Schematics > Video and TV > UHF Preamplifier
UHF PreamplifierThis circuit is designed to work at UHF frequencies in the range 450-800MHz.
It has a gain of around 10dB and is suitable for boosting weak TV signals. The
circuit is shown below:-
UHF Preamplifier circuit
The MPSH10 transistor used is available from Maplin Electronics order code
CR01B. Alternatives that may be used instead are BF180 and BCY90. The tuned
circuit comprising the 15nH inductor and 2.2pF capacitor resonate in the centre
of the UHF band. The 2.2pF capacitor may be exchanged for a 4.7pF or a trimmer
capacitor of 2-6pF to improve results. The approximate frequency response is
shown below. N.B. This is a simulated response using the TINA program produced
by using a swept 20uV input swept over the frequency range 400-800MHz. Output
was measured into a 1k source and the frequency generator has a 75ohm impedance.
Construction
The coil is half a turn of 18-20 SWG copper wire bent around a half inch
drill bit. This ensures a low Q and therefore broad tuning. High frequency work
requires special construction techniques to avoid instability (unwanted
oscillations) caused by feedback from output to input. Veroboard is not suitable
for this project as the capacitance between tracks is around 0.2pF. A better
approach is to use tag-strip or a PCB. The circuitry should be enclosed in a
metal case and a screen made between input and output. As the transistor is used
in common base mode,its low input impedance is a good match for 50-75 ohm coax
cable, whilst at the same time providing full voltage gain to the upper
frequency limit of the device. The 15nH inductor load, having almost a short
circuit impedance at DC, has an impedance of 56ohms at 600MHz. This inductance
and 2.2pF capacitor form a tank circuit at the transistors collector, providing
maximum gain at resonance. Note however that the voltage gain will be reduced
under load, when the circuit is connected to the input of a TV set or a very
long piece of coaxial cable for example. Hence the simulated Tina plot.
Title: UHF Preamplifier
Source: unknown
Published on: 2006-01-15
Reads: 683
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