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Unwanted oscillations in a Transimpedance Amplifier (TIA) using a OPA637

Adriaan Carter
Prodigy 10 points
Other Parts Discussed in Thread: OPA637

Dear Forum,

I am trying to implement a typical transimpedance amplifier to amplify the current from a photodiode using an OPA637, as shown below. However I am getting rail to rail square wave oscillations. I am powering the circuit with plus/minus 12V batteries, which should be very quite. The oscillations occur at a wide variety of Rf (2K Ohm, 20K Ohm, 100K Ohm) and a wide variety of feeback capacitors  across Rf (5 pF, 18 pF, 100 pF, 1800 pF). The largest capacitor turns the  output into a triangle wave. Removing the photodiode makes no difference. The circuit is perfectly well behaved when I substitute the OPA637 with OPA128. What am I missing?

Adriaan

  • 0
    TI__Expert 3555 points

    Adrian,

    I suspect that the combination of your feedback cap and photo diode cap is causing this problem along  with the minimum allowable closed loop gain of your amplifier.

    The circuit stability can quickly be determined by comparing the value of the minimum stable gain of the OPA637 to (1 + Cin/Cf) where Cin  = Cdm +Ccm+Cdoide+Cpcb.

    Let me explain these variables.

    Ccm - amplifier input common mode capacitance

    Cdm -  differential amplifier input capacitance

    Cpcb- pub stray capacitance around the amplifier input pins.

    If the am minimum stable gain is less than (1+Cin/Cf) your circuit will oscillate.

    I would love to calculate this for you. What is the value of your photo diode junction capacitor equal to?

  • 0 in reply to Bonnie Baker - WEBENCH Design Center
    TI__Expert 3555 points

    Adriaan,

    I made it back to my desk tonight and I am able to give you a better answer.

    The OPA637 can be designed into a photodiode amplifier design with the following configuration

    Rf = 160 k

    Cf = 1.3 pF

    Max photodiode current = IpdMax = 50 uA

    Photodiode junction capacitance - Cpd = 50 pF

    Here are some calculated statistics about this configuration

    Phase margin = 65 degrees, signal bandwidth = 738 kHz, VoutMin = - 4V, VoutMax = 4V, Vref (at the non-inverting amplifier input) = - 4 V