场效应管放大电路的直流偏置电路及偏置电路工作原理详解

偏置电路概述

晶体(ti)管(guan)(guan)构成的放(fang)大器要(yao)做到不失真(zhen)地将信(xin)号电(dian)(dian)压(ya)放(fang)大，就(jiu)必须(xu)保证晶体(ti)管(guan)(guan)的发(fa)(fa)射结(jie)(jie)正(zheng)偏、集(ji)电(dian)(dian)结(jie)(jie)反偏。即应该(gai)设(she)置(zhi)它的工(gong)作点。所(suo)谓工(gong)作点就(jiu)是通(tong)过外部电(dian)(dian)路(lu)的设(she)置(zhi)使晶体(ti)管(guan)(guan)的基极、发(fa)(fa)射极和(he)集(ji)电(dian)(dian)极处于所(suo)要(yao)求(qiu)的电(dian)(dian)位（可(ke)根据(ju)计算获得(de)）。这些(xie)外部电(dian)(dian)路(lu)就(jiu)称为偏置(zhi)电(dian)(dian)路(lu)。

偏置电路工作原理

稳定静(jing)态工作点原理(li)：

由于(yu)流过(guo)发射极偏置电(dian)(dian)阻（Re）的(de)电(dian)(dian)流IR远(yuan)大(da)于(yu)基极的(de)电(dian)(dian)流Ib（Ie>>Ib），因此，可以认为基极电(dian)(dian)位Vb只取决于(yu)分压电(dian)(dian)阻Re的(de)阻值大(da)小(xiao)，与三极管参数无关，不受温度影响。

静态(tai)工作(zuo)点的稳定(ding)是由(you)Vb和Re共同作(zuo)用实现，稳定(ding)过程如下：

设(she)温度升高(gao)→Ic↑→Ie↑→VRe↑→Vbe↓→Ib↓→Ic↓

其中：Ic↑→Ie↑是由并联电(dian)路电(dian)流方程 Ie = Ib+Ic得(de)出(chu)(chu)(chu)，Ie↑→Vbe↓是由串(chuan)联电(dian)路电(dian)压方程Vbe= Vb－Ie×Re得(de)出(chu)(chu)(chu)，Ib↓→Ic↓是由晶体三极(ji)管电(dian)流放(fang)大原理 Ic =β×Ib （β表(biao)示三极(ji)管的放(fang)大倍数(shu)） 得(de)出(chu)(chu)(chu)。

由(you)上述分析不难得(de)出，Re越大(da)稳(wen)定性越好。但事物总是(shi)具有两面性，Re太(tai)大(da)其功率损(sun)耗也(ye)大(da)，同时Ve也(ye)会增加(jia)很多，使Vce减小导致三极管工作范围变窄，降低(di)交流放大(da)倍数。因(yin)此Re不宜取得(de)太(tai)大(da)。在小电(dian)流工作状态下，Re值为几(ji)百欧到几(ji)千欧；大(da)电(dian)流工作时，Re为几(ji)欧到几(ji)十欧。

场效应管偏置电路

场效(xiao)应管偏(pian)(pian)置(zhi)电(dian)(dian)路(lu)(lu)为了使放大电(dian)(dian)路(lu)(lu)正(zheng)常地工作能把输(shu)入信号(hao)不失(shi)真地加(jia)(jia)以(yi)(yi)放大，必(bi)须有一个合适(shi)而稳定的(de)静(jing)(jing)态工作点为放大电(dian)(dian)路(lu)(lu)提供直(zhi)流电(dian)(dian)流和直(zhi)流电(dian)(dian)压(ya)的(de)电(dian)(dian)路(lu)(lu)。叫做直(zhi)流（静(jing)(jing)态）偏(pian)(pian)置(zhi)电(dian)(dian)路(lu)(lu)，简(jian)称偏(pian)(pian)置(zhi)电(dian)(dian)路(lu)(lu)由于各种电(dian)(dian)子(zi)电(dian)(dian)路(lu)(lu)对(dui)偏(pian)(pian)置(zhi)电(dian)(dian)路(lu)(lu)有不同的(de)要求(qiu)，所以(yi)(yi)在实(shi)际(ji)电(dian)(dian)路(lu)(lu)中加(jia)(jia)设(she)的(de)偏(pian)(pian)置(zhi)电(dian)(dian)路(lu)(lu)也有所不同。

