![]() Therefore, in practice the output often is routed through either a voltage follower ( common-drain or CD stage), or a current follower ( common-gate or CG stage), to obtain more favorable output and frequency characteristics. Another major drawback is the amplifier's limited high-frequency response. As seen below in the formula, the voltage gain depends on the load resistance, so it cannot be applied to drive low-resistance devices, such as a speaker (having a resistance of 8 ohms). ![]() However, the FET device's output resistance typically is not high enough for a reasonable transconductance amplifier ( ideally infinite), nor low enough for a decent voltage amplifier ( ideally zero). As a voltage amplifier, input voltage modulates the current flowing through the FET, changing the voltage across the output resistance according to Ohm's law. As a transconductance amplifier, the input voltage is seen as modulating the current going to the load. The analogous bipolar junction transistor circuit may be viewed as a transconductance amplifier or as a voltage amplifier. The only terminal remaining is the source. In this example, the signal enters the gate, and exits the drain. The remaining terminal is what is known as "common". The easiest way to tell if a FET is common source, common drain, or common gate is to examine where the signal enters and leaves. In electronics, a common-source amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage or transconductance amplifier. ![]() Figure 2: Basic N-channel JFET common-source circuit with source degeneration. ![]()
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