Amplifiers – Modulator-demodulator-type amplifier
Reexamination Certificate
2000-05-09
2002-01-15
Pascal, Robert (Department: 2817)
Amplifiers
Modulator-demodulator-type amplifier
C330S20700P, C330S251000, C375S238000
Reexamination Certificate
active
06339360
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to digital amplifiers and more particularly to a pulse width modulated (PWM) audio amplifier employing a pulse insertion circuit for maintaining circuit operation in overdriven conditions and for preventing damage to amplifier components when the output is grounded.
Conventional linear amplifiers modulate load power by varying the conduction through output devices such as transistors or vacuum tubes. This continuous variation implies operation of the output devices in a region of linear response. Dissipated power is equal to the product of voltage across and current through the device. As a result, such output devices in linear amplifiers must dissipate considerable power. The dissipated power represents a loss in the efficiency of the amplifier. In addition, the energy lost must be removed from the device in order to avoid overheating. As a result, linear amplifiers have intrinsic efficiency limitations, and require complicated and expensive heat dissipation schemes.
In contrast, digital amplifiers, for example class D devices, operate by using an incoming analog signal to modulate a digitally switched signal. The digital signal is amplified and demodulated to produce an amplified version of the analog input. Modulation switching transistors in saturation are cut off, with only brief transitions between each cycle through the linear region of operation. Consequently, thermal losses due to dissipated power through the output device tend to be much smaller than corresponding losses in an analog amplifier.
Digital amplifiers, however, generally have higher component counts and are more complex than corresponding linear amplifiers. These factors result in higher direct costs for components, higher development costs, increased process risks during manufacture and increased failure risks thereafter. It is therefore desirable to reduce the component count of an effective digital amplifier while maintaining performance advantages offered by digital switching.
FIG. 1
shows a general block diagram of a conventional pulse-width modulation system
100
.
FIGS. 2A and 2B
show exemplary waveforms of the system
100
under normal and overmodulated conditions, respectively.
An oscillator
101
produces a square wave clock signal or reference pulse R output at a frequency FR, and a sawtooth output S, at a frequency 2FR. An input signal IN is amplified through a gain stage
102
and a feedback stage
103
. The output of the feedback stage
103
is coupled to one input of a comparator
104
as CIN signal. The sawtooth signal S is coupled to other input of the comparator
104
.
The comparator
104
generates a pulse-width modulated (PWM) gate drive signal G. The gate drive G toggles between high and low states, as shown in FIG.
2
. The PWM gate drive signal G is coupled via drivers
106
and
107
to MOSFET power switches
108
and
109
for amplification. The amplified PWM signal is coupled to a load
111
, e.g., speaker via an output filter
110
, where the carrier signal is removed. Negative feedback in the form of a sample FS from the output filter
110
is coupled to an input to the feedback stage
103
as shown to reduce signal distortion.
The above-described modulator works satisfactorily under input signal conditions illustrated in FIG.
2
A. However, if the input to the comparator signal is overdriven, i.e., IN exceeds the peak amplitude of the sawtooth S, as shown in
FIG. 2B
, the gate drive signal G fails to toggle and is maintained in one state or the other. The condition known as overmodulation causes the gate drive signal G to reduce to a DC level in the regions OM, instead of a pulse with modulated signal. This condition leaves one or the other of the MOSFETS
108
and
109
in an ON state until such time as the input IN returns below the peak amplitude of the sawtooth signal S. The impedance of the output load
111
, which is generally 4 or 8 ohms, limits the current through the ON MOSFET during the over-modulated period and prevents component failure.
An optional gain control circuit
105
, coupled between the output of the comparator
104
and a gain control input
102
A of the gain stage
102
, may be employed to reduce the gain of the gain stage
102
and thereby reduce or limit overmodulation. However, the gain control
105
is only operable over a finite dynamic range. Accordingly, if the input signal IN is increased beyond this amount, the gain control
105
ceases to function and the overmodulation condition can once again occur.
Although the amplifier
100
can function when this condition occurs, if the output load is shorted to ground, the overmodulated condition results in the ON MOFSET being current limited only by its internal ON-resistance, plus the impendance of the output filter
110
, which is generally less than about 250 millihoms. Under these conditions the continuous current through the ON MOSFET will likely exceed its continuous current rating and cause component failure.
U.S. Pat. No. 4,724,396 describes a gain control circuit that reduces the gain of the gain stage when overmodulation occurs. The patent suffers from the above-described limitations.
U.S. Pat. No. 5,917,369 describes a pulse width modulated amplifier that employs output limiting logic and an automatic gain control circuit. The patent describes a circuit for inserting pulses into an overmodulated region of a PWM signal to prevent filter saturation when the amplitude of the input signal exceeds that of a modulating triangle wave reference. The square wave, synchronized to the modulating triangular wave triggers a monostable vibrator whereby pulses of fixed duration are generated. The arrangement requires a complex drive circuit and a high component count insertion circuit.
SUMMARY OF THE INVENTION
The invention is based upon the discovery that toggling pulses may be inserted in the output of a PWM amplifier during overmodulated periods so that the duty cycle of output switch can be limited. The arrangement lowers the current through the output device, e.g., MOSFET, and component failure is thereby avoided. The present invention inserts pulses using a simplified circuit whereby series connected circuits produce opposite polarity pulses for a insertion gate circuit.
An input muting circuit limits the current through the MOSFETs to reduce thermal losses during short circuit conditions thereby avoiding over-temperature conditions.
REFERENCES:
patent: 4479175 (1984-10-01), Gille et al.
patent: 4992751 (1991-02-01), Attwood et al.
patent: 5387875 (1995-02-01), Tateno
patent: 5389829 (1995-02-01), Milazzo
patent: 5815581 (1998-09-01), Anderson
patent: 5917369 (1999-06-01), Nguyen
patent: 5929702 (1999-07-01), Myers et al.
patent: 6016075 (2000-01-01), Hamo
patent: 54-10653 (1979-01-01), None
patent: 4-315312 (1992-11-01), None
Rimskikh, E.I., Turov, A.I. Dronov A.P., et al., “Pulsewidth modulator with two sided pulsewidth reduction” Bul. 35/25 Sep. 1976, Dec. 23, 1974 as 089412, p. 840., Sverd Natural Econ 23.12.74-SU-089412 (Dec. 17, 1976) H03K-07/08 *abstract*.
David Arthur Sauer, “Correction Scheme for Non-Linear Ramp Tip In A Pulse-Width Modulator”, TN No.: 1257 (Sep. 8, 1980), Technical Notes: A Publication of RCA/Princeton, NJ (2 pages).
Vodolagin, Yu A., “Pulsed secondary supply control pulse-width modulator”, SU 1001-459-A, 83/849855/51, U22 U24, Mar. 12, 1980, SU-211699 *abstract*.
Harris Semiconductor, (http://www.zettweb.com/CDROMs/cdrom006/drivers/audioamp.htm), Class D Audio Amplifier Evaluation Board HIP4080AEVAL2, Mar. 29, 2000 (3 pages).
Dykema Gossett PLLC
Nguyen Patricia T.
Peavey Electronics Corporation
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