场效应管放大电路的基本结构

场效应(ying)管(guan)与(yu)晶(jing)体(ti)管(guan)一样，也(ye)具(ju)有放大(da)作用，但与(yu)普通晶(jing)体(ti)管(guan)是电流控制型器(qi)件(jian)相(xiang)反，场效应(ying)管(guan)是电压控制型器(qi)件(jian)。它具(ju)有输入阻抗高(gao)、噪声低的特点。

场效(xiao)应(ying)管(guan)的3个电极(ji)(ji)(ji)，即栅极(ji)(ji)(ji)、源(yuan)极(ji)(ji)(ji)和(he)漏极(ji)(ji)(ji)分别相当于晶(jing)(jing)体(ti)管(guan)的基极(ji)(ji)(ji)、发射(she)极(ji)(ji)(ji)和(he)集电极(ji)(ji)(ji)。图5-21所(suo)示(shi)是场效(xiao)应(ying)管(guan)的3种组态电路(lu)，即共源(yuan)极(ji)(ji)(ji)、共漏极(ji)(ji)(ji)和(he)共栅极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)。图5-21（a）所(suo)示(shi)是共源(yuan)极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，它相当于晶(jing)(jing)体(ti)管(guan)共发射(she)极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，是一种最常用的电路(lu)。图5-21（b）所(suo)示(shi)是共漏极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，相当于晶(jing)(jing)体(ti)管(guan)共集电极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，输(shu)(shu)入(ru)信号(hao)从漏极(ji)(ji)(ji)与(yu)栅极(ji)(ji)(ji)之(zhi)(zhi)间(jian)(jian)输(shu)(shu)入(ru)，输(shu)(shu)出(chu)(chu)信号(hao)从源(yuan)极(ji)(ji)(ji)与(yu)漏极(ji)(ji)(ji)之(zhi)(zhi)间(jian)(jian)输(shu)(shu)出(chu)(chu)，这(zhei)种电路(lu)又(you)称为源(yuan)极(ji)(ji)(ji)输(shu)(shu)出(chu)(chu)器(qi)或(huo)源(yuan)极(ji)(ji)(ji)跟随器(qi)。图5-21（c）所(suo)示(shi)是共栅极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，它相当于晶(jing)(jing)体(ti)管(guan)共基极(ji)(ji)(ji)放(fang)(fang)大(da)(da)(da)器(qi)，输(shu)(shu)入(ru)信号(hao)从栅极(ji)(ji)(ji)与(yu)源(yuan)极(ji)(ji)(ji)之(zhi)(zhi)间(jian)(jian)输(shu)(shu)入(ru)，输(shu)(shu)出(chu)(chu)信号(hao)从漏极(ji)(ji)(ji)与(yu)栅极(ji)(ji)(ji)之(zhi)(zhi)间(jian)(jian)输(shu)(shu)出(chu)(chu)，这(zhei)种放(fang)(fang)大(da)(da)(da)器(qi)的高频特性比较好。

偏置(zhi)电路(lu)

绝缘栅型场(chang)效应(ying)管的(de)(de)输入电(dian)(dian)阻很(hen)(hen)高，如果在栅极上感应(ying)了(le)电(dian)(dian)荷(he)(he)，很(hen)(hen)不容易泄放(fang)，极易将(jiang)(jiang)PN结击穿(chuan)而造成损坏。为(wei)了(le)避(bi)免发生PN结击穿(chuan)损坏，存放(fang)时(shi)应(ying)将(jiang)(jiang)场(chang)效应(ying)管的(de)(de)3个极短接；不要将(jiang)(jiang)它(ta)放(fang)在静电(dian)(dian)场(chang)很(hen)(hen)强(qiang)的(de)(de)地方，必要时(shi)可放(fang)在屏蔽(bi)盒内。焊接时(shi)，为(wei)了(le)避(bi)免电(dian)(dian)烙铁带有感应(ying)电(dian)(dian)荷(he)(he)，应(ying)将(jiang)(jiang)电(dian)(dian)烙铁从电(dian)(dian)源上拔下。焊进电(dian)(dian)路(lu)板后，不能让栅极悬空。

