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THS7530: Input stage of THS7530

Matth. B.
Expert 1135 points
Part Number: THS7530
Other Parts Discussed in Thread: VCA810, VCA2612, LMH6401, VCA820

Hello,

I'm looking about the THS7530 to make a high gain fully diifferential amplifier. Regarding the datasheet, I see that THS7530 used a modified Gilbert cell structure.

- Could everyone explain me with more details what is the advantage of this architecture? Is this use is madatory because of the variable gain control?

- Also, what about the stability fo this opamp, no issue?

Thanks for your help.

  • 0
    Guru 48195 points

    Kind of an open ended query Matthieu, 

    But some general comments, 

    Parts like THS7530 provide a continuously variable gain under voltage control in a dB/V sense. They can have pretty low input voltage noise as this one does. 

    This is a diff to diff version, there are also alternate diff to single ended versions, like the VCA810 and then the VCA82x series. 

    There are also a whole series of CMOS TGC type VGA's for Ultrasound originally, like the VCA2612 which are usually multi-channel 

    Usually, these part maintain a constant BW with gain control, often they are not particularly good DC precision wise, 

    as time went on, a vast range of much higher speed DVGA came out (Digital Variable gain amps) driven by the cellular market. e.g LMH6401

    Normally, parts like the THS7530 etc that are open loop Gilbert cell types have no stability issues in the front end, you can get into trouble with the closed loop output stages at times if (VCA820 type) if you treat them improperly, but the THS7530 itself does not have that option - as always, output cap load can cause issues. 

  • 0
    TI__Mastermind 23855 points

    Hello Matthieu,

        A gilbert cell structure is not mandatory for variable gain control, there are other VGA architectures. Here is a research paper which I believe you will find useful which lists common VGA architectures and their differences which can be found on page 98.

    Thank you,

    Sima

  • 0
    Expert 1135 points

    Hello,

    Thanks a lot for your answers.

    About the stability, do you have a curve of an open loop of Gilbert cell?

    And on the THS7530, in fact there is no closed loop, right?

    My final goal is to use this amplifier to build a low noise amplifier with diff input and single ended output, high gain (40 dB) and high bandwidth (100 MHz)

    Thanks

  • +1 in reply to Matth. B.
    TI__Mastermind 23855 points

    Hello Matthieu,

       The THS7530 would fulfill those requirements. Gilbert cell is a modified version of open loop; I believe this is discussed also in the research paper I shared earlier. The gain/frequency plots can be located after the table specifications especially look at Figure 1. 

    Thank you,

    Sima