. Electronic apparatus for biological research . 7V2V (a) (b) Figure There remains one thing to do. So far as signals are concerned, Kg is part of the load in a manner analogous to the cathode resistor in a concertina phase-splitter with valves, and introduces negative feedback. Where such feedback is undesirable it may be removed by shunting i?, with a low impedance to alternating currents; an electrolytic capacitor of the order of 100 [xP is suitable, to give Figure as the complete circuit for the stage. The base potential-divider slightly reduces the input resistance of the sta
. Electronic apparatus for biological research . 7V2V (a) (b) Figure There remains one thing to do. So far as signals are concerned, Kg is part of the load in a manner analogous to the cathode resistor in a concertina phase-splitter with valves, and introduces negative feedback. Where such feedback is undesirable it may be removed by shunting i?, with a low impedance to alternating currents; an electrolytic capacitor of the order of 100 [xP is suitable, to give Figure as the complete circuit for the stage. The base potential-divider slightly reduces the input resistance of the stage, since the 10 and the 40 k resistors are both effectively in parallel with it; a rather bigger coupling capacitor is therefore called for. Some input signal power is lost in R^ and 7?2 5 their values should therefore not be chosen too low, but equally they must not be too high or the stabilization can be shown to be adversely affected. The values arbitrarily allotted are typical. Other transistor parameters and equivalent circuits We now enumerate some of the other ways of considering transistors which are extant, showing how to equate the alternative approaches to that used in this chapter. The current generator in Figure is sometimes replaced by a voltage generator of output (5/iQ^^ (Figure ). R^^ is a 'mutual resistance'; —• vWW -nAAV Hn^ --^S^KT R, Figure that is, the transistor is regarded as introducing so many volts in the collector circuit per amp in the base circuit {cf. mutual conductance, so many amps in the anode circuit per volt at the grid, with valves). The necessary conversion factor is just R^ — ^R^. 691
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