 swg wire. The 4 turns are spaced to occupy roughly
half the coils circumference. The approximate inductance of the secondary is
20uH. To cover 5 to 20 Mhz a capacitor tuning from around 3pF to 200pF is
required. A standard capacitor of 400 or 500pF (full mesh) can be used by
including a series capacitor, C2 in the above Capacitors. Capacitors in series
behave the same as resistors in parallel. The smallest capacitance is just less
than the smallest capacitor in series and highest value also less than the
highest capacitance. With a 220pF capacitor for C2 and a 500pF variable
capacitor (that tunes down to 5pF) the effective capacitance tunes 143pF to
about 4.8pF. This is roughly correct and not critical as the gain of the FET
will amplify frequencies outside the tuned circuit range. The 2N3819 FET
operates in common source. The series base resistor R1 is included to even out
the response, the internal gate source impedance is thus increased by R1 at
higher frequencies. The drain circuit includes a 2.5mH choke. A 4.7mH can also
be used. As the Q factor of these coils are high, a series resistor R3 is
introduced to flatten the response. The frequency response is shown below
calculated at 10% increments of VC1:
The output impedance from the FET is high, so is buffered by the BC108C in
emitter follower mode. Current drain is around 3mA from a 9 Volt battery. As
with any RF circuit, the circuit is sensitive to noise and interference. A metal
or aluminum box would be a good choice for this project. However, on my trusty
breadboard, this circuit preformed well, and weak signals were boosted well.
Parts List:R1 100k
R2 1k
R3 330
R4 47k
R5 68k
R6 4.7k
VR1 4.7k
C1 100n
C2 220p
C3 100n
C4 1n
C5 10n
VC1 500pF
L1 2.5m
J1 2N3819
Q1 BC108B
Title: SW RF Pre-Amplifier
Source: unknown
Published on: 2006-01-15
Reads: 473
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