场效应功放电路

场效应管具有输入(ru)阻抗(kang)高、频率特性好、稳(wen)定性好(无二(er)次击穿现象)

场效应管功(gong)放(fang)电路自制的(de)场效应功放电路图

低噪声、低失真等特点，已被广泛地(di)应用在(zai)音响电路(lu)中，用VMOS功(gong)(gong)率场效应管(guan)(guan)制成(cheng)的功(gong)(gong)放(fang)，音色(se)优(you)美，音色(se)比双极型(xing)晶体管(guan)(guan)功(gong)(gong)放(fang)暖，与(yu)电子管(guan)(guan)功(gong)(gong)放(fang)相似，失真小且制作容易，因此很受音响爱好(hao)者的喜爱。［]能拥有(you)自制的高(gao)品(pin)质(zhi)功(gong)(gong)放(fang)更是很多发烧(shao)友的梦想，因为(wei)自己动手制作功(gong)(gong)放(fang)。

下面介绍一(yi)款简洁易制的场(chang)效应管功(gong)放(fang)电路。

如图所示(shi)(图中只画(hua)出一(yi)个声(sheng)道)。为减小失真，输入级(ji)采用差动(dong)放(fang)大电路。V1用对管(guan)2SC1583，稳定(ding)性(xing)和对称性(xing)好；V2接成恒(heng)流源(yuan)，为本级(ji)提(ti)供稳定(ding)的静态(tai)工作电流，采用恒流源作(zuo)差动(dong)放(fang)(fang)大器(qi)的射极(ji)电阻(zu)，可提高差动(dong)放(fang)(fang)大器(qi)的共模抑制比和动(dong)态范围，从(cong)而进一步改善失真。c1为输入(ru)耦合电容，R1、c2构成(cheng)低通滤波器(qi)，阻(zu)止(zhi)前(qian)级的超音(yin)频(pin)干扰(rao)信号窜入(ru)功(gong)放(fang)(fang)；R2决定了(le)功(gong)放(fang)(fang)的输入(ru)阻(zu)抗(kang)。合形式，是从(cong)成(cheng)本(ben)上(shang)考(kao)虑的。若全(quan)用场(chang)效(xiao)应管，效(xiao)果更好(hao)。

各管的射(she)(阴)极都加有本级电流(liu)(liu)负反馈电阻，起稳定静(jing)态工作(zuo)(zuo)点(dian)的作(zuo)(zuo)用。有利于改善失真。整机负反馈则由R18、R19、C6、C7组(zu)成，总增益约为26．8dB。C7是隔(ge)直电容，使前后级形成直流(liu)(liu)全负反馈，保证(zheng)输出中点(dian)静(jing)态零电位。

c3、c6是(shi)为了抑(yi)制高(gao)频自激振荡而设(she)置。放(fang)(fang)大器(qi)的(de)电(dian)压增益大部分由(you)V3获得，而c3可产生(sheng)高(gao)频负(fu)反馈，降低放(fang)(fang)大器(qi)的(de)高(gao)频增益，破(po)坏(huai)高(gao)频自激的(de)幅度条(tiao)(tiao)件(jian)。但c3又使(shi)高(gao)频相(xiang)位更加滞后(hou)，所以(yi)在反馈回路(lu)中加入C6，进(jin)行相(xiang)位超前(qian)补偿，破(po)坏(huai)高(gao)频自激的(de)相(xiang)位条(tiao)(tiao)件(jian)。

C5、R17组成相移校正(zheng)电(dian)路，使(shi)负(fu)载(zai)近(jin)于纯(chun)电(dian)阻。防止高(gao)频自激。由于扬声(sheng)器阻抗中的电(dian)感(gan)分量在高(gao)频时明显增加，使(shi)放大(da)器的负(fu)载(zai)呈电(dian)感(gan)性，引起输(shu)电(dian)流滞(zhi)后(hou)于输(shu)m电(dian) 压。若放大(da)器的高(gao)频增益(yi)较高(gao)，还容易产生高(gao)频自激振荡。

