. Electronic apparatus for biological research . 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
. Electronic apparatus for biological research . 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. The biasing scheme is automatic in the sense that as the valve wears out and the anode current fafls, the bias is automatically reduced and a measure of compensa- tion obtained. FREQUENCY RESPONSE OF INTERVALVE COUPLINGS We have already seen that whilst the frequency response of a direct-coupled pair of valves extends down to zero, the response of an coupled pair is that of a simple high-pass filter, that is, the response curve is 3 dB's down at 154
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