A high-input-impedance buffer

The input impedance of high-input-impedance buffers with a bootstrapped biasing circuit and supply rails may attain values much larger then it would correspond to the open-loop gain of the operational amplifier used. Similarly, the efficiency of suppression of capacitance connected to the inputs is unexpectedly high, although not that of the opamp input capacitance itself. Good agreement of experimental and calculated properties is achieved using a multiple feedback model, taking into account, besides the open-loop gain, also the common-mode and power supply rejections of the operational amplifier and the voltage transfer to the supply rails. The combined open loop and parasitic gains result in an effective gain that may attain very high, theoretically even infinitely high, value. The bootstrap of the biasing resistor multiplies the noise, although without affecting the signal-to-noise ratio.