R13、R14串(chuan)接在栅(zha)极(ji)(ji)是(shi)防止VMOS管产生高频自(zi)激。(]由(you)(you)于(yu)栅(zha)极(ji)(ji)的(de)高阻抗，加上接线及分布电(dian)(dian)容、电(dian)(dian)感(gan)和栅(zha)极(ji)(ji)分布电(dian)(dian)容的(de)影响，VMOS管在工作中可会出(chu)现高频自(zi)激振荡。解(jie)决V3为第二级(ji)电(dian)(dian)压放大管，V5接成恒流源，为本级(ji)提供稳的(de)静态(tai)工作电(dian)(dian)流和高的(de)负载(zai)阻抗，由(you)(you)于(yu)V5的(de)存(cun)在，v3的(de)压增益大为提高，这样(yang)，就不(bu)必用自(zi)举电(dian)(dian)路(lu)。

V4、VR和R9接在V3、V5集电(dian)极(ji)之间(jian)构成Vbe扩(kuo)大(da)电(dian)，调节(jie)VR可改变末(mo)级大(da)功率(lv)管(guan)(guan)(guan)的(de)静态工作电(dian)流(liu)。V4还起度(du)(du)补偿作用(yong)，当功率(lv)管(guan)(guan)(guan)的(de)温度(du)(du)升高时，V4的(de)发射结压降(jiang)小，于是V4的(de)集电(dian)极(ji)一发射极(ji)电(dian)压也降(jiang)低(di)，从而降(jiang)低(di)了功管(guan)(guan)(guan)的(de)静态电(dian)流(liu)，作用(yong)与二极(ji)管(guan)(guan)(guan)相似，但比(bi)二极(ji)管(guan)(guan)(guan)更灵，安装时应与功率(lv)管(guan)(guan)(guan)一起装在散(san)热(re)器上(shang)，电(dian)气(qi)上(shang)要绝(jue)。

V6一(yi)(yi)V9等组成输(shu)出级(ji)，采(cai)用双极(ji)型晶体管与(yu)场效应管混的办(ban)法(fa)是加入阻(zu)尼电阻(zu)，即在(zai)栅极(ji)串接一(yi)(yi)只电阻(zu)(一(yi)(yi)般不超过1kQ)。

二、用场效应管构成的功放电路

本文介绍了(le)一款采用场(chang)(chang)效(xiao)应(ying)管(guan)做前级(ji)(ji)放大制(zhi)作的(de)(de)(de)功率(lv)放大器，音(yin)响效(xiao)果很理想。()由于该功放的(de)(de)(de)后级(ji)(ji)电(dian)路(lu)是(shi)一个直流耦合的(de)(de)(de)电(dian)路(lu)，各级(ji)(ji)工(gong)作点的(de)(de)(de)选(xuan)择(ze)，尤其第(di)一级(ji)(ji)场(chang)(chang)效(xiao)应(ying)管(guan)的(de)(de)(de)工(gong)作点的(de)(de)(de)选(xuan)择(ze)，将对(dui)电(dian)路(lu)的(de)(de)(de)性能有较大的(de)(de)(de)影响，因此本文着重叙述电(dian)路(lu)参数的(de)(de)(de)计(ji)算及(ji)调试。

1．电路原理

场效应(ying)管功(gong)放电路用(yong)场效应(ying)管构成的功(gong)放电路图(tu)

