Voltage feedback (VFB) op amps normally have both differential and common-mode input impedances specified. Current feedback (CFB) op amps normally specify the impedance to ground at each input. Different models may be used for different voltage feedback op amps, but in the absence of other information, it is usually safe to use the model in Figure 1 below. In this model the bias currents flow into the inputs from infinite impedance current sources. I B+ I B
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