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The left figure is one-stage low pass filter whose cutoff fre-
quency fc depends on lhe capacitance C and the resitance R.
1
l c =
2rCR
using a variable resistor changes the cutolf frequency fc
randomely.
FETs (Field Effect Transistor) have a characteristics that the
resistance belween source and drain varies in accordance with
gate voltage.
V g = h i g h . . . . R s d = s m a l l
V g = | o w . . . . . R s d = l a r g e
VCF (Voltage Controlleo rllfffl
O u t f f i l n
{ c R
h
Flg. A
o u t * r n
iR
Flg. B
Vg
T Gate
Source T-JT--
Drain
Rsd
Flg. G
Therefore, putling a FETinstead ol avariable resistormakes
the
same circuit as figure B.
Ftg. R
Since the FETs gate voltage is provided lrom DAC1, the cutoff frequency varies by the strength ol breath.
Namely, stronger the breath, higher the cutoff frequency.
Weak breath
T
T
Strong breath
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