Good feature of op-amp summing amplifier is that there is direct current pa th provided from every input to the output. For that reasons input voltage sources don't need to have current sink capability. Good feature of op-amp non inverting summing amplifier is that output volta ge is proportional with the sum of the input signals, and with the same sig n. Bad feature is that there is no direct current path from the inputs to t he output, so input voltage sources must have sinking current capability.
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