Current to Voltage amplifier

Discussion in 'Homework Help' started by sjkim0329, Dec 13, 2008.

  1. sjkim0329

    Thread Starter Member

    Oct 19, 2008
    12
    0
    Hi, guys
    I'm trying to design a current to voltage amplifier for high speed photodetector(response>25MHz).
    I have a basic idea of current to voltage amplifier but I'm just kind of lost.
    How would this high response would affect my circuit?

    Thank you for your time.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The gain is the ratio of the input current to that through the feedback resistor. Think of a resistor as a current to voltage converter.

    As far as frequency response goes, you will find there aren't too many op amps around that will give much gain at 25 MHz. You will need to do a parametric search to find one. That is a function of the op amp design more than of the circuit (yours is very incomplete).

    There should be roughly a zillion photodiode circuits in existence.
     
  3. sjkim0329

    Thread Starter Member

    Oct 19, 2008
    12
    0
    Thanks beenthere,

    As you suggested, I was able to do some more research and after some calculations I was able to complete the circuit.
    Using Multisim, I plotted frequency response at the output of the amplifier.
    My understanding is that magnitude should stay pretty much the same.
    But mine is actually decreasing. Any ideas what I'm doing wrong here?
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    The placement of C1 & 2 is a bit odd. Are they supposed to be decoupling capacitors? Also, what does the data sheet say about gain at higher frequencies? Remember that your op amp is not ideal, so gain is increasingly limited with frequency.
     
  5. sjkim0329

    Thread Starter Member

    Oct 19, 2008
    12
    0
    Large Signal Bandwidth
    440 MHz, G = +2
    320 MHz, G = +10
    Small Signal Bandwidth (–3 dB)
    1 GHz, G = +1
    700 MHz, G = +2

    Gain is +2 when it's 50MHz.

    more info, http://www.analog.com/static/imported-files/data_sheets/AD8009.pdf

    I realized they were pointless so I got rid of those capacitors and frequency response looks better now. Magnitude stays constant but according to my calculation magnitude suppose to be close to 1V but for some reason, it's very small(1.95e-243). Did I possibly set up AC current source wrong? Also, I'm doing AC analysis 50MHz to 100Mhz, Octave(sweep type), log(vertical scale).

    Thank you very much for your help so far.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    You have a current of 10^-7 and a gain of 10^5, for a resulting output of 10 mv. You might consider adding another op amp for an additional gain of 100.
     
  7. sjkim0329

    Thread Starter Member

    Oct 19, 2008
    12
    0
    Yes, I thought changing the gain would change the output value but it didn't work.
    I was getting error message: iteration limit reached

    I tried to change this value at options but it is still giving me this error.
    It works when current is 0.1uA and resistor is 1Kohms.
    Any ideas? Thank you guys.
     
  8. eblc1388

    Senior Member

    Nov 28, 2008
    1,542
    102
    Hi sjkim0329,

    Does the following two circuits work in the same way?

    Which one would you use?
     
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