Baohulu,
Quote:I think we are saying the same thing.
No we are not. You are saying
Quote:thus, the TIA needs very high unit gain bandwidth
but I am saying,
Quote: do a thorough analysis on your own system to see whether you need more loop-gain bandwidth
They don't seem to be the same to me.
Those impractical numbers you showed don't constitute a thorough analysis. You need to actually analyze the IIPn out of the amplifiers loop-gain bandwidth. Which, as I have said multiple times, may be good.
One reason that the linearity can be good outside of the loop-gain bandwidth is because the dominant path to the output is directly through the feedback path (not through the opamp), which is a passive network. Think about it.
Quote:if you look at you figure carefully, you will find that at interference B and C frequency, you opam still has enough gain.
I drew the figure, so probably I was looking at it carefully when I drew it. I already mentioned,
Quote: I haven't analyzed the Op-Amp input above the loop-gain bandwidth of the Op-Amp
Also, you read too much into my figure.
Quote:I think maybe your system signal has not that big bandwidth. if in TV system, the signal bandwidth is 8MHz if in LOW IF system, that means the low pass RC corner is higher than 10MHz, then the opam -3dB bandwidth is higher than 10MHz, if the opam has 60dB DC gain, then the opam unit gain bandwidth will be higher than 10GHz, this is not that practical in products.
My figure is only illustrating that out-of-band interferers are attenuated before amplification in a LPF-based TIA. It is not trying to tell you how to design the op-amp or filter response.
You have presented an arguement that you definitely need more loop-gain bandwidth. I am saying that
you may or may not, and have presented you with a situation in which you don't need more loop-gain bandwidth. So now all you need to do is properly analyze your system to see whether you really need more loop-gain bandwidth. However, it seems that you are intent on extending the loop-gain bandwidth even if it means you end up overdesigning. So maybe somebody else can help you with that.
Considering my signature (at the bottom), I probably should have given up on this discussion by now...
Aaron
P.S. If I have come across as impolite, I appologize. But I have the flu right now, and it is frustrating having to say the same thing over and over again.