Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 CATHODE FOLLOWER from a special battery which is returned to the cathode. This method has the advantage that the screen current is kept out of the load (Figure ). Advantage of high input impedance If our source of input voltage has a high internal resistance r, and the valve into which it works has input resistance R and input capacitance C (Figure ), then there occurs: ^R B] Figure (1) A loss of voltage deUvered to the input terminals AB, due to the
Electronic apparatus for biological research Electronic apparatus for biological research . electronicappara00dona Year: 1958 CATHODE FOLLOWER from a special battery which is returned to the cathode. This method has the advantage that the screen current is kept out of the load (Figure ). Advantage of high input impedance If our source of input voltage has a high internal resistance r, and the valve into which it works has input resistance R and input capacitance C (Figure ), then there occurs: ^R B] Figure (1) A loss of voltage deUvered to the input terminals AB, due to the poten- tial divider action of R and r\ the fraction of the transferred is Rl{r + R). (2) An additional loss at high frequencies due to the shunting effect of C. We have a simple low-pass filter turning over at 1 ft>. = U+''/ R To get fo^ high we need r, R and C low. We have no power to alter r must be ^r for reason (1), so C must be small. With the cathode follower C is small and R is large, so it forms the ideal stage to follow generators of very high internal resistance. A conventional amplifier may then follow the cathode follower. Output impedance The output impedance of a device is its internal resistance regarded as a generator, and may be inferred by seeing what happens to the output voltage when some kind of external load is connected across it. In the case of the cathode follower, suppose as the result of the connection of a load R^ (Figure ) the output voltage falls dV^. The grid potential is unaffected, so dVgj,^dVj^ and an extra current flows in the valve f^dVj(R + O where R = R„ in parallel with Rj^. R^ is invariably much smaller than Rj,, so the extra current is approximately /nd VJ(Rl + ^a) and nearly all flows in the external load. The output impedance is Change of output voltage ^ d Vj, RL + r, Rl 1 /^ gm Change of output current f^^Vk RL + r, 173
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