电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)主要由(you)两级(ji)(ji)(ji)差动(dong)放大及三级(ji)(ji)(ji)射极(ji)(ji)(ji)跟随电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)组成(cheng)，如图1所示。N沟道场效应管VT1、VT2构成(cheng)第一级(ji)(ji)(ji)共(gong)(gong)源(yuan)(yuan)差动(dong)放大器，而(er)(er)VT3、VT4分(fen)别又(you)与VT1、VT2构成(cheng)共(gong)(gong)栅(zha)共(gong)(gong)源(yuan)(yuan)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)。众所周知差动(dong)放大器具有(you)共(gong)(gong)模抑制(zhi)比(bi)高、失调和漂移小的(de)(de)(de)(de)优良性能。而(er)(er)在(zai)(zai)共(gong)(gong)栅(zha)共(gong)(gong)源(yuan)(yuan)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)中，后级(ji)(ji)(ji)的(de)(de)(de)(de)输入电(dian)(dian)(dian)(dian)(dian)(dian)阻就是(shi)前(qian)级(ji)(ji)(ji)的(de)(de)(de)(de)负载电(dian)(dian)(dian)(dian)(dian)(dian)阻，由(you)于(yu)(yu)共(gong)(gong)栅(zha)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)的(de)(de)(de)(de)输入电(dian)(dian)(dian)(dian)(dian)(dian)阻较(jiao)小，使前(qian)级(ji)(ji)(ji)共(gong)(gong)源(yuan)(yuan)极(ji)(ji)(ji)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)增益变(bian)小，但组合(he)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)增益主要由(you)共(gong)(gong)栅(zha)极(ji)(ji)(ji)决(jue)定(ding)，输出电(dian)(dian)(dian)(dian)(dian)(dian)阻则主要由(you)共(gong)(gong)栅(zha)极(ji)(ji)(ji)决(jue)定(ding)。因为前(qian)级(ji)(ji)(ji)共(gong)(gong)源(yuan)(yuan)极(ji)(ji)(ji)的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)增益变(bian)小，所以特别适宜(yi)于(yu)(yu)高频(pin)工作。R3、VT5、R4构成(cheng)1mA的(de)(de)(de)(de)恒流(liu)(liu)(liu)源(yuan)(yuan)，此电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)在(zai)(zai)VD3、R7上(shang)产生22V的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)，从而(er)(er)使VT3、VT4的(de)(de)(de)(de)栅(zha)极(ji)(ji)(ji)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)稳(wen)定(ding)在(zai)(zai)22V。由(you)于(yu)(yu)栅(zha)源(yuan)(yuan)极(ji)(ji)(ji)间电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)很小，VT3、VT4．的(de)(de)(de)(de)源(yuan)(yuan)极(ji)(ji)(ji)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)即VT1、VT2的(de)(de)(de)(de)漏源(yuan)(yuan)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)就稳(wen)定(ding)在(zai)(zai)22V。VT3、VT4的(de)(de)(de)(de)漏源(yuan)(yuan)电(dian)(dian)(dian)(dian)(dian)(dian)压(ya)(ya)(ya)也稳(wen)定(ding)在(zai)(zai)约22V。对(dui)直流(liu)(liu)(liu)而(er)(er)言VT3、VT4的(de)(de)(de)(de)栅(zha)极(ji)(ji)(ji)电(dian)(dian)(dian)(dian)(dian)(dian)位为22V，对(dui)交(jiao)流(liu)(liu)(liu)而(er)(er)言VT3、VT4的(de)(de)(de)(de)栅(zha)极(ji)(ji)(ji)电(dian)(dian)(dian)(dian)(dian)(dian)位为0V，因此为共(gong)(gong)栅(zha)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)。R11、VT6构成(cheng)第一级(ji)(ji)(ji)差动(dong)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)的(de)(de)(de)(de)恒流(liu)(liu)(liu)源(yuan)(yuan)，其(qi)作用(yong)是(shi)提高交(jiao)流(liu)(liu)(liu)阻抗，提高共(gong)(gong)模抑制(zhi)比(bi)。R5、C2是(shi)相位补偿元件，用(yong)于(yu)(yu)防止(zhi)高频(pin)振荡。 VT8、VT9构成(cheng)第二级(ji)(ji)(ji)差动(dong)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)，VT7为其(qi)恒流(liu)(liu)(liu)源(yuan)(yuan)。VT0、VT11为比(bi)例式镜(jing)像(xiang)恒流(liu)(liu)(liu)源(yuan)(yuan)电(dian)(dian)(dian)(dian)(dian)(dian)路(lu)(lu)，VT11的(de)(de)(de)(de)集电(dian)(dian)(dian)(dian)(dian)(dian)极(ji)(ji)(ji)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)与VT10电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)之比(bi)等(deng)于(yu)(yu)R22／R23，由(you)于(yu)(yu)R22=R23，因此差动(dong)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)两臂的(de)(de)(de)(de)电(dian)(dian)(dian)(dian)(dian)(dian)流(liu)(liu)(liu)是(shi)相等(deng)的(de)(de)(de)(de)。

