Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2003-03-27
2004-09-14
Shingleton, Michael B (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C327S131000, C327S132000, C327S137000
Reexamination Certificate
active
06791405
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a Class-D amplifier mainly used for power amplification of an audio signal and in particular to a triangular wave generating circuit used to convert an analog audio signal to a pulse signal.
FIG. 6
is a block diagram showing an exemplary configuration of a Class-D amplifier. In this figure, a numeral
101
represents an analog signal input terminal,
102
a triangular wave generating circuit,
103
an integrator,
104
a voltage comparator for comparing the output of the integrator
103
with the output of the triangular wave generating circuit
102
,
105
a pulse amplifier,
106
,
106
′ switching elements controlled to turn ON/OFF by the output of the pulse amplifier
105
, and
109
,
109
′ positive and negative power sources. A numeral
110
represents a resistor for feeding back a PWM signal obtained at the junction of the switching elements
106
and
106
′ to the integrator
103
. The resistor
110
and a resistor
111
determine the feedback amount. A capacitor
112
is a DC cutoff capacitor. A numeral
107
represents an LPF (low-pass filter) and
108
represents a load.
FIG. 7
is a waveform diagram showing the waveform of each section of FIG.
6
.
FIG. 7A
shows the waveform of the output S
1
of the triangular wave generating circuit
102
,
FIG. 7B
the waveform of an analog signal S
2
input to the input terminal
101
,
FIG. 7C
the waveform of a signal (PWM signal) on the non-inverted output terminal of the voltage comparator circuit
104
,
FIG. 7D
the waveform of an output signal S
4
of the LPF
107
.
FIG. 8
is a waveform diagram showing the waveforms of the output S
1
of the triangular wave generating circuit
102
, signals at the output terminals R
1
, R
2
of the voltage comparator
104
, and a signal at the junction Q of the switching elements
106
,
106
′. In
FIG. 8
, a sign P represents the output of the integrator
103
and a sign P′ an ideal waveform of the output of the integrator
103
.
As shown in these figures, the analog input signal S
2
is supplied to the voltage comparator via the integrator
103
. The signal is compared with the output S
1
of the triangular wave generating circuit
101
and converted to a PWM-modulated pulse signal in the voltage comparator
104
(see FIG.
8
B and FIG.
8
C). Then, the signal is amplified by the pulse amplifier
105
and switching-amplified by the switching elements
106
,
106
′. The switching-amplified signal is turned into the analog signal S
4
by the LPF
107
and output to the load
108
.
FIG. 9
is a circuit diagram showing the details of the Class-D amplifier shown in FIG.
6
. The integrator
103
comprises an operational amplifier
121
and a capacitor inserted between the non-inverted input terminal and the output terminal of the operational amplifier
121
. The LPF
107
comprises a coil
124
and a capacitor
125
.
In the Class-D amplifier, the precision of a triangular wave generated in the triangular wave generating circuit
103
has a great effect on the distortion of amplification. Thus, it is quite important to generate a high-precision triangular wave with negligible variations in the peak value and a negligible offset deviation.
FIG. 10
is a circuit diagram showing an exemplary configuration of a related art triangular wave generating circuit. In
FIG. 10
, a numeral
1
represents an input terminal where a clock pulse of a duty ratio of 50 percent is input,
2
an amplifier,
3
a resistor,
4
an operational amplifier,
5
a capacitor, and
6
an output terminal. In this circuit, when a pulse input to the input terminal goes high and low alternately, recharging/discharging of the capacitor
5
takes place accordingly, and an output voltage Vout changes in a shape of a triangle.
FIG. 11
is a circuit diagram showing another exemplary configuration of a related art triangular wave generating circuit. In
FIG. 11
, numerals
1
,
2
,
4
through
6
are same as those in FIG.
10
. Numerals
11
,
12
represent switch elements controlled to turn ON/OFF by the output of the amplifier
2
. Numerals
13
,
14
represent are respectively constant-current circuits. In this circuit, when the switch element
11
is turned ON and the switch element
12
is turned OFF, the capacitor
5
is recharged by a current I
1
. When the switch element
11
is turned OFF and the switch element
12
is turned ON, the capacitor
5
is recharged in a direction opposite to the above direction by a current I
2
. This operation is repeated to cause the output voltage Vout to be changed in a shape of a triangle.
In the circuit of
FIG. 10
, the output voltage Vout is obtained using the following expressions:
Q=CV (Q: electric charge of the capacitor
5
; C: capacitance of the capacitor
5
; V: voltage of the capacitor
5
)
it=CVout (i: current flowing in the capacitor
5
)
Vout=it/C=(RVin)/C (R: value of the resistor
3
; Vin: input voltage)
As understood from the calculation, the output voltage Vout depends on the value R of the resistor
3
, the capacitance C of the capacitor
5
and the amplitude and frequency of the input clock pulse. In general, the value R of the resistor
3
and the capacitance C of the capacitor
5
are varied so that the peak value of the output voltage Vout does not stay constant.
In the circuit shown in
FIG. 11
, an offset is generated on the output voltage Vout by a slight deviation of the duty ratio of the input clock pulse and the current value of the constant-current circuits
13
,
14
, as shown in FIG.
12
B.
FIG. 12A
shows a triangular wave free from an offset deviation.
SUMMARY OF THE INVENTION
The invention has been proposed under such circumstances and aims at providing a triangular wave generating circuit used in a Class-D amplifier which can generate a high-precision triangular wave free from variations in the peak value or offset deviation.
In order to solve the aforesaid object, the invention is characterized by having the following arrangement.
(1) A triangular wave generating circuit used in a Class-D amplifier, comprising:
an integrating unit including an amplifier and a capacitor inserted between the input terminal and output terminal of the amplifier;
a first constant-current unit which recharges the capacitor so that the output of the amplifier approaches a first predetermined voltage;
a second constant-current unit which recharges the capacitor so that the output of the amplifier approaches a second predetermined voltage;
a current setting unit which sets currents of the first and second constant-current units;
a first switch unit which makes ON/OFF control of the current flowing in the first constant-current unit;
a second switch unit which makes ON/OFF control of the current flowing in the second constant-current unit;
a first comparing unit which compares the output of the amplifier with the first predetermined voltage and outputs a signal when the output of the amplifier coincides with the first predetermined voltage;
a second comparing unit which compares the output of the amplifier with the second predetermined voltage and outputs a signal when the output of the amplifier coincides with the second predetermined voltage; and
a flip-flop whose output signal is inverted depending on the output of the first and second comparing unit, the flip-flop making ON/OFF control of the first and second switch units.
(2) The triangular wave generating circuit according to (1), wherein the current setting unit includes:
a phase comparing unit which compares the phase of an externally supplied clock pulse with the phase of the output of the flip-flop,
a low-pass filter for removing the high-frequency component of the output of the phase comparing unit, and
a current control unit which controls the currents of the first and second constant-current unit.
(3) A triangular wave generating circuit used in a Class-D amplifier, comprising:
an integrating unit including an amplifier and a capacitor inserted between the input terminal and output term
Noro Masao
Tsuji Nobuaki
Pillsbury & Winthrop LLP
Shingleton Michael B
Yamaha Corporation
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