Design of High Impedance Front Ends with Bootstrap

Interfacing electronics with very high impedance devices requires elimination of loading effects due to the current flowing into the front end circuit, which unacceptably reduces signal levels. This unwanted attenuation can be so high as to make it almost impossible to detect the signal. This paper discusses techniques for interfacing electronic circuits with very high internal impedance voltage sources.

In particular, the design of high-impedance buffers is illustrated, with particular attention to the problem of increasing the input impedance by bringing the positive feedback gain close to unity without giving rise to self-oscillation. A circuit with femtofarad level input capacitance and wide bandwidth is presented as a case study. A high-output impedance current source is discussed as well, as an example of use of the buffer. Finally, fields of application of circuits for high impedance sensing and sourcing are surveyed.