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Post amplifier for OPA847 ?

liu tianyi
Prodigy 100 points

Other Parts Discussed in Thread: OPA847, TLC339, THS4508

Hi,  all

I am designing a  transimpedance amplifier (TIA), using OPA847. see My Tina schematic.  The photodiode capacitance is 50 pF, the bandwidth requirement is roughly 70 MHz.

Since it is single ended and  most of the Limiting amplifier has a  differential  input,  Does anybody have a suggestion how to choose a post amplifier?

Should I use a  comparator?   Just like design example in "optical detector with analog & TTL output" in TINA "examples"?

Thanks in advance.


0211.70MHz TZA.TSC

  • 0
    TI__Expert 5400 points

    Hi Liu,

    It is unclear to me what specific functions you are hoping to perform with a second amplifier stage.  Are you simply trying to further amplify the signal from your first amplifier stage, without reducing its bandwidth?  Or do you need to perform a single ended to differential conversion, since you mention differential inputs to a limiting amplifier?  It would also be helpful if you could describe the overall application you're working to implement.

    With some more information, we can make a better decision as to what amplifier best suits your needs -- 

  • 0 in reply to Bart Stiller
    Prodigy 100 points

    Thanks, Bart

    What I need is to receive the small analog input of the TIA and get a digital output

    It seems I have two options:

    1,  TIA +  single ended to differential + Limiting amp (with differential input) +CDR

    2,   TIA+  fast comprartor (just like the TLC339 in the design example "optical detector_1" in TINA)

    Which one should I use? or do you have any suggestions?

  • 0 in reply to liu tianyi
    TI__Expert 5400 points

    Liu,

    It sounds like you are trying to detect some sort of input threshold current, and set a corresponding binary output, is that correct?  If so, I would say a comparator would be the appropriate choice, as it will require fewer components and the threshold voltage could be set more easily.  Your first proposal seems unnecessarily complex to perform this function.  Let me know if I have misinterpreted your application --  

  • 0 in reply to Bart Stiller
    Prodigy 100 points

    Thanks, Bart

    I put my question in the figure, try to make it clear.

    I have designed the first stage of the TIA using OPA847 and feedback resistor and capacitor. 

    the question is which kind of the amplifier should I use for the next stage for single end to differentail and output buffer?  (showing in the Figure )

  • 0 in reply to liu tianyi
    TI__Expert 5400 points

    Since your first TINA simulation used such a small source signal, with only 2.4k transimpedance gain, I will assume you need additional gain from subsequent stages.  As such, I would recommend an FDA with a large gain bandwidth product, such as the THS4508.  I will attach a TINA simulation that demonstrates such a circuit.

    2211.OPA847 + THS4508.TSC

  • 0 in reply to Bart Stiller
    Prodigy 100 points

    Thanks very much, Mr. Bart.

    your circuits definitely works.  I realize it may too much to ask. However, I have two little concerns:

    1,  the output common voltage  of out1 and out2 is different. (should it be the same?)

    2,  since my input is  small than 1mV,  is it possible  the offset (1-4 mV) of the Amp decreases the sensitivity of the TIA?