请教！平衡式低噪声放大器的电桥问题！

要90d，否则反射回到input port的phase不能抵消。

那别人用传输线变压器做的平衡式低噪声放大器是怎么回事 啊?[/COLOR]

什么事平衡式放大器?

both 90deg and 180deg coupler can be used for balanced amplifier design. The condition is when you used a 90deg splitter, you need 90deg combiner; and when you are using 180deg splitter, you need a corresponding 180deg combiner.
For both design, input and output reflection can be canceled theoretically. but for real situations, this is hard to obtain in band edges. But totally speaking, the return loss for both input and output are better than unbalanced design.

谢谢各位的解答！
请问ksyao，我只找到在平衡电路中运用90度相差3dB耦合器的资料，没有找到运用180相差耦合器的相关资料，你有吗?还是你做过这样的电路?
假如用180度相差的3dB耦合器做，在其中一条链路中需要加四分之一波长的延长线吗?
谢谢！[/COLOR]

引入90度相差的话，一来一回正好180度，入射波和反射波正好的端口抵消，改善驻波，但3dB电桥假如是180度的相差，理论上怎么改善驻波呢?
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用180度的功分功和器的似乎应该是"推挽"而不是"平衡",它只起抵消偶次谐波的作用,而不起抵消反射的作用.您所看到的CATV放大器中用的是磁环绕的180度分配器是用作推挽,因为带太宽,高次谐波的抑制很重要.

我来顶浦哥的..

the typical 180 deg coupler is rat-race coupler, you can check any textbook for the information of it; the rat-race couplers are used in balanced amplifier design for its wide band and better matching / isolation performance;
I used such design previously; the only problem is that the PCB layout is troublesome

谢谢各位！

rat-race coupler应该就是波导混合环吧?好像尺寸太大
我想做一个400MHZ的分支线3dB耦合器，但频率低，四分之一线尺寸也有点长，我想在分支线的四个端口加电容，从而减小分支线的长度，有人做过类似的实物吗?可行吗
用ADS原理图仿真好像可行！[/COLOR]