However, when considering higher frequency effects, it would be wrong to assume the input impedance remains zero at higher frequencies because it. When a large impedance is combined with even a tiny amount of capacitance, the result is a large τ. The basic op-amp transimpedance amplifier looks like this, with the op-amp's non-inverting (+) input grounded, and a feedback resistor Rfbetween inverting (-) input and output: The input current flows entirely through the feedback resistor, and the op-amp adjusts its voltage output to keep its inputs at equal voltages. From the perspective of Kirchhoff's Current Law at the op-amp's inverting input node, it is indistinguishable as to whether the current contribution comes from a resistor VinRin=iin (from the previous op-amp inverting amplifier section) or simply from an externally-specified input current iinas in the transimpedance amplifier. A voltage-controlled voltage source (VCVS) model lets us examine more fine-grained behavior of the transimpedance amplifier and its limitations. As we did in the inverting amplifiersection, we'll replace the ideal op-amp with a VCVS model.
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