Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2002-04-11
2004-06-15
Mottola, Steven J. (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P
Reexamination Certificate
active
06750705
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to class D amplifier circuits, and more particularly, to a method and a circuit to improve the power supply rejection ratio of a class D amplifier circuit.
2. Description of the Prior Art
Class D amplifiers are used in a variety of applications. Increasingly, MOSFET-based, class D amplifiers are used in audio applications. Referring now to
FIG. 1
, a simplified schematic for a class D audio amplifier is shown. A CMOS buffer B
1
14
is used as the audio amplifier. The buffer B
1
14
receives the input signal IN
10
and generates the output signal OUT
18
. The output signal OUT
18
drives an audio speaker SPK
22
, though the coupling may be indirect, such as through a capacitor.
As shown in the diagram, the CMOS buffer
14
converts the input signal IN
10
into the output OUT
18
. Note that the output voltage
18
varies directly with the battery voltage BATT
26
powering the buffer
14
. During the first signal pulse, the battery BATT
26
level is V
1
. However, during the second pulse, the battery level becomes V
2
. The output level OUT
18
increases due to the increase in the BATT
26
level. The energy transferred to the speaker SPK
22
by the class D amplifier is proportional to the output OUT
18
voltage multiplied by the ON time. Therefore, the energy transfer for the first pulse is proportional to the area under the signal curve (A
1
) while the energy transfer of the second pulse is proportional to the area A
2
. It can be clearly seen that the variation in battery voltage BATT
22
causes a large difference in energy transfer which translates into a large difference in speaker SPK
22
loudness. The variation in BATT
22
may be due to a large signal DC shift, as shown, or due to a large signal AC component that has been coupled onto the battery supply. Either way, the prior art circuit displays poor power supply rejection ratio (PSRR). The analysis of the simplified, prior art class D amplifier shows that either scenario may result in significant speaker SPK
22
loudness variation.
Several prior art inventions describe class D amplifiers. U.S. Pat. No. 6,107,875 to Pullen discloses a class D amplifier. A frequency compensation and gain control circuit is used to improve frequency performance. An integrator is used in the circuit. U.S. Pat. No. 6,262,632 to Corsi et al describes a class D amplifier using a ramp generator to create a delay to eliminate cross-talk. U.S. Pat. No. 5,815,581 to Andersson teaches a class D amplifier having a feedback loop. U.S. Pat. No. 5,672,998 to Wittlinger discloses a class D amplifier having a feedback loop using a pulse width modulated signal to reduce distortion.
SUMMARY OF THE INVENTION
A principal object of the present invention is to provide an effective and very manufacturable class D amplifier circuit.
A further object of the present invention is to provide a class D amplifier circuit with energy control to improve the power supply rejection ratio (PSSR).
A still further object of the present invention is to provide an energy control circuit for a class D amplifier using an analog feedback loop.
Another still further object of the present invention is to provide an improved H-bridge driver based on a class D amplifier having energy control.
Another further object of the present invention is to provide a method to improve a class D amplifier through energy control.
In accordance with the objects of this invention, an energy control circuit for a class D amplifier is achieved. The energy control circuit comprises, first, a means of generating an energy accumulation signal proportional to an output drive signal of the class D amplifier. Last, a means of receiving the energy accumulation signal and of interrupting the output drive signal when the energy accumulation signal exceeds a reference level is used.
Also in accordance with the objects of this invention, an H-bridge amplifier circuit is achieved. The circuit comprises, first, a first class D amplifier. The first class D amplifier comprises, first, a first output driver for receiving a first input signal and for generating a first output drive signal corresponding to the first input signal. Second, a first integrator generates a first energy accumulation signal proportional to the first output drive signal of the first class D amplifier. Finally, a first comparator receives the first energy accumulation signal and generates a first reset signal to interrupt the first output drive signal when the first energy accumulation signal exceeds a reference level. Second, a second class D amplifier comprises, first, a second output driver that receives a second input signal and that generates a second output drive signal corresponding to the second input signal. Second, a second integrator generates a second energy accumulation signal proportional to a second output drive signal of the second class D amplifier. Finally, a second comparator receives the second energy accumulation signal and generates a second reset signal to interrupt the second output drive signal when the second energy accumulation signal exceeds a reference level. The first output drive signal and the second output drive signal are coupled with a load therebetween.
Also in accordance with the objects of the present invention, a method to perform energy control for a class D amplifier is achieved. The method comprises, first, generating an energy accumulation signal proportional to an output drive signal of the class D amplifier. Second, the output drive signal is interrupted when the energy accumulation signal exceeds a reference level.
REFERENCES:
patent: 5672998 (1997-09-01), Wittlinger
patent: 5815581 (1998-09-01), Andersson
patent: 6107875 (2000-08-01), Pullen et al.
patent: 6262632 (2001-07-01), Corsi et al.
patent: 6362683 (2002-03-01), Miao et al.
Smedley et al., “One Cycle Control of Switching Converters”, Proc. of the Annual Power Elec. Spec. Conf., Mass., Jun. 25-27, 1991, NY, IEEE, U.S., vol. 22, Jun. 24, 1991, pp. 888-896, XP010048415.
Zheren Lai et al., “A Low Distortion Switching Audio Power Amplifier”, IEEE No. 0-7803-2730-6/95, 1995, pp. 174-180, XP002198564.
Ackerman Stephen B.
Dialog Semiconductor GmbH
Mottola Steven J.
Saile George O.
Schnabel Douglas R.
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