TV Preamplifier via UHF Band

Overview

The design of the circuit is aimed to create a preamplifier for television systems that would operate in the UHF frequency range of 450 MHz to 800 MHz and increase the weak TV signals.

Terminology

• Ultra High Frequency (UHF) – operates at a frequency range of 300 MHz to 3 GHz
• MPSH10 – an NPN RF transistor designed for use in low noise UHF/VHF amplifiers, in high output UHF oscillators, and in low frequency drift; has collector currents in the 100 uA to 20 mA range in common base or common emitter mode of operations

Circuit Explanation

For the circuit to be able to oscillate in the middle of the UHF band with maximum amplitude, know as resonance, the circuit can be tuned with the use of the 2.2 pF capacitor and 15 nH inductor. The inductor has a standard wire gauge of 18-20 wound around a half inch drill bit where the coil is half turned. To achieve better results, a 4.7 pF capacitor may replace the 2.2 pF capacitor. Similarly a trimmer capacitor with 2 to 6 pF capacitor may be used to set the fine adjustments in the capacitance of the circuit. The impedance produced by the 15 nH inductor plays around 56 ohms within the 600 MHz frequency and the overall gain of the circuit is 10 dB, which is enough to power up the inadequate signals. To obtain the maximum resonance and gain, the transistor collector is being supplied by the tank circuit where the inductance and capacitance are coupled. The transistor provides the device’s upper frequency limit with the full voltage gain. Also, it complements with the impedance of the coaxial cable by producing a low input impedance while conducting in the common base mode. The voltage gain might be reduced upon connecting the circuit to a longer coaxial cable or on a TV set which may cause under load to the circuit. this can be measured using a graph know as TINA or Toolkit for Interactive Network Analysis.

The inductor is responsible in making sure that the tuning will be distinct by producing a low quality factor or Q. this Q determines the time it will take for the oscillation to fade out by comparing the amplitude of the system to its oscillating period. This is necessary to avoid feedbacks from output to input that will result to instability of the circuit especially when trying to obtain high frequencies. To physically avoid these feedbacks, a screen should be placed between the output and input while the whole circuitry should be enclosed in a metal casing.

Application

The UHF TV preamplifier is mainly used for providing an increase in the incoming signals by amplifying the weak UHF or VHF television signals for those areas with poor reception. It also helps on eliminating snow on the TV. In creating this preamplifier circuit, choosing which channels are strong and which channels are weak should be taken into consideration, so as not to interfere with the weaker stations.

Source:www.zen22142.zen.co.uk/Circuits/rf/uhfpreamp.htm