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OPA1622: What is the minimum stable gain that can be achieved with the OPA1622

Part Number: OPA1622

Hello,

We are using the OPA1622 for an amplifier application. When we use 2.2k input resistors, and 10k feedback resistors for a gain of 4.54x, the circuit performs flawlessly. However when we swap the resistors to 33k for the input, and 47k for the feedback, to achieve a gain of only 1.42x, we are having problems that we suspect is oscillation. Should this be expected? If so, what is the minimum gain that can be used for stable performance?

Thanks in advance! 

  • Hi Mike,

    The OPA1622 is unity gain stable, so I don't suspect that a gain=1.42V/V would show any issue. 

    What is your input signal? The input pins of the OPA1622 are protected from large differential voltage with anti-parallel protection diodes. In most circuit applications, the input protection circuitry has no consequence. However, in low gain circuits, fast-ramping input signals can forward bias these diodes because the output of the amplifier cannot respond quickly enough to the input ramp. If the input signal is fast enough to create this forward-bias condition, the input signal current must be limited to 10 mA or less.

    -Tamara

  • Hi Tamara,

    The input signal is 1v differential sine wave. It's the exact same level from the same source that works flawlessly when the 2.2k input resistors, and 10k feedback resistors are used. The PCB engineer testing the boards only has a scope. I have a proper analyzer, but can't test them until I receive them. But would rather get the issue sorted while he still has them.

    Here's the scope traces with 10khz input with 2.2k/10k resistors:

    Here is 10khz input with 33/47k resistors. But @ 1khz input, it looks fine just like the plot above. 

    Also, with the 33/47k resistors, the amplitude reduces with a rise in frequency. With the 33k/47k resistors, the amplitude at 10khz is only half of the amplitude at 1khz. With the 2.2k/10k resistors, it stays flat with a rise in frequency like it's supposed to.

  • Hi Mike,

    I looked into this a bit more. What you are seeing is an input bias current error. The OPA1622 is a bipolar input amplifier so large value resistors will interact with the input bias current, resulting in overall error.

    Try reducing the 47k/33k values to a smaller ratio, like 470-ohm and 330-ohm.

    -Tamara

  • Okay but I need to have an input impedance of at least 20K with this gain stage. Otherwise it will be too tough of a load for some sources to drive. Closer to 50K would be even better. So are you saying this will not be possible with this opamp? 

  • Hi Mike, 

    You can try adding a resistor in series with the the non-inverting input to balance the ib effect.

    The parallel combination of 47k and 33k is approximately 19k:

    -Tamara