hi all,
 in passive mixer, a common structure is switch+TIA,due to parasitic cap of the switch, the TIA feedback factor is small, maybe 1/10. so the TIA loaded bandwidth is small. however, in a receiver system, there is often a requirement like IIP3 for N+6, N+12 channel,  if the channel bandwidth is wide, then TIA can't not have a high gain at 6*bandwidth & 12*bandwidth, then the IIP3 can not be good enough.
I want to know how can the TIA be to have enough bandwidth with a good phase margin. I have heard that the TIA can use feedforward method to compensate , who have a detailed description or reference?
thanks

TIA?

Acronym?

TIA = Time Interval Analyzer - but what does it mean here?

Hi,


First of all, I guess you already know, but the intermodulation distortion is due to the amplification in the op-amp. I suggest you analyze your system very thoroughly. You may find that the switch capacitance or even the low-pass nature of the TIA has the effect of filtering the signal before amplification. In that case, the IIPn can be good even well outside the TIA loop-gain bandwidth. Especially if you turn the TIA into a LPF. Of course, increasing the power in your op-amp's output stage will also help.


regards,
Aaron

Obviously widening the loop gain bandwidth will improve the linearity at higher offset frequencies. I'm not disagreeing with that. However, I suggest that you properly analyze your system to check the out of channel IIPn before going to great efforts to improve your loop gain-bandwidth. The reason is because the out-of-channel signal may be heavily attenuated at the input of the op-amp. I've attached a diagram of what I'm refering to ((a) shows a normal TIA and (b) shows a LPF TIA). I haven't analyzed the Op-Amp input above the loop-gain bandwidth of the Op-Amp (that's why I said you need to check!).

I believe - He's talking about out of band signals where the linearity is compromised due to amplifier bandwidth, but the results are not consequential because the content is out of the passband and is being attenuated due to that.

aaron_do,
I think we are saying the same thing. if you look at you figure carefully, you will find that at interference B and C frequency, you opam still has enough gain. your opam -3dB bandwidth is higher than your low pass RC corner.
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.
thus, the TIA needs very high unit gain bandwidth

thanks

Baohulu,


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No we are not. You are saying

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but I am saying,

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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.

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I drew the figure, so probably I was looking at it carefully when I drew it. I already mentioned,

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Also, you read too much into my figure.

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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.

Baohulu,


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yes. thanks for pointing that out.

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Incorrect. Compared to the normal TIA case, the effect of adding the LPF response is the same regardless of whether the LPF transfer function or the op-amp open-loop gain has a higher bandwidth. Compared to the normal TIA case, the signal effectively sees the LPF response in front of the op-amp. Therefore the improvement is very significant.

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yes. But it is not my method. I didn't invent the active-RC LPF.

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Incorrect. You need to analyze your system to see whether you need to improve the loop-gain bandwidth. As you already agreed, the LPF TIA shows better (actually much better) linearity than the standard TIA.

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that is true. And that is why its better to analyze your system to see if you really need that high bandwidth. That is what I was always saying.

Also note that you have control over the TIA DC gain which will affect the linearity. You can always reduce the DC gain to improve the linearity. Since the output is filtered, the linearity requirements of the following stages will be greatly relaxed.


Aaron