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The goals of this lab were to introduce the common source FET amplifier, perform LTSpice simulation, add a CE amp, perform a second simulation, and breadboard the RF amplifier circuit.
AM signals received by antennas are fairly weak. An RF amplifier is needed to boost the signal before the audio signal can be effectively extracted by the detector. The RF amplifier in this lab is a two stage CS/CE (common source/common emitter) amplifier. The first stage is constructed with a field effect transistor (FET), has high input impedance, and a low gain. The CE stage is used to increase the overall gain.
The FET used for the CS was an n-type JFET. It behaves very similarly to the n-type MOSFET and is less static sensitive. The FET has three main regions of operation: pinchoff, triode, and saturation. For the purpose of the CS amplifier, the transistor needs to operate in the saturation region. We cando this by creating a DC biased network as seen in the schematic.
Since the gain of the CS amplifier is porportional to the resistance looking in to the drain terminal, it is greatly diminished by the parallel combiation of the load resistor and small drain resistors. This can be fixed by adding a fairly large inductor, which looks like an open circuit from an AC stanpoint, in series with the drain resistor. When simulated with the RFC (large inductor) in place, the gain of the CS inreases to about 15 V/V from 0.98 V/V withou the RFC. With the CE amp added as a second stage the gain increased to about 50 V/V.
With the completion of the RF amplifier, the AM radio is now complete with the exception of the antenna.