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楼主 |
发表于 2004-11-30 14:54
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懂英文的可参考这个帖子,我刚看到的
我没时间,没怎么细看。。。相信结论跟我们的差不了太多。嘿嘿
http://www.diyaudio.com/forums/showthread.php?s=&threadid=16796&perpage=10&highlight=&pagenumber=1
<img src="attachments/dvbbs/2004-11/2004113014385331.png" 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/2004-11/2004113014385331.png\');}" onmousewheel="return imgzoom(this);" alt="" />
andy_c []
diyAudio Member
Offline
Registered: Apr 2003
Location: Here, ahhh, I think
Unstable VAS current in amp from Slone book Post #1
I've been reading Randy Slone's "High-Power Audio Amplifier Construction Manual", which I picked up right after reading Douglas Self's amplifier design book. Douglas Self described some distortion reduction techniques for the VAS, including having a beta-enhancing emitter follower at the VAS input, as well as using a cascode at the VAS output. These designs were used in conjunction with an input stage diff amp having a current mirror load to balance it. I was disappointed that Self didn't provide data on the combination of the beta enhancement and cascode techniques (it was an "either or" option), nor did he discuss any complementary versions of his circuit at all. In fact, he acts as if fully complementary designs don't even exist. I don't think he ever mentions them at all.
On reading Slone's book, I was quite happy to find that not only did he have a fully complementary design using current mirrors on the diff amp outputs, but also combined this approach with both a beta-enhanced VAS input and a cascoded VAS output. Nice! That is, it was nice until I tried to calculate the bias current of the last transistor of the VAS. Then I realized there was a serious flaw in the design. That bias current is subject to large variations and is essentially uncontrolled. To illustrate, I've modified his design a little bit with different current sources and resistor values to avoid copyright issues, but preserved the basic topology. I entered the schematic into LTSpice, and I show the schematic below. It's easy to see how to calculate the currents in the diff amp of course. Likewise, the emitter follower bias current can be easily seen to be one Vbe drop divided by its emitter resistor. No problem. But check out that last stage! How do you calculate its bias current? LTSpice calculates a value of about 50 mA, which is close to the value Slone reports, but it's not clear how this happens. Apparently it just rises until it loads down the emitter follower enough that it can't support any further rise. As an experiment, I tried raising the supplies to +/-90 V from +/-60 V. The VAS output stage bias current goes from 50 mA to 140 mA. That's really bad.
I began to think that the whole idea of using current mirrors on the diff amp outputs of a fully complementary design was a terrible idea. You end up with the diff amp output being taken from back-to-back collectors, so the voltage at that point is no longer controlled. Self avoids this problem with a single-ended design by biasing the VAS with a constant-current load. So the voltage at the diff amp output is controlled by the Vbe drops of the VAS, plus the forced VAS current times the VAS emitter resistor, and the current of the VAS is forced by a constant-current load. But with a fully complementary design, this isn't possible.
Is it possible somehow to use current mirror diff amp loading with a fully complementary design and still get stable, easily controllable VAS bias current? It turns out that you can. I'll show a schematic in the next post.
注:楼主是SPICE的高手,好象他任职的公司是出品电路分析计算机软件的
其中,第二段中间有这么一句话:Then I realized there was a serious flaw in the design. That bias current is subject to large variations and is essentially uncontrolled.
我意识到这个设计有个严重的缺陷,就是受外界大的改变时,电压放大级的电流实际上是不受控制的。(As an experiment, I tried raising the supplies to +/-90 V from +/-60 V. The VAS output stage bias current goes from 50 mA to 140 mA. That's really bad.从+/-60V电源改为+/-90V时,电流从50MA变为140MA)
第三段开头的话:I began to think that the whole idea of using current mirrors on the diff amp outputs of a fully complementary desi |
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