单极点、单级放大器

wensan

单极点、单级放大器

在现实的世界中，
无论是音频、射频，还是微波，
所有的放大器都有频宽的限制，
无限频宽的放大器并不存在！

也就是说，
所有的放大器都可以视为一个滤波器！
而一般音频放大器则相当于低通滤波器，
要深入了解放大器必须先了解滤波器。

基本的滤波器是由电阻R、电感L、电容C所组成。
电阻是消耗能量的组件，
电感和电容则是不会消耗能量，
而把能量储存起来的储能组件。

下图为基本的一阶RC低通滤波器的电路图，
-3db频宽设定在100Hz。
<img src="attachments/dvbbs/2004-11/2004111072913810.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072913810.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

其频率─振幅响应和频率─相位响应如下图所示：
<img src="attachments/dvbbs/2004-11/20041110729493.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/20041110729493.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />


一阶低通滤波器是单极点的频率响应，
上图中，振幅在100Hz处下降了3db，
相位延迟了45度。
100Hz以上的频率以频率每增加一倍、振幅衰减6db的斜率下降，
相位延迟则渐渐趋近90度。

下图为一阶RC低通滤波器的步阶(方波)瞬时响应：
<img src="attachments/dvbbs/2004-11/2004111072920211.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072920211.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111072928409.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072928409.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
其输出方波的前后缘呈指数函数的方式上升和下降。

下面这几张图为基本的一阶RL低通滤波器的电路图和频率响应、方波响应，
<img src="attachments/dvbbs/2004-11/2004111072956991.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072956991.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111072951131.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072951131.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/200411107302707.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411107302707.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/200411107307494.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411107307494.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
它的表现都跟一阶RC低通滤波器一样。

如果把两个一阶低通滤波器串接起来会如何？
下图为两个一阶低通滤波器串接的电路及其频率响应：
<img src="attachments/dvbbs/2004-11/2004111072945415.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072945415.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111072938519.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111072938519.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

由于两个一阶低通滤波器的-3db频宽都设定在100Hz，
因此在100Hz处的振幅下降了6db，
相位延迟了90度。
100Hz以上的频率以频率每增加一倍、振幅衰减12db的斜率下降，
相位延迟则渐渐趋近180度。

其步阶(方波)瞬时响应如下：
<img src="attachments/dvbbs/2004-11/2004111081539439.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111081539439.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111081640625.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111081640625.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

其输出方波的前后缘呈现出二重积分的现象。

如果把电容和电感一起做成滤波器又如何？
这样就形成二阶LC滤波器。

下图为二阶LC低通滤波器的电路图和频率响应：

<img src="attachments/dvbbs/2004-11/2004111081933385.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111081933385.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111082019786.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111082019786.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

上图中，振幅在100Hz处下降了3db，
相位延迟了90度。
100Hz以上的频率以频率每增加一倍、振幅衰减12db的斜率下降，
相位延迟则渐渐趋近180度。

跟前面两个一阶低通滤波器串接的电路相比，
差别在振幅响应一个在100Hz处的振幅下降了6db，
另一个在100Hz处的振幅下降了3db。
为什么？

因为LC滤波器中的储能源件─电容和电感，
在释放其储存的能量时，
其释放的能量并不完全被电阻吸收，
有一部分被另一个储能组件所吸收储存起来，
释放的能量没有全部被消耗掉所致。

这种现象就是「谐振」！

如果电阻消耗的能量越少，
其谐振尖锐度(Q值)越高，
谐振的现象就越明显。

像下图中，
把电阻的阻值增加三倍，
其频率响应在100Hz处振幅不但没有下降，
反而上升了一些。
<img src="attachments/dvbbs/2004-11/200411108229743.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411108229743.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111082225130.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111082225130.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

下面这几张图为二阶LC低通滤波器的步阶(方波)瞬时响应：
<img src="attachments/dvbbs/2004-11/2004111082335469.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111082335469.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111082348901.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111082348901.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/200411108244186.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411108244186.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111082454443.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111082454443.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />

从二阶LC低通滤波器的方波瞬时响应可以发现波形的突起甚至铃振的现象，
这是因为谐振时产生的「自然谐振频率」所造成！

即便是谐振Q值小到让方波波形没有突起，(如下图所示)
<img src="attachments/dvbbs/2004-11/200411108385395.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411108385395.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/200411108391654.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411108391654.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
这也只是谐振时产生的「自然谐振频率」很小，
肉眼难以观察而已，
并不表示完全没有产生「自然谐振频率」出来。


如果把两个一阶低通滤波器串接的电路，
后端输出的讯号回授到前面起来会如何？
电路及其频率响应、方波响应如下：

<img src="attachments/dvbbs/2004-11/2004111083955990.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111083955990.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/200411108405525.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/200411108405525.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111084016354.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111084016354.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
<img src="attachments/dvbbs/2004-11/2004111084039596.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111084039596.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
[upload=jpg]UploadFile/2004-11/200411108405350.jpg[/u

小鬼头

以下是引用wensan在2004-11-13 21:12:00的发言：
小鬼头兄所提的应该属于晶体管极际电容与寄生电感的谐振现象。

如果真要对「反馈」做很广义的解释，
那么其实「米勒效应」就是一种反馈效应，
这样说来，「反馈」根本是无所不在的！
只是这已经远远超出了一般对「反馈放大器」所定义的范围。

