Appeal No. 2000-1939
Application 08/791,281
is fixed (virtual ground of an operational amplifier) and the
other end of the capacitor C3 is also connected to a fixed
(ground) voltage reference via the transistor M2, which
transistor is still in a conduction state." Id. at 4,
lines 6-11.
In contrast, when transistor M4 is turned off, which occurs
while transistor M1 is still on, the charge stored in the channel
of transistor M4 is injected into capacitor C3 in an amount that
is dependent on the magnitude of the input voltage VIN. Id.
at 4, lines 15-21. More particularly,
[t]he circuit node B sees, toward ground, an
intrinsic parasitic capacitance Cp which is due to the
source and drain diffusions of the two transistors M1
and M2, and a conduction resistance RON of the
transistor M1.
As those familiar with the art will readily
recognize, both the capacitance Cp and the resistance
RON have a non-linear pattern versus voltage.
Since the voltage at the circuit node B varies
with the voltage of the input signal VIN, it follows
that the impedance seen toward ground from the circuit
node B, and consisting of a parallel of the parasitic
capacitance Cp and the conduction resistance RON of the
transistor M1, will also vary with the input voltage.
This causes the amount of the charge injected into
the capacitor C3 by the transistor M4 to be non-
constant and vary with the input signal Vin [sic, VIN],
thereby adversely affecting the system linearity and
introducing harmonic distortion in the output signal.
Id. at 4, line 22 to page 5, line 3.
-4-
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