场效应管放大电路的直流偏置电路

由场(chang)效(xiao)应管组成放(fang)大电(dian)(dian)路(lu)(lu)时(shi)，也要建立合(he)适(shi)的静态工作点Q，而(er)且场(chang)效(xiao)应管是电(dian)(dian)压(ya)控制(zhi)器件，因此(ci)需要有(you)合(he)适(shi)的栅-源偏置(zhi)电(dian)(dian)压(ya)。常(chang)用(yong)的直流偏置(zhi)电(dian)(dian)路(lu)(lu)有(you)两(liang)种形式，即自偏压(ya)电(dian)(dian)路(lu)(lu)和分压(ya)式自偏压(ya)电(dian)(dian)路(lu)(lu)。

1．自偏压电路(lu)

偏置电(dian)路

图(tu)1（a）所示电(dian)(dian)(dian)(dian)(dian)路(lu)是一(yi)个自(zi)偏(pian)(pian)压电(dian)(dian)(dian)(dian)(dian)路(lu)，其中场效(xiao)应管的栅(zha)(zha)极(ji)(ji)通(tong)(tong)过电(dian)(dian)(dian)(dian)(dian)阻Rg接地，源(yuan)极(ji)(ji)通(tong)(tong)过电(dian)(dian)(dian)(dian)(dian)阻R 接地。这种(zhong)偏(pian)(pian)置(zhi)方式靠漏极(ji)(ji)电(dian)(dian)(dian)(dian)(dian)流ID在源(yuan)极(ji)(ji)电(dian)(dian)(dian)(dian)(dian)阻R上产生的电(dian)(dian)(dian)(dian)(dian)压为(wei)栅(zha)(zha)-源(yuan)极(ji)(ji)间提(ti)供一(yi)个偏(pian)(pian)置(zhi)电(dian)(dian)(dian)(dian)(dian)压VGS，故称为(wei)自(zi)偏(pian)(pian)压电(dian)(dian)(dian)(dian)(dian)路(lu)。静态时(shi)，源(yuan)极(ji)(ji)电(dian)(dian)(dian)(dian)(dian)位(wei)VS=IDR。由于(yu)栅(zha)(zha)极(ji)(ji)电(dian)(dian)(dian)(dian)(dian)流为(wei)零，Rg上没(mei)有(you)电(dian)(dian)(dian)(dian)(dian)压降，栅(zha)(zha)极(ji)(ji)电(dian)(dian)(dian)(dian)(dian)位(wei)VG=0，所以栅(zha)(zha)源(yuan)偏(pian)(pian)置(zhi)电(dian)(dian)(dian)(dian)(dian)压VGS= VG–VS= –IDR 。耗(hao)尽(jin)型(xing)MOS管也可采用这种(zhong)形(xing)式的偏(pian)(pian)置(zhi)电(dian)(dian)(dian)(dian)(dian)路(lu)。

图(tu)1（b）所(suo)示电(dian)路(lu)是自偏压电(dian)路(lu)的(de)特例，其(qi)中(zhong)VGS=0。显然(ran)，这(zhei)种偏置电(dian)路(lu)只适用于(yu)耗尽型MOS管(guan)，因为在栅源电(dian)压大于(yu)零、等(deng)于(yu)零和小(xiao)于(yu)零的(de)一定(ding)范围内，耗尽型MOS管(guan)均能(neng)正常工作(zuo)。

增强(qiang)型MOS管只有在栅(zha)-源(yuan)电压达到其(qi)开启电压VT时，才有漏极电流ID产生，因此这类管子不能(neng)用(yong)于图(tu)1所(suo)示的自偏压电路中。

2．分压式(shi)偏置电路

分(fen)(fen)压式偏置(zhi)电(dian)路(lu)是(shi)在自(zi)偏压电(dian)路(lu)的基础(chu)上(shang)加接(jie)分(fen)(fen)压电(dian)路(lu)后构成的，如(ru)图2所示(shi)。静(jing)态时，由(you)于栅极(ji)电(dian)流为零(ling)，Rg3上(shang)没(mei)有电(dian)压降，所以栅极(ji)电(dian)位由(you)Rg2与Rg1对电(dian)源VDD分(fen)(fen)压得到，即

偏置电(dian)路(lu)

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