Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 INTERSTAGE COUPLING Where the gain of the simple vohage ampHfier is insufficient we achieve the requisite amphfication in several 'stages' by coupUng the output of one valve to the input of the next. Again, when the input swing required by a power amplifying valve in order that the load be fully operated is greater than the available source of signal can provide we interpose a voltage amplifying stage. In both cases the problem arises as to how to couple adjacent valve
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 INTERSTAGE COUPLING Where the gain of the simple vohage ampHfier is insufficient we achieve the requisite amphfication in several 'stages' by coupUng the output of one valve to the input of the next. Again, when the input swing required by a power amplifying valve in order that the load be fully operated is greater than the available source of signal can provide we interpose a voltage amplifying stage. In both cases the problem arises as to how to couple adjacent valves. There are two broad divisions in the ways in which this can be carried out, depending on the frequency response required. If the response has to extend down to zero frequency the stages have to be 'direct coupled', but if it is possible to allot a lower frequency limit to the desired response the stages may be ' coupled'. As valves are voltage-operated devices it would be more logical if the term could be amended to ' coupled', but the notion of the coupled amplifier is well established and is hkely to remain. As coupled amplifiers are much easier to design and use it would be fooHsh to use a direct coupled apparatus for, say, action potential recording from a nerve, where a frequency response extending down to 100 c/s would be ample. On the other hand if one is interested in the absolute value of a membrane potential one has no alternative but to use direct coupling*. Figure DIRECT COUPLING METHODS Climbing amplifier By having an HT battery provided with a very large number of taps the grid of the second valve may be connected directly to the anode of the first, in the manner shown in Figure , which shows two voltage amplifying stages. * Except one, a special kind of amplifier called a carrier amplifier, with which we deal in Part IV. 11 149
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