Cathode Follower and Negative Capacitance as High Input Impedance Circuits

Action potentials recorded across membranes may have a maximum rate of rise of up to 103 v/sec and the impedance of the electrode plus the generator is of the order of 10 MΩ. Therefore, to reduce distortion of the recorded signal, it is necessary to diminish the effective capacitance of the input circuit to about 1 μμf. It is also requisite to reduce the currents which pass through the biological specimen, both the input grid current (< 10-13 A) and the current charging the input capacitance (< 10-9 A). The performance of a cathode follower and a negative capacitance as to reduction of input capacitance was measured by the damping factor a and the time constant T' of the second-order transfer function. An equivalent input time constant Teq = aT' of 10 μsec ensures small distortion and negligible current through the cell. Whether a cathode follower or a negative capacitance is the more suitable depends on the value of that part of the input-ground capacitance Cg, which cannot be removed by screening. When Cg is large (microelectrode deeply immersed in the specimen) a negative capacitance is advantageous; with a small value of Cg (electrode immersed < 1 mm) the cathode follower may neutralize to a Teq = 30 μsec as does a negative capacitance with a cutoff frequency of 200 kc.