. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. INTERSTAGE COUPLING of amplification. A reasonable design procedure is as follows: Take the lowest frequency the amplifier is required to handle and divide it by the number of RC couplings there are going to be. This gives w^. Take the reciprocal, getting the minimum product CRg for each coupling. There is usually a maximum value of Rg which the makers of V^ will allow. It is called the maximum grid resistance and is given in the manufacturers' litera- ture. Having found this, the economic
. Electronic apparatus for biological research. Electronic apparatus and appliances; Biology -- Research. INTERSTAGE COUPLING of amplification. A reasonable design procedure is as follows: Take the lowest frequency the amplifier is required to handle and divide it by the number of RC couplings there are going to be. This gives w^. Take the reciprocal, getting the minimum product CRg for each coupling. There is usually a maximum value of Rg which the makers of V^ will allow. It is called the maximum grid resistance and is given in the manufacturers' litera- ture. Having found this, the economic minimum value of C follows. There is no point in making C larger, as 'microfarads cost money'. Automatic grid bias—^With RC coupled amplifiers it is possible to dispense with the bias batteries by using 'automatic grid bias', which is illustrated in Figure The bottom of the grid resistance is taken to earth and instead HT +. Figure of making the grid slightly negative to earth we make the cathode slightly positive, by causing the cathode current to flow down a resistance. The value of resistance Rj^ required is simply Requisite bias Rt.^ = ^ Cathode current Notice the use of the term cathode current rather than anode current. In Figure a triode is shown and the two are the same thing. If the valve had been a pentode or tetrode, the cathode current would have been the sum of the anode and screen currents. The screen current is usually small but it ought not to be forgotten. The cathode current fluctuates with the input signal and produces a fluctuating bias voltage across it. This is fre- quently undesirable; the fluctuation in bias may be 'ironed out' by connecting across the bias resistance a large capacitance C^, the 'cathode bypass capaci- tance'. This must be chosen so that its reactance is always much lower than the resistance of i?^, even at the lowest frequency the amplifier is to transmit. Values of the order of 100 microfarads are often necessary. Th
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