Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2002-08-06
2003-11-11
Ton, My-Trang Nu (Department: 2816)
Amplifiers
Modulator-demodulator-type amplifier
Reexamination Certificate
active
06646502
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention applies to class-D amplifiers, and in particular, class-D audio amplifiers operating in a purely digital signal environment and, therefore, suitable for integration as part of a system on a chip.
2. Description of the Related Art
Referring to
FIG. 1
, as is well known in the art, class-D amplifiers receive an analog input signal
1
(e.g., depicted as a pure sine wave) and generates a digital output signal
3
(e.g., a bipolar, or three-level, pulse width modulated signal) having a low frequency component that is proportional to the input signal
1
. As is well known, one advantage of a class-D amplifier over a linear amplifier (e.g., class-AB) is greater efficiency, often approaching 100%. One common application for a class-D amplifier is as a driver for a loudspeaker. Such high efficiency makes class-D audio amplifiers quite suitable for integration as part of a system on a chip. One example of such a system on a chip would be a baseband processor for cellular or cordless telephones.
Referring to
FIG. 2
, a class-D amplifier
4
has often been implemented using a signal comparison circuit
6
, a reference signal source
8
, a non-inverting output driver
10
a
and an inverting output driver
10
b
interconnected substantially as shown. The analog input signal
1
is compared against a triangular reference signal
9
produced by the reference signal source
8
. The resultant comparison signal
7
is buffered by the output driver amplifiers
10
a
,
10
b
to produce the drive signals
11
a
,
11
b
for the loudspeaker
12
. Usually, a low pass filter (not shown) is also placed between the output signal
11
and loudspeaker
12
.
This amplifier
4
can be implemented using only a few simple analog circuit blocks. However, such a circuit
4
requires a stable power supply voltage VDD for the output buffer amplifiers
10
a
,
10
b
. Accordingly, since there is no feedback from the actual digital output signals
11
a
,
11
b
any variations in the power supply voltage VDD will be reflected in the output signals
11
a
,
11
b
. Hence, such a circuit
4
has a poor power supply rejection ratio (PSRR). Further, since the output signal switching frequency is not very high, the external low pass filter (not shown) is usually necessary.
Referring to
FIG. 3
, a higher PSRR can be achieved with a class-D amplifier circuit
14
in which a linear class-AB amplifier
16
is used. Such a circuit
14
includes the class-AB amplifier
16
, a current sensing circuit
18
(e.g., an electronic equivalent of an ammeter) and a digital output amplifier
20
, interconnected substantially as shown. The analog input signal
1
is buffered by the differential class-AB amplifier circuit
16
operating as a voltage follower circuit. The amplified input signal
17
passes through the current sensing circuitry
18
, the main output signal
19
a
of which provides the feedback for the class-AB amplifier
16
and some amount of drive for the loudspeaker
12
.
The current sensing output
19
b
of the current sensing circuitry
18
drives the digital output amplifier
20
. It is this output
21
of the digital output buffer amplifier
20
that provides the majority of the drive current for the loudspeaker
12
. Hence, the output
19
a
from the class-AB amplifier circuit
16
need only provide that relatively small amount of current necessary for maintaining the signal to the loudspeaker
12
at the desired level. A low pass filter, such as an inductor
22
, is necessary to provide isolation between the output terminals of the class-AB
16
and output
20
amplifiers.
While this circuit
14
provides an improved PSRR, it nonetheless continues to require an external low pass filter
22
, as well as an analog input signal
1
. Accordingly, implementation of this type of circuitry
14
in fully integrated form (e.g., for use as part of a system on a chip) remains problematic.
Referring to
FIG. 4
, another conventional class-D amplifier circuit
30
uses a delta-sigma modulator (analog)
32
, a signal slicer
34
and output buffer amplifiers
36
a
,
36
b
, interconnected substantially as shown, to drive the loudspeaker
12
. A differential analog input signal
31
is processed by the delta-sigma modulator
32
to produce a three-level output signal
33
(having values of −1, 0 or +1). This signal
33
is processed by the signal slicer
34
to produce the drive signals
35
a
,
35
b
for the output buffer amplifiers
36
a
,
36
b
. These output signals
35
a
,
35
b
are binary in that they have one of two states, depending upon the value of the slicer input signal
33
. For example, as indicated in
FIG. 4
, if the slicer input signal
33
has a value of +1, the Out+ signal
37
a
equals the positive power supply voltage VDD, and the Out− signal
37
b
equals the potential of the negative power supply voltage terminal VSS. The output drive signals
37
a
,
37
b
also serve as the feedback signals for the delta-sigma modulator
32
(in accordance with well known delta-sigma modulator circuit principles).
This type of circuit
30
has a good PSRR since the output buffer amplifiers
36
a
,
36
b
form part of the feedback loops for the delta-sigma modulator
32
. Accordingly, variations in the power supply voltage VDD or other voltage drops in the output amplifiers
36
a
,
36
b
are compensated by virtue of the feedback loops. Additionally, no external low pass filtering is required when a high oversampling ratio (OSR) is used in combination with the three-level output signal
33
generated by the delta-sigma modulator. However, the delta-sigma modulator
32
must still function as an analog circuit in order to compensate for analog variations in the power supply voltage VDD and other voltage drops in the output amplifiers
36
a
,
36
b.
Accordingly, it would be desirable to have a class-D amplifier circuit with a very high PSRR, no requirement for external filtering, and the capability for operating with a digital input signal.
SUMMARY OF THE INVENTION
In accordance with the presently claimed invention, a class-D amplifier is provided with a high power supply rejection ratio (PSRR) while accepting a digital input signal and not requiring an output signal filter, thereby being ideally suited for integration as part of a system on a chip. The input signal is converted by a first delta-sigma modulator to provide a first multivalue digital signal representing the desired output. This first multivalue digital signal is combined with a second multivalue digital signal provided by a second delta-sigma modulator to provide a third multivalue digital signal. This third multivalue digital signal is converted to binary digital output signals for differentially driving a load. These binary digital output signals are also fed back and combined with the first multivalue digital signal to provide the feedback signal for the second delta-sigma modulator.
In accordance with one embodiment of the presently claimed invention, a class-D amplifier includes delta-sigma modulation circuitry, signal combining circuitry and signal conversion circuitry. The delta-sigma modulation circuitry receives and converts a digital input signal to a first multivalue digital signal corresponding to the digital input signal, and receives a feedback signal and in response thereto receives and converts an analog input signal to a second multivalue digital signal corresponding to the feedback signal. First signal combining circuitry, coupled to the delta-sigma modulation circuitry, receives and combines the first and second multivalue digital signals and in response thereto provides a third multivalue digital signal corresponding to a sum of the first and second multivalue digital signals. The signal conversion circuitry, coupled to the first signal combining circuitry, receives and converts the third multivalue digital signal to first and second binary digital signals with first and second binary signal values that vary in relation to the third mult
National Semiconductor Corporation
Nu Ton My-Trang
Vedder Price Kaufman & Kammholz P.C.
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