
393
Let us now see what happens at 470 kHz. At this frequency the
reactance of c
be
is 1.7 kQ. And
c
b
>
c
is 68 kQ, which in parallel with
r
b
>
c
is very nearly equivalent to the same capacitance in series with
2.3 kQ of resistance. The total impedance (Sec. 6.8) is hardly more
than 68 kQ. We begin as before with 1 V of ec, and assuming the
same load we get the same AF and FE (Fig. 22.7). But DA is very
different both in magnitude and phase angle. The feedback
impedance is still large enought for r
be
to be neglected in
comparison with it, but is much less than before, so DA is 1000/68
= 14.7 μΑ; and by drawing the impedance triangle (or by
calculation ...