. Electronic apparatus for biological research . Figure Figure kinds of interference, by using differential amplification, but it is chiefly with the user that the onus of obtaining unadulterated signals at the amplifier input rests. Interference is borne in three ways: Electric fields—Most interfering electric fields arise in the manner shown in Figure ; the point P, driven by the interference generator Gj, alter- nates in potential with respect to earth in the neighbourhood of the amplifier input terminals. The equivalent circuit is shown in Figure Q is the coupling
. Electronic apparatus for biological research . Figure Figure kinds of interference, by using differential amplification, but it is chiefly with the user that the onus of obtaining unadulterated signals at the amplifier input rests. Interference is borne in three ways: Electric fields—Most interfering electric fields arise in the manner shown in Figure ; the point P, driven by the interference generator Gj, alter- nates in potential with respect to earth in the neighbourhood of the amplifier input terminals. The equivalent circuit is shown in Figure Q is the coupling capacitance between P and the 'live' input lead, Qn the amplifier input capacitance, R^ the input resistance, and Rq is the signal generator resistance—in electrophysiology an electrode. Clearly the effect produced by P is minimal when C<. is small (which is simply saying that P should be removed as far as possible) or when Qn is large, or when Rq or R\^ are small. Since the efficiency of signal transfer of the arrangement is poor unless R^^ is much greater than Rq, it follows that in practice it is Rq rather than R^^ which will be the determining factor. It follows that experiments using silver wire or wick electrodes are easier to carry out than those employing microelectrodes; for the former we have Rq of the order of ohms X 10^, whereas for the latter the figure is about a thousand times greater. Furthermore, with the low resistance electrode, C\^ may be allowed to be quite large, perhaps 100 pF, but the same value in 656
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