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High speed amplifier ??

Discussion in 'Electronic Design' started by rain, Feb 17, 2006.

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  1. rain

    rain Guest

    I am a student of Northwestern Polytechnical University in China.
    want to use a photodiode (AEPX65:Responsivity 0.35A/W,Dark Current
    2nA,NEP 6.8e-14WHz-½,Capacitance 6pF,Risetime 1ns) to receive abou
    10nsec wide pulses but low intensity about 10e-6mJ,then the output curren
    is converted to voltage by an amplifier circuit,the voltage signal i
    transmitted to the oscilloscope (Tektronix 100 MHz BW,1 GS/s Sample Rate
    so that to dispaly the pulse wave.Now I have the photodiode and th
    oscilloscope, but I don't know which amplifier is suitable.OPA657
    EL2075?maybe other more suitable? And I else don't know how to design th
    voltage feedback circuit. If someone has some suggestion or som
    schematic, it would be great.Hoping to hear from you soon.
    Thank you in advance

    Jiang Yajun
  2. Phil Hobbs

    Phil Hobbs Guest

    By '10e-6mJ' do you mean 10 nJ or 1 nJ? Either way, those pulses are
    *huge*, with peak currents of 350 mA or 35 mA. You're going to saturate
    that poor photodiode completely. In any case, you won't need an
    amplifier: just connect the PD cathode to a +5V supply through a 1k
    resistor, connect a 47 nF capacitor between the PD cathode and the
    centre conductor of a piece of coax, and connect the coax shield and
    power supply common terminal to the PD anode. Then put a neutral
    density filter in front of the poor photodiode to knock the peak current
    down to 1 or 2 mA. Use the 50-ohm input setting on the scope.

    Your pulses will be 50 or 100 mV tall, easily seen on the 10 mV/div
    range. If you want more signal, any fast amplifier is fine--you don't
    need an OPA657. A Mini-Circuits MMIC amp is a good possibility. Keep
    the impedance levels low, because this isn't a low-light application.

    (In fact if the duty cycle were high enough you could easily power the
    amplifier off those pulses!)


    Phil Hobbs
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