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In electronics, a common-gate amplifier is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a current buffer or voltage amplifier. In this circuit, the source terminal of the transistor serves as the input, the drain is the output, and the gate is connected to some DC biasing voltage (i.e. an ...
Another major drawback is the amplifier's limited high-frequency response. 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.
The upper FET gate is electrically grounded, so charge and discharge of the stray capacitance, C dg, between drain and gate is simply through R D and the output load (say R out), and the frequency response is affected only for frequencies above the associated RC time constant τ = C dg R D //R out, namely f = 1/(2πτ), a rather high frequency ...
In analog amplifiers this curtailment of frequency response is a major implication of the Miller effect. In this example, the frequency ω 3dB such that ω 3dB C M R A = 1 marks the end of the low-frequency response region and sets the bandwidth or cutoff frequency of the amplifier.
For the case when the common-base circuit is used as a voltage amplifier, the circuit is shown in Figure 2. The output resistance is large, at least R C || r O, the value which arises with low source impedance (R S ≪ r E). A large output resistance is undesirable in a voltage amplifier, as it leads to poor voltage division at the output.
In this circuit, the base terminal of the transistor serves as the input, the collector is the output, and the emitter is common to both (for example, it may be tied to ground reference or a power supply rail), hence its name. The analogous FET circuit is the common-source amplifier, and the analogous tube circuit is the common-cathode amplifier.
The most common type of FET amplifier is the MOSFET amplifier, which uses metal–oxide–semiconductor FETs (MOSFETs). The main advantage of a FET used for amplification is that it has very high input impedance and low output impedance .
In electronics, a common-drain amplifier, also known as a source follower, is one of three basic single-stage field-effect transistor (FET) amplifier topologies, typically used as a voltage buffer. In this circuit (NMOS) the gate terminal of the transistor serves as the signal input, the source is the output, and the drain is common to both ...