. Electronic apparatus for biological research . Figure n n But this is by definition n, so the output in terms of «is t——j^ «y —^ ,thatis, 1 -\- AB AB the effect of the feedback is to make the noise in the output AB times less. Cathode follower noise An important application of the above is to the cathode followers which frequently precede the first amplifying valve in electrophysiology. Here B equals 1 and A is the gain, without feedback, gm^^, between 30 and 100. Thus the noise generated by the cathode follower is only between 1/30 and 1/100 of the noise it would produce if it were an
. Electronic apparatus for biological research . Figure n n But this is by definition n, so the output in terms of «is t——j^ «y —^ ,thatis, 1 -\- AB AB the effect of the feedback is to make the noise in the output AB times less. Cathode follower noise An important application of the above is to the cathode followers which frequently precede the first amplifying valve in electrophysiology. Here B equals 1 and A is the gain, without feedback, gm^^, between 30 and 100. Thus the noise generated by the cathode follower is only between 1/30 and 1/100 of the noise it would produce if it were an amplifying stage. If the cathode follower is preceded by a glass microelectrode, as is usually the case, its noise is quite unimportant. CASCODE CIRCUIT There is an interesting circuit which has triode-like noise but pentode-like gain. It is called the cascode and is shown in Figure Two triodes are used, usually similar, the grid of the upper one being returned to a fixed positive potential in a similar manner to a pentode screen grid. Both valves run under space charge limited conditions and so there is reduced shot noise from each but no partition noise. We have for the lower valve di^- and for the upper valve MR + O ....(1) .(2) From 2 dV. /* 257
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