. Electronic apparatus for biological research . Stabilized 68k supply out 2 5k Figure' is applied via i\ to V^, which controls V^ in the same way as in Figure The reference potential is applied not direct to V^ cathode but to V^ grid. Vi and K3 form a differential amplifier with the 47k resistor as common 'tail'. Clearly when Pg slider is at the top there is no positive feedback, but as P2 slider is moved down feedback comes into action; for an increase in output voltage raises V^ grid and hence the common cathode potential. Kg anode current therefore falls, reducing the voltage dro
. Electronic apparatus for biological research . Stabilized 68k supply out 2 5k Figure' is applied via i\ to V^, which controls V^ in the same way as in Figure The reference potential is applied not direct to V^ cathode but to V^ grid. Vi and K3 form a differential amplifier with the 47k resistor as common 'tail'. Clearly when Pg slider is at the top there is no positive feedback, but as P2 slider is moved down feedback comes into action; for an increase in output voltage raises V^ grid and hence the common cathode potential. Kg anode current therefore falls, reducing the voltage drop across that fraction of /*2 above the slider and hence increasing the slider potential. This increase in potential must be communicated down the chain to V^ grid, hence the feedback is positive. In use, /*! is used to set the output voltage to the required value, whilst P2 determines the output impedance. After altering P2, Pi will require re-adjustment. The output impedance can be made zero, or even negative, but instabihty can occur if this is done. An important advantage possessed by this circuit, and not by those of Figures and , is that the use of differential amplification between the reference potential and P^ slider confers the usual advantage that the circuit is relatively unaff"ected by the variations in cathode emission which follow from feeding V^ from an unstabilized 6-3 V supply. 589
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