Virtual earth current measurement

Figure 11.7 Low output impedance system. A, Buffered voltage supply to provide constant drain voltage B, FET, e.g. 2N3819 note the gate connection in reality is short (u) C, cell D, feedback amplifier to give low measurement impedance , constant-current source X, virtual earth. A3 is in a feedback loop with the reference electrode and cell. This provides appropriate values of Vq to give Fog and /d, i.e. the operating point is independent of the transistor used. and are kept constant, this being the case Iq and Fqs are also constant under steady state conditions. Change in potential is monitored through the calomel reference electrode. A similar feedback system was used by Janata et ai (11).

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