However, the junction transistor was a relatively bulky device that was difficult to manufacture on a mass-production basis, which limited it to a number of specialised applications. FETs were theorized as potential alternatives to junction transistors, but researchers were unable to build practical FETs, largely due to the troublesome surface state barrier that prevented the external electric field from penetrating into the material. A breakthrough came with the work of Egyptian engineer Mohamed M. Atalla in the late s.
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WhatsApp Advertisement FM transmissions can be received within a range of 40 km. If you are in fringe areas, you may get a very weak signal. It costs less and is simple to design. Field effect transistors Field-effect transistors FETs are superior to bipolar transistors in many applications as these have a much higher gain—approaching that of a vacuum tube. On comparing the FETs with a vacuum tube, the gate implies the grid, the source implies the cathode, and the drain implies the plate. In a transistor, the base implies the grid, the emitter implies the source, and the collector implies the drain.
In dual-gate FETs, gate 1 is the signal gate and gate 2 is the control gate. The gates are effectively in series, making it easy to control the dynamic range of the device by varying the bias on gate 2. The resistance between the gate and rest of the device is extremely high because these are separated by a thin dielectric layer.
The isolation between the gates G1 and G2 is relatively high in mixer applications. This reduces oscillator pulling and radiation.
The oscillator pulling is troublesome particularly in shortwave communications. It is a characteristic in many unsophisticated frequency-changer stages, where the incoming signal, if large, pulls the oscillator frequency slightly off the frequency set by the tuning knob and towards a frequency favourable to the large incoming signal. In this configuration, it is used for FM radio band. The quadratic input characteristic of the FET input stage gives better results than the exponential characteristic of a bipolar transistor.
Gate 1 is meant for input and gate 2 is for gain control. The input from the antenna is fed to gate G1 via C1 and L1. Trimmer VC1 is used to tune and select the input frequencies.
Capacitor C4 kpF at the gain control electrode gate 2 decouples any variation in G2 voltage at radio frequencies to maintain constant gain. Set preset VR 47k to adjust the gain or connect a fixed resistor for fixed gain. The output of the circuit is obtained via capacitor C5 and fed to the FM receiver amplifier. You may use a 9V battery without the transformer and diode 1N, or any 6VV power supply to power the circuit refer Fig.
The RF output can be taken directly through capacitor C5. For TV boosters, two types of mountings are employed: The fixed tuned booster is mounted on the mast of the antenna.
Use an epoxy PCB. After soldering, clean the PCB with isopropyl alcohol. Use a suitable enclosure for the circuit. All component leads must be small. Avoid shambled wiring to prevent poor gain or self oscillations.
BF964 Vishay Semiconductors, BF964 Datasheet
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