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anode or drain voltage, and this loss may sometimes need to be taken
into account.
If only a simple resistor is used, it carries not only the steady (or
z.f.) component of current but also the signal current, which
produces a voltage in opposition to the input. We have, in fact,
negative feedback (Sec. 19.15). If that is wanted, well and good; but
if not, then something has to be done to provide a path of relatively
negligible impedance for the signal currents, without disturbing the
z.f. resistance. A by-pass capacitor, C
s
in Fig. 13.8, serves the
purpose; for r.f. a value of 0.1 μΡ or even less is enough, but for the
lowest audio frequencie ...