教科书的三极管模型里，就已经考虑了极间电容的“密勒”效应——而教科书，已经算是比较”理论“化一些的了[em11]

wensan

小鬼头兄所提的应该属于晶体管极际电容与寄生电感的谐振现象。

如果真要对「反馈」做很广义的解释，
那么其实「米勒效应」就是一种反馈效应，
这样说来，「反馈」根本是无所不在的！
只是这已经远远超出了一般对「反馈放大器」所定义的范围。

wode

最简单的共射放大器（没有射极电阻），也存在反馈吗？

小鬼头

就拿增益为1的跟随器来说，一般观点是属于无“反馈”性质（当然也可认为是100%的负反馈）。

但一些MOSFET做的（源极跟随器），却很容易产生自激振荡，三极管做的（射极跟随器）有时也会在高频段产生振荡。。。

wensan

放大器的振荡或谐振来自延迟的反馈讯号形成正反馈，
而被反复循环放大！

有一种比较广义的「反馈」，
像BJT共射级放大电路在射极串一个电阻，
称之为「自身反馈」。
在这种广义的「反馈」电路中，
除非串联的电阻是「负电阻」，
不然是无法产生被反复循环放大的正反馈。

所以，
「无反馈放大器」不管输出讯号的延迟有多少，
既然它不会把延迟的讯号反馈回去，
自然就不会形成被反复循环放大的正反馈，
也就没有自激的可能。

wode

接反馈网络，电路中的分布参数也可能引起反馈，是吗？

aeon

是反馈，没反馈应该不能自激振荡

wode

这些电容电感存在的原因所以受到限制的，是吧？
从WENSAN兄文章中的自激判断法，是否可以推论出电路如果没有反馈网络，就永远没有自激的结论？

wensan

wode

好羡慕你们DX，懂得这么多，呵呵 。放大器不能是无限频宽是因为有电容和电感这两种因素的存在吧？

wensan

AD797也是只有一个电压放大级
<img src="attachments/dvbbs/2004-11/2004111018229535.jpg" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/2004111018229535.jpg\');}" onmousewheel="return imgzoom(this);" alt="" />
PASS的XA后级也是


「电流模」Current Feedback放大器跟一般的运算放大器之间，
只是将反相输入端设计成低阻抗输入，
跟是不是一级电压放大无关。
Current Feedback放大器可以设计成一级电压放大，
也可以不是。

locky_z

LM6361视频运放
[upload=gif][/upload]
还有LM359 诺顿运放

小鬼头

“一段增幅”放大器架构的线路图。。。不过这是来自DIYAUDIO的，呵呵

<img src="attachments/dvbbs/2004-11/20041110114915589.gif" border="0" onload="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onmouseover="if(this.width>screen.width*0.7) {this.resized=true; this.width=screen.width*0.7; this.style.cursor=\'hand\'; this.alt=\'Click here to open new window\nCTRL+Mouse wheel to zoom in/out\';}" onclick="if(!this.resized) {return true;} else {window.open(\'attachments/dvbbs/2004-11/20041110114915589.gif\');}" onmousewheel="return imgzoom(this);" alt="" />

DarkHorse

HQ

能看图太好了.
鬼版功劳不少啊.[em11][em11][em11]

小鬼头

按照晶体管的模型，每一个晶体管放大级（包括跟随器的形式）都会产生一个极点。

从工程的角度考虑，当两个极点的距离拉得比较开时（即两个放大级的频率下降点相差较大时），可视作是单极点放大器。

早期的标准OTL、OCL放大器，包括著名的741运放，他们的开环增益都相当高，而且在没有补偿前，两个主要的极点距离较近，造成相移速度快。一旦加入了负反馈后，这个相移速度快就会产生问题，也就是放大器的增益下降到1之前，相移已经到达180度，从而产生了振荡。

也因此，早期的放大器必须进行补偿。最常用的方法就是所谓的“单极点”补偿方法，在产生主极点的那一级（通常是电压放大级）的B、C极间加一个补偿电容（也叫密勒电容），利用密勒效应将这个极点扯低，从而拉大了两个极点的距离，避免了上述的振荡问题。但这种做法有一个明显的敝端，就是导致开环频响过窄，以741为例，开环频响仅数十HZ。而按照TIM理论，开环频响越窄，负反馈越深，TIM失真就越严重。

后期出现了双极点补偿法（是这个名吧？记不清了[em12]）。其方法是故意在放大器上加一个补偿网络，这个网络产生一个主极点和一个零点，这个零点与放大器的原主极点共轭对消，网络的主极点成为放大器的主极点。零点与极点对消后，能够令相移速度减慢，这样，就可以使用较高的主极点频率而不会振荡。也就是说，这种方法，可以取得较宽的开环频率，通常是KHZ的水平——那些输入级负载上加有RC串联小网络、主放大级没有使用密勒电容的放大器，多是这种补偿方法。

还有第三种补偿方法，很少使用。。。我把它忘了。[em10]

文山兄这种线路结构的线路，由于使用了多只晶体管，实际上也存在着多个极点，但是，第二极点（通常应该是输出级产生的）与第一极点的距离大，可视作是比较“天然”的单极点放大器，也因此具有相应的“天然”良好特性[em05]

小鬼头

妈呀，花了两个小时。。。[em10][em10][em10]

还请文山兄复核一遍，看看我修改后的图片有没有错（文字没改动过）[em05]

BTW：文山兄所给出的单极点线路原理图，与日本音响杂志上的“一段增幅”放大器有异曲同工之妙，也是我比较看好的线路[em09]，呵呵

蓝子风

说实话,WENSAN电路功底真的另我等汗颜的

wensan

唉呀！
可别把焦点转移了。

「非平衡转平衡讯号的探讨」跟「单极点、单级放大器」可是我这二十年来最重要的两篇研究论文。
虽然博士帽与我今生无缘，
但这两篇文章是我二十年功力的展现啊！