RF Preamplifier Operating in Band 2 VHF Range


RF Preamplifier Operating in Band 2 VHF Range

Circuit Project Description

The circuit design employs a VHF amplifier adjusted in such a way that it will operate in the frequency range of 88 to 108 MHz, as part of the Band 2 Radio Spectrum or the FM radio broadcasting range.

TerminologyVery High frequency (VHF) – the radio frequency range from 30 MHz to 300 MHzBand 2 VHF – an FM radio broadcasting band that operates within the frequency range of 88 to 108 MHzField Effect Transistor (FET) – a type of transistor that controls the current and the shape of an electric field where the flow of current or the conductivity of material is only through a single type of semiconductor materialCircuit Explanation

The 2N3819 is an all purpose JFET which offers good performance at mid to high frequencies and a low cost type. It has low noise and leakage, very high system sensitivity, high speed switching capability, wideband high gain at 100 MHz, high low-level signal amplification and high quality of amplification. In this circuit, the two 2N3819 is configured in cascode to increase the current-source performance of the circuit by obtaining higher gain, higher output impedance, higher input-output isolation, higher input impedance, or higher bandwidth. Cascode is a two-stage amplifier that reduces the sensitivity of the circuit to the voltage supply fluctuations and extends the operating voltage beyond its recommended 5.5 V rating. The only disadvantage is that it requires high supply voltage and at all times two transistors.

Located in the upper part of the circuit is the transistor that conducts at common gate where the input signal is applied to the source terminal and the output is taken from the drain for high frequency gain, low input resistance, high output resistance, and a current gain almost at unity. This configuration can be used as a current buffer or voltage amplifier which is applicable to CMOS RF receivers.

The lower transistor conducts at common source mode for high input impedance and high voltage gain. This configuration maybe used as a transconductance amplifier where the current directed to the load is being modulated by the voltage; or as a voltage amplifier where the voltage across the output resistance is being altered because of the modulation of flowing current by the input voltage. Tuning is done on this transistor to achieve the peak for a precise station.

The inductors used have implemented a standard wire gauge of 18 that is wound 4 turns with air space on a 1 cm diameter rod. The inductor L1 has a tap on earth end by 1 turn while inductor L2 is interwound from the supply end by 1 turn. The coil was coupled so that the magnetic field of each coil will overlap the other coil. Enamel coated wires should be used to allow a very thin installation while allowing the turns to touch each other without creating shorted turns.

Application

There are several services where VHF radio band is being allocated such as amateur radio or ham radio, which are utilized for self-training, recreation or public service; in FM radio broadcast where it uses 87.5 MHz to 108 MHz; in marine communications operating at 156 to 174 MHz, where they play vital roles in rescue operations and providing transmission with other harbors and marinas; in television broadcast; in air traffic control communications where ground control used VHF radio to get in touch with an aircraft, vehicle or a person for clearance in the area; in land mobile stations that provide military, business or emergency operations; and in air navigation systems being used Instrument Landing System (ILS), Distance Measuring Equipment (DME), and VHF Omni-directional Radio (VOR).


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