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Electric charge flows through a semiconducting channel between source and drain terminals. By applying a reverse bias voltage to a gate terminal, the channel is pinched, so that the electric current is impeded or switched off completely. A JFET is usually conducting when there is zero voltage between its gate and source terminals.
Cross-sectional view of a MOSFET type field-effect transistor, showing source, gate and drain terminals, and insulating oxide layer. The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET ...
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. The remaining terminal is what is known as "common". In this example, the signal enters the gate, and exits the drain. The only terminal remaining is the source. This is a common-source FET circuit.
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 ...
Top: source, bottom: drain, left: gate, right: bulk. Voltages that lead to channel formation are not shown. In field-effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an on state or an off state at zero gate–source voltage.
The drain-to-source resistance of the JFET (R DS) and the drain resistor (R 1) form the voltage-divider network. The output voltage can be determined from the equation V out = V DC · R DS / (R 1 + R DS). An LTSpice simulation of the non-linearized VCR design verifies that the JFET resistance changes with a change in gate-to-source voltage (V ...
The threshold voltage, commonly abbreviated as V th or V GS(th), of a field-effect transistor (FET) is the minimum gate-to-source voltage (V GS) that is needed to create a conducting path between the source and drain terminals. It is an important scaling factor to maintain power efficiency.
Once the drain-source voltage reaches a certain minimum value, the JFET enters saturation where current is approximately constant. This configuration is known as a constant-current diode , as it behaves much like a dual to the constant voltage diode ( Zener diode ) used in simple voltage sources.