模电ach5-场效应管放大电路3

1、5.3 结型场效应管(JFET)5.3.1JFET的结构和工作原理5.3.2JFET的特性曲线及主要参数5.3.3JFET放大电路的小信号模型分析法15.3.1 JFET的结构和工作原理两边扩散高浓度的P型区栅结的正偏方向1.结构D导电沟道NP+P+GJFET symbols: (a) n-channel; (b) p-channel.S2N沟道JFETJFET operation can be compared to a water spigot.The source of watrere is theaccumulation of electrons at the negative pol

2、e of the drain-source voltage.The drain of water is the electron deficiency(or holes) at the voltage.itive pole of the appdThe control of flow of water is the gatevoltaget controls the width of the n-channel and, therefore, the flow of charges from source to drain.35.3.1 JFET的结构和工作原理2. 工作原理（以N沟道JFET

3、为例） vGS对沟道的控制作用当vGS0时PN结反偏 耗尽层加厚沟道变窄。vGS继续减小，沟道继续变窄。当沟道夹断时，对应的栅源电压vGS称为夹断电压VP 。对于N沟道的JFET，VP 0。5.3.1 JFET的结构和工作原理 vDS对沟道的控制作用当vGS=0时， vDS iD G、D间PN结的反向电压增加，使靠近漏极处的耗尽层加宽，沟道变窄，从上至下呈楔形分布。当vDS增加到使vGD=VP 时，在紧靠漏极处出现预夹断。 沟道电阻此时vDS 夹断区延长iD基本不变55.3.1 JFET的结构和工作原理综上分析可知沟道中只有一种类型的多数载流子参与导电，所以场效应管也称为单极型三极管。JFET

4、栅极与沟道间的PN结是反向偏置的，因此iG0，输入电阻很高。JFET是电压控制电流器件，iD受vGS控制。预夹断前iD与vDS呈近似线性关系；预夹断后，iD趋于饱和。65.3.2 JFET的特性曲线及主要参数2vvDSvGS1. 输出特性iD IDSS 1 GSvconst.DVGSP IDSS and VP values can be found in a specification sheet.2. 转移特性vconst.DDS与MOSFET类似3. 主要参数7Summaryn-channelSummaryFor all FETs：iG 0 AiD iSFor JFETs and depl

5、etion-type MOSFETs：2v IDSS1 GS iDVPFor enhancement-type MOSFETs：i K (vV)2DnGST耗尽增强模式模式SummarySummaryJFETs differ from BJTs：Nonlinear relationship betn input (vGS) and output (iD).FETs are voltage controlled deviwhereas BJTs are currentcontrolled devi.FETs have a higher input impedance. FETs are less

6、 sensitive to temperature.iC iB1 vGS 2DDSSVP5.3.3 JFET放大电路的小信号模型分析法1. FET低频小信号模型122.固定偏置的共源电路JFET Common-Source (CS) Fixed-Bias Configuration（1）Q点的计算Mathematical ApproachIG 0AGSQGG2VIDSS 1 VDD IDQIDGSVPIVDSDSQThe input is on the gate and the output is on the drain.132.固定偏置的共源电路JFET Common-Source (CS

7、) Fixed-Bias Configuration（1）Q点的计算Graphical ApproachThe input is on the gate and the output is on the drain.142.固定偏置的共源电路JFET Common-Source (CS) Fixed-Bias Configuration（2）动态指标分析Input impedance:Output impedance:Voltage gain:| rd Vo gA(r|vmViDor 10R V DdoViAv r 10R153.分压偏置的共源电路JFET Common-Source (CS)

8、 Voltage-Divider Biasing（1）Q点的计算Mathematical ApproachVGSRg2IVRDDR RDg1g2饱和区ID2v IDSS1 GS VP源极电阻稳定Q点注意：ID IDSSand VGS isitive. IDSSand VGS is negative.D) ID IDSSand VGS isitive.315)The JFET is a .A) voltage-controlled devicecurrent-controlled devicefrequency-controlled deviceD)er-controlled device6)The terminal of the JFET is the equivalentof the collector terminal of a BJT.gatedrainsourceanode327)As the channel width of a JFET decreases, thesource-to-drain.increasesdecreasesremains constantis not affected8)Which of the following is usually used to controlthe channel wid