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variable gain transimpedance amplifier

Other Parts Discussed in Thread: OPA657, LMH6629, LMH6505, LMH6503

Hi everyone,
 
Can someone help me on designing a photodiode amplifier with the variable gain. The op-amp I want to use is OPA657 due to it's wide bandwidth with the feedback resistor about 3M OHM. Photodiode has the capacitance of 15 pF. Some suggested about using the JFET on the amplifier but can someone help to design it and explain about it?

Thanks. 

  • Hello David,

    As a broad response, it'd seem to me that you'd be better off if you followed / succeeded your TIA (Transimpedance amplifier) with a post amp which would then provide you gain control. Is that possible in your setup?

    With your conditions (Cin= 15pF, RF= 3Mohm) and using the OPA657 (1.6GHz GBWP), you are going to be limited to around 2-3MHz of bandwidth (equations below from the LMH6629 datasheet):

    So, a variety of post amplifiers would work for you to set your gain. Examples:

    LMH6505, LMH6503, etc.

    Or, alternatively you could use a circuit similar to what I've attached below (taken from my colleague Kristoffer Flores 2011 E2E post), where he has shown the response to various gains and has tried to compensate and to minimize the variations:

    2664.OPA695 VGA attenuator 6-8-2011.TSC

    In your case, you would not be going up to the 200-300MHz bandwidth regions that Kristoffer's setup is meant for, and you'd have much less impact on bandwidth as you vary gain.

    If a post amp idea does not work for you, you'd probably have to employ an analog switch in your TIA feedback path and then have to compensate for its off capacitance, etc. But then again, with your relatively low bandwdith, this may not be too difficult to do.

     

    Hope this helps you.

     

    Regards,

    Hooman