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Unformatted text preview: Lecture 4: Introduction to electronic analog circuits 361-1-3661 1 Our main aim is to build all the possible practical amplifiers by using an FET transistor and resistor. 2.1. FET transistors: symbols, physical structures, analytical models, and graphical characteristics The symbols of the n-channel junction field-effect (JFET) and metal-oxide-silicon field-effect (MOSFET) transistors and their physical structures are given in Fig. 1. We will analyze in the lectures only n-channel transistors. The only difference between the n-channel and p-channel transistors is in their static states: the static state of the p-channel transistors is inverse to that of the n-channel ones because of their opposite structures. There will be no difference in the small-signal models. The circuits analyzed in home exercises, the lab, and the exam will comprise both n-channel and p-channel transistors. FET transistors have a channel, either built-in or induced, between the source and drain terminals (see Fig. 1). In JFETs the channel is isolated from the gate by the reverse biased p- n junctions, whereas in MOSFETs, the channel is isolated from the gate by a thin layer of silicon oxide, SiO 2 . Note that the substrate (body) in MOSFETs should always be reverse biased to be isolated from the source, drain, and channel. Let us start the analysis of the transistor physical structures assuming that their sources and bodies are grounded, and their drain potentials are zero. The channels of the depletion-mode FETs are built in, whereas the channel of the enhancement-mode MOSFET is absent at v GS =0. To obtain the channel in the enhancement- mode MOSFET, a great enough positive potential, v GS , should be applied to its gate relative to the source, body, and drain. The positive gate attracts the electrons from the source, drain, where they are major charge carriers, and also from the body, where they are a minority. Let us denote the value of v GS at which the channel just starts forming as V t . Let us assume that for a v GS =2 V t the enhancement-mode MOSFET has the same channel width as the depletion-mode transistors for v GS =0. Note now that the only difference between the enhancement- and depletion-mode FETs is in a 2 V t shift of their v GS voltages. Note also that the electric field across the gate-channel region controls the channel width of the FETs. This field (or voltage) is applied to the depletion regions of the JFETs and to the oxide layer between the gate and the channel of the MOSFETs. Other names for the depletion- and enhancement-mode transistors are normally-on ( n / on ) and normally-off ( n / off ) transistors since the first type does have a built-in channel at v GS =0 and the other does not have it....
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