. Electronic apparatus for biological research . HT+ * ^ri Figure oscillation is limited only by the maximum excursions of which the valve anodes are capable, from HT positive (valve cut off) to a level of anode potential corresponding to zero, or slightly positive, grid bias, We shall call this latter state 'conducting hard'. This circuit and those which we shall consider in this and the next chapter are all non-linear ones in which the anode potential of a valve is not a replica of the grid potential, as it has been in most of the valve circuits we have dealt with up to now. With t
. Electronic apparatus for biological research . HT+ * ^ri Figure oscillation is limited only by the maximum excursions of which the valve anodes are capable, from HT positive (valve cut off) to a level of anode potential corresponding to zero, or slightly positive, grid bias, We shall call this latter state 'conducting hard'. This circuit and those which we shall consider in this and the next chapter are all non-linear ones in which the anode potential of a valve is not a replica of the grid potential, as it has been in most of the valve circuits we have dealt with up to now. With the non-linear or 'pulse' circuits we often find valves being used in the rather simple role of switches. They are either 'off' (or 'cut off') or 'on' (or 'conducting hard'). Design procedure for pulse circuits may be carried out along the same general lines as for amplifier circuits. That is to say, a working region is constructed on the valve anode characteristic and a load line drawn. Because non-linear operation is intended the working region extends to the anode voltage axis and somewhat outside the characteristic for zero grid bias. 'Off' and 'on' are represented by the blobs on the load line {Figure ). High speed of operation on transients in pulse circuits is secured by, amongst other things, the use of anode loads of rather low resistance, of the order 5-10 kQ instead of 50-100 This has the effect of making the load line steep, and in order to secure a swing of anode voltage comparable with that of Figure a high anode current and anode dissipation become * Except in cases where the frequency has to be very stable, when it is best to square up the output of a crystal oscillator. 227
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