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Bruce Trump

You have voted. Unity-gain-stable op amps won in a landslide—they’re far more popular than decompensated op amps. What’s this all about?

Click Here to read on EDN Magazine site.

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  • Mangchi-- Just to clarify terminology, the OPA637 is a decompensated amplifier. An uncompensated amplifier would require external components for stable operation in virtually any common configuration.

    The noise gain of a transimpedance amp when driven from a pure current source is unity (beta=1). Feedback makes the impedance at the summing junction low, but this does not mean that beta is zero. In fact, it is the strong feedback that causes the impedance to be low.

    A typical DAC does not have an infinite output impedance. It's not really a current source. In fact, its output impedance is code-dependant and this creates a code-dependent noise gain. Furthermore there may be a significant output capacitance which affects the high-frequency noise gain and therefore the stability. All to say, it's a more complicated situation and depends on the specifics of the DAC. I suggest that if you have further questions you should post on our E2E forums as this will be highly product-dependent.  -- Bruce

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  • Mangchi-- Just to clarify terminology, the OPA637 is a decompensated amplifier. An uncompensated amplifier would require external components for stable operation in virtually any common configuration.

    The noise gain of a transimpedance amp when driven from a pure current source is unity (beta=1). Feedback makes the impedance at the summing junction low, but this does not mean that beta is zero. In fact, it is the strong feedback that causes the impedance to be low.

    A typical DAC does not have an infinite output impedance. It's not really a current source. In fact, its output impedance is code-dependant and this creates a code-dependent noise gain. Furthermore there may be a significant output capacitance which affects the high-frequency noise gain and therefore the stability. All to say, it's a more complicated situation and depends on the specifics of the DAC. I suggest that if you have further questions you should post on our E2E forums as this will be highly product-dependent.  -- Bruce

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