VT12、VT13为(wei)末(mo)三级射极跟随电路提供合(he)适的(de)工作电流点。

在(zai)输(shu)入信号(hao)为正时，R29、R30的中点(dian)以及输(shu)出O点(dian)电(dian)(dian)(dian)位为正，因此均可作为VT1、VT2差动级的负反馈(kui)电(dian)(dian)(dian)压(ya)。R38、C9构(gou)成低通滤波器，角频率为1Hz时增益即(ji)下降到1／根号(hao)2。集(ji)成电(dian)(dian)(dian)路TL072构(gou)成输(shu)出点(dian)直流稳零跟随器，其输(shu)出Uo与其输(shu)入Ui的关系为Uo=Ui+1／T∫Uidt，即(ji)Uo为Ui的比例积(ji)分，Uo作用于(yu)VT1、VT2差动级负反馈(kui)，能使O点(dian)直流电(dian)(dian)(dian)位为士10mV以下。在(zai)开环(huan)增益足够(gou)大的情(qing)况(kuang)下，整个后级电(dian)(dian)(dian)路的闭(bi)环(huan)电(dian)(dian)(dian)压(ya)增益等于(yu)R14／R12。

场效应管放大电路的静态分析

根据(ju)偏置电(dian)路(lu)形式(shi)，场(chang)效应管放大(da)电(dian)路(lu)的直流(liu)通(tong)路(lu)分为(wei)自给(ji)偏压电(dian)路(lu)和分压式(shi)偏置电(dian)路(lu)。

一、自给偏压电路

用N沟道(dao)结型场(chang)效应管组成的自给偏(pian)压(ya)电路如图Z0217所示。

自给偏压(ya)原理：在(zai)正常(chang)工(gong)作(zuo)范围内，场效应(ying)管的(de)栅极几乎不取电流，IG= 0，所以，UG = 0，当有IS = ID流过RS时，

必(bi)然(ran)会产(chan)生一个电压Us＝IsRs＝IdRs，从而有(you)

UGS = UG- US= - IDRS

依(yi)靠场效应管自(zi)身(shen)的电(dian)流ID 产生了栅(zha)极(ji)所需的负偏(pian)压(ya)，故称为自(zi)给偏(pian)压(ya)。

为了(le)减小(xiao)RS对放大倍数的(de)影响，在RS 两(liang)端并联了(le)一个旁路电容 Cs。

估算(suan)静态工作点,由(you)图Z0217所示电路的直流通路可得：

UGS = UG- US= - IDRSGS0223

UDS = ED - ID（RS + Rd） GS0224

结型场效(xiao)应管的转(zhuan)移特性可近似表示为(wei)：

式中IDSS为饱和(he)漏电流(liu)，VP为夹(jia)断电压。

联立求(qiu)解GS0223～GS0225各式，便可求(qiu)得静态(tai)工作点Q（ID，UGS，UDS）。

二、分压式偏(pian)置电路

由于参数IDSS ，VP 等与(yu)温度(du)有关，因此，场效(xiao)应管(guan)放(fang)大电路(lu)也要设法稳(wen)定静态工作点。

实际(ji)上，自给(ji)偏压电(dian)(dian)路就(jiu)具有一定(ding)(ding)(ding)的(de)(de)(de)稳定(ding)(ding)(ding)Q点的(de)(de)(de)能力(li)。例(li)如(ru)：温(wen)度升高(gao)(gao)使(shi)ID增(zeng)加时(shi)，US也随之增(zeng)加，从而使(shi)UGS 更负，反(fan)过来又(you)抑制(zhi)了ID的(de)(de)(de)增(zeng)大。但如(ru)果对温(wen)度稳定(ding)(ding)(ding)性(xing)要求(qiu)更高(gao)(gao)时(shi)，单纯靠增(zeng)大RS来稳定(ding)(ding)(ding)Q点，势(shi)必会导致(zhi)Au下(xia)降，甚(shen)至产生严重的(de)(de)(de)非线性(xing)失真。图Z0218所(suo)示的(de)(de)(de)分(fen)压式偏置电(dian)(dian)路，通(tong)过R1与R2分(fen)压，给(ji)栅极一个固定(ding)(ding)(ding)的(de)(de)(de)IE电(dian)(dian)压，这样就(jiu)可以把RS选(xuan)的(de)(de)(de)比较大，而Q点又(you)不(bu)致(zhi)于过低。图中Rg的(de)(de)(de)主(zhu)要作用是增(zeng)大输入电(dian)(dian)阻，进一步减小栅极电(dian)(dian)流。

对分压式(shi)偏(pian)置电(dian)路(lu)，在确定静悉工(gong)作点时，同(tong)样可(ke)用图解法和计(ji)算法。与自给(ji)偏(pian)压电(dian)路(lu)不同(tong)之处是UG≠0。只需(xu)将(jiang)栅源回路(lu)直流(liu)负载(zai)线方(fang)程(cheng)改为(wei)：