A HIGH-INPUT-IMPEDANCE BUFFER WITH MULTIPLE FEEDBACK

Štefan Lányi - Marco Pisani

   The properties of bootstrapped high-input-impedance unity gain buffers have been analysed. It turned out that the input impedance and the noise may attain values much larger than it would correspond to the opamp open-loop gain. Correspondingly, the efficiency of suppression of capacitances connected to the inputs was unexpectedly high, though not of the opamp input capacitance itself. Good agreement of experimental and calculated properties was achieved using a multiple feedback model, taking into account, besides the open-loop differential gain, also the common-mode and power supply gains of the operational amplifier and, independently, the voltage transfer to the supply nodes.

Keywords: unity gain buffer, bootstrap, multiple feedback, noise, input impedance

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