Lecture 4 - Lecture 4 Introduction to electronic analog...

Info icon This preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
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. n+ n+ n+ n+ n+ p p V DS =| V t | V DS >| V t | V DS = V t V DS > V t D D D G G G B B G S D G S D B B S S S MOSFET n / on (depletion type) MOSFET n / off (enhancement type) V GS =2 V t G S D n+ p p v GS =0 V DS =| V t | V DS >| V t | D G S JFET n / on (depletion type) V t V t | V t | | V t | | V t | v GS = V GS v GS =0 v GS = V t V DS =| V t | - |V GS | v DS v DS v DS v DS V DS = 0 V DS = 0 p p | V t | V DS >| V t | - |V GS | V DS > 0 V
Image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}

What students are saying

  • Left Quote Icon

    As a current student on this bumpy collegiate pathway, I stumbled upon Course Hero, where I can find study resources for nearly all my courses, get online help from tutors 24/7, and even share my old projects, papers, and lecture notes with other students.

    Student Picture

    Kiran Temple University Fox School of Business ‘17, Course Hero Intern

  • Left Quote Icon

    I cannot even describe how much Course Hero helped me this summer. It’s truly become something I can always rely on and help me. In the end, I was not only able to survive summer classes, but I was able to thrive thanks to Course Hero.

    Student Picture

    Dana University of Pennsylvania ‘17, Course Hero Intern

  • Left Quote Icon

    The ability to access any university’s resources through Course Hero proved invaluable in my case. I was behind on Tulane coursework and actually used UCLA’s materials to help me move forward and get everything together on time.

    Student Picture

    Jill Tulane University ‘16, Course